Microbial Mercury Methylation in Aquatic Environments: A Critical

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Critical Review

Microbial mercury methylation in aquatic environments – a critical review of published field and laboratory studies Olof Regnell, and Carl J Watras Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02709 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 15, 2018

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Environmental Science & Technology

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Microbial mercury methylation in aquatic environments – a critical review of

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published field and laboratory studies

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Olof Regnell*1 and Carl. J. Watras*2, 3

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1 Department

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Bureau of Water Quality, Wisconsin Department of Natural Resources, Madison, WI

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Center for Limnology, University of Wisconsin-Madison, 3110 Trout Lake Station Drive,

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Boulder Junction, Wisconsin 54512, USA.

of Biology/Aquatic Ecology, Lund University, SE-223 62 Lund

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TOC Art

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Abstract

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Methylmercury (MeHg) is an environmental contaminant of concern because it biomagnifies in

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aquatic food webs and poses a health hazard to aquatic biota, piscivorous wildlife and humans.

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The dominant source of MeHg to freshwater systems is the methylation of inorganic Hg (IHg) by

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anaerobic microorganisms; and it is widely agreed that in situ rates of Hg methylation depend on

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two general factors: the activity of Hg methylators and their uptake of IHg. A large body of

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research has focused on the biogeochemical processes that regulate these two factors in nature;

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and studies conducted within the past ten years have made substantial progress in identifying the

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genetic basis for intracellular methylation and defining the processes that govern the cellular

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uptake of IHg. Current evidence indicates that all Hg methylating anaerobes possess the gene pair

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hgcAB that encodes proteins essential for Hg methylation. These genes are found in a large

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variety of anaerobes, including iron reducers and methanogens; but sulfate reduction is the

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metabolic process most often reported to show strong links to MeHg production. The uptake of

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Hg substrate prior to methylation may occur by passive or active transport, or by a combination

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of both. Competitive inhibition of Hg uptake by Zn speaks in favor of active transport and

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suggests that essential metal transporters are involved. Shortly after its formation, MeHg is

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typically released from cells, but the efflux mechanisms are unknown. Although methylation

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facilitates Hg depuration from the cell, evidence suggests that the hgcAB genes are not induced or

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favored by Hg contamination. Instead, high MeHg production can be linked to high Hg

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bioavailability as a result of the formation of Hg(SH)2, HgS nanoparticles, and Hg-thiol

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complexes. It is also possible that sulfidic conditions require strong essential metal uptake

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systems that inadvertently bring Hg into the cytoplasm of Hg methylating microbes. In

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comparison with freshwaters, Hg methylation in open ocean waters appears less restricted to

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anoxic environments. It does seem to occur mainly in oxygen deficient zones (ODZs), and 3 ACS Paragon Plus Environment


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possibly within anaerobic micro-zones of settling organic matter, but MeHg (CH3Hg+) and

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Me2Hg ((CH3)2Hg) have been shown to form also in surface water samples from the euphotic

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zone. Future studies may disclose whether several different pathways lead to Hg methylation in

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marine waters and explain why Me2Hg is a significant Hg species in oceans but seemingly not in

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most freshwaters.

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The toxicity of MeHg (CH3Hg+) to humans (especially developing fetuses) was well established

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after incidents in Minamata, Japan during the 1950s and 60s and in Iraq in the 1970s1. Although

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early toxicology focused on highly contaminated systems, it became increasingly evident during

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the years from 1960 to 1990 that fish from many remote lakes showed elevated Hg levels2 and

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that the dominant chemical species of Hg in fish was MeHg.3, 4 These findings caused issuance of

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widespread fish consumption advisories across North America and Scandinavia, and they begged

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questions about the source(s) of the MeHg in seemingly pristine aquatic environments. Perhaps

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the first step toward answers was the report of an experiment in which inorganic Hg added to

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sediment was converted to MeHg by microbiological processes.5

Introduction

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In 1985, it was demonstrated that dissimilatory sulfate reducing bacteria (SRB) were the main Hg

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methylators in sediment taken from an estuary.6 Further studies showed that Hg methylation was

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catalyzed by enzymes and that a corrinoid protein was involved.7 Evidence suggested that

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inorganic Hg somehow entered the pathway of acetyl-CoA synthase production in which a

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corrinoid/iron sulfur protein is a methyl group donor.8 However, it was later found that some

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SRB that do not use the acetyl-CoA pathway methylate Hg.9 Almost two decades later, the genes


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hgcA and hgcB were identified that have been proven essential for Hg methylation. These genes

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encode a corrinoid protein and an iron-sulfur cluster protein, respectively.10

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Early field studies provided insight into the aquatic cycle of MeHg, and over the past +20 years

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the body of literature on the formation and distribution of MeHg across aquatic ecosystems has

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grown considerably. Initially, progress toward understanding the aquatic cycle of MeHg was

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hindered by analytical difficulties; but a series of analytical breakthroughs in the late 1980s

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enabled the measurement of MeHg in natural waters at picomolar concentrations.11-13 Coupled

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with trace metal clean sampling protocols,14 it then became possible to compare concentrations of

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MeHg among various waterbodies and to construct mass balances for MeHg in relatively pristine

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natural environments.15-17 Studies of lakes remote from industrial discharges suggested that

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MeHg was produced in anoxic hypolimnetic waters during the course of summer, reaching

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concentrations roughly 100 times those in aerobic mixed layers.18 Interestingly, lakes highly

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contaminated by industrial Hg discharge displayed concentrations of MeHg in water and

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sediment that were disproportionately low (Table S1). Data compiled by Schaefer et al. (2004)

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indicate that the ratio MeHg/total Hg decreases by about two orders of magnitude as waterborne

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total Hg increases from 1 ng/L in pristine sites to 1000 ng/L in contaminated sites, going on

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average from ~10% MeHg to ~0.1% MeHg.19 Cossa et al. (2014) presented similar results for

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sediment.20

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The refinement of mass spectrometric methods enabled the determination of methylation rates in

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sediment and water samples using isotopically enriched Hg as a tracer.21, 22 Furthermore, studies

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of in situ Hg isotope composition and how the different isotopes are fractionated by abiotic and

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biotic processes have provided valuable information about Hg biogeochemistry.23-25 5 ACS Paragon Plus Environment


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Mechanistic explanations of field and laboratory observations of Hg methylation are rapidly

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beginning to reach a high level of detail. Often cited reviews on Hg methylation include those by

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Morel et al. (1998); Ullrich et al. (2001); Hsu-Kim et al. (2013); and Lehnherr et al. (2014).26-29

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Here, we focus largely on recent findings that have firmly established the genetic and molecular

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basis of methylation and advanced our understanding of the pathways of Hg uptake, but also

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include field data that provide a broader perspective on Hg methylation. We start with methods

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for measuring rates of methylation, because such empirical data form the basis of the current

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understanding of mechanisms and controlling factors.

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A first approximation of Hg methylation activity in situ can be obtained by calculating the ratio

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of MeHg to total Hg (THg) at field sites.30-32 However, in most cases it will not suffice to simply

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measure MeHg and THg, because MeHg levels (e.g. in sediment and water) are affected not only

Estimating and evaluating measurements of Hg methylation rates

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by methylation, but also by biological, and photochemical demethylation,19, 33, 34 and

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translocation of MeHg.35-37 For lakes, in situ MeHg production has been approximated by

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measuring changes in storage over time and constructing mass balance budgets that compile an

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inventory of sinks (losses via outflowing water and demethylation) and inputs from external

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sources.34, 38 Potential sources of error include temporal variation in input and output not covered

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by sampling, and estimates of demethylation.

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At finer temporal and spatial scales, in situ Hg methylation rates can be estimated using

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incubation experiments where an isotopically enriched IHg tracer (labelled IHg) is added to the

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samples of interest. One can account for the demethylation rates by adding MeHg with an 6 ACS Paragon Plus Environment

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isotopic label differing from that of the added IHg. This enables simultaneous methylation and

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demethylation assays.39, 40

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Assuming first order reactions for methylation and demethylation, the net rate of MeHg

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formation can be expressed as

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d[MeHg]/dt = km[IHg] – kd[MeHg]

Eq.1

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where km = the integrated average Hg methylation rate constant (t-1) for the IHg species present,

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kd = the integrated average MeHg demethylation rate constant (t-1) for the MeHg species present,

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[IHg] = the concentration of inorganic Hg and [MeHg] = the concentration of MeHg.

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For methylation assays in which labelled MeHg ([MeHg*]) is initially zero, the analytical

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solution to Eq. 1 is

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[MeHg*](t) = [IHg*]t=0 km (1-e-(km+kd)t) /(km+kd)

Eq. 2

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where [IHg*]t=0 is the initial concentration of labelled inorganic Hg.

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At equilibrium, that is when [MeHg*]/dt = 0, it follows directly from Eq. 1 that

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km/kd = [MeHg*]/[IHg*]

Eq. 3

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A question often encountered in methylation assays is whether demethylation can be disregarded

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when calculating km. Using Eq. 2, it is possible to explore the effect of km and kd on [MeHg*]

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over time. It is also possible to explore how the time to reach near equilibrium [MeHg*] (e.g. 90

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% of [MeHg*]t=∞) decreases with increasing values of the sum of the rate constants (km + kd) and

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how [MeHg*]t=∞ is determined by km/kd and [THg*] (Figure S1).

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For demethylation assays in which [IHg*] is initially zero, the analytical solution of Eq. 1 is

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[MeHg*](t) = [MeHg*]t=0 (km + kd e-(km+kd)t) /(km+kd)

Eq. 4

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where [MeHg*]t=0 is the initial concentration of labelled MeHg.

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The equations above do not take into account that shifts in Hg speciation could alter km and kd

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during incubation. It is common to inoculate environmental samples or bacterial cultures with

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HgO, HgCl2 or Hg(NO3)2, all of which can be methylated at a high km. Over time, they can form

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other Hg(II) species with lower km-values through ligand exchange, sorption to particles,

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precipitation as HgS(s), reduction to Hg(0), or binding to cell constituents not involved in Hg

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methylation.41-46 Another reason for unstable methylation (and demethylation) rates is that the

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microbial community is subject to changes during incubation.47 These problems can be

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minimized by keeping incubation times short and Hg additions low. Also, unless km and/or kd are

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too high, a short incubation time makes it possible to ignore the competing reaction (e.g.

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demethylation in methylation assays), leading to simplification of Eq. 1 and its solutions.48 A

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remaining source of uncertainty is that methylation and demethylation may not be first order

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The use of Hg tracers that differ from the ambient Hg species is likely to result in Hg methylation

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rates that differ from those in situ. Therefore, the rates determined are usually viewed as potential

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rates. When the ratio [MeHg]/[IHg] (calculating [IHg] as [THg – [MeHg]) in ambient water or

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sediment is found to be lower than the experimentally determined km/kd ratio using Eq. 3, a likely

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reason is a higher km of the added Hg(II) tracer than of the ambient Hg.48 For example, when

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isotopically labeled Hg(II) was added to a whole lake to simulate direct atmospheric deposition,

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methylation was initially much faster than of ambient IHg, but it declined rapidly presumably

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because of speciation changes.49 It is possible to determine the extent to which ambient and

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added Hg differ with respect to km and kd by both analyzing the isotopes of the Hg tracers and an

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additional isotope of the ambient Hg.48 In one study, individual km values for different Hg species

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found to be present in brackish sediment were determined. Knowing their ambient

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concentrations, it was possible to provide an estimate of the ambient methylation rate for all

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species combined.42 Another approach is to obtain more ambient-like conditions by preincubating

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the Hg tracers with sterilized natural water or sediment.50, 51

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When evaluating effects of changing environmental conditions on km, it is important to consider

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both the methylation capacity (activity of Hg methylators) and the bioavailability of IHg

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substrate.52 When the organic matter (OM) present is highly accessible to heterotrophs, Hg

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speciation likely has a strong influence on km. Liem-Nguyen et al. (2016) demonstrated that Hg

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methylation in sediment enriched with algal carbon was strongly influenced by Hg speciation.53

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Conversely, Hg methylation rates in boreal lake sediment increased with algal-derived OM

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content but not with more refractory terrigenous OM, suggesting that microbial activity was

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limiting MeHg production.54 Terrestrial OM is likely more available to heterotrophs closer to its 9 ACS Paragon Plus Environment


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source where it is fresher,55 potentially explaining high MeHg levels and Hg methylation rates in

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northern wetlands.31, 56, 57 Because of the low content of nitrogen and phosphorus in terrigenous

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OM,58 nutrient delivery as well as priming effects of more labile OM likely play an important

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role in making terrigenous OM accessible.59, 60 These effects could help explain the presence of

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MeHg hotspots at the fringes of peatlands that receive nutrients via runoff water from uplands61

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and in surficial pulp fiber sediment where the microbes have access to settling algal matter and

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dissolved nutrients in the overlying water.62 In addition, OM has a strong effect on Hg speciation

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and bioavailability (Section 5).

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Both incubation studies and field studies clearly show that microbes within anaerobic microbial

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communities are responsible for Hg methylation in freshwater environments.22, 35 A gene pair,

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hgcA and hgcB, has been identified10 and shown to be present in all tested anaerobes that

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methylate Hg and not present in those that do not.63, 64 Deleting one or both of these genes in two

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model organisms commonly used in recent Hg methylation studies, Desulfovibrio desulfuricans

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ND132 (sulfate reducer) and Geobacter sulfurreducens PCA (iron reducer), caused both

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organisms to lose their ability to methylate Hg.10

Hg methylation genes

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Evidence indicates that the gene hgcA encodes a corrinoid protein that donates a methyl group to

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Hg(II).10, 65 In addition to the cobamide-binding domain facing the cytoplasm, the HgcA protein

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has a transmembrane domain that may be involved in the uptake of Hg and/or release of MeHg

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from the cell, or it may simply anchor the protein in the cytoplasmic membrane.10, 66, 67 The gene

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hgcB, with few exceptions present directly downstream of hgcA,64, 67 encodes an iron-sulfur

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cluster protein (HgcB) whose likely function is to provide electrons to the cobalt ion of HgcA, 10 ACS Paragon Plus Environment

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which is necessary for maintaining the methylation/demethylation cycle.10, 67 It is also possible

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that HgcB plays a role in the transfer of Hg(II) to HgcA.67 In the bacterial strains Desulfovibrio

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desulfuricans ND132 and Geobacter sulfurreducens PCA, deletion of hgcA and hgcB did not

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affect growth in the culture mediums used.10 Moreover, a proteome study of Geobacter

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sulfurreducens PCA indicated that deleting these genes does not affect proteins involved in

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central metabolic processes suggesting that they do not control any basal metabolic function, and

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that the methylating apparatus they encode has a specialized role.69 A finding that somewhat

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speaks against HgcA having as its sole purpose to carry out Hg methylation is that certain point

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mutations in hgcA resulted in increased Hg methylation relative to wild type Desulfovibrio

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desulfuricans ND 132.67

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The hgcAB genes have been identified in several different anaerobes, including dissimilatory

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sulfate reducers (SRB) and iron reducers (FeRB) belonging to the class δ-Proteobacteria within

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the phylum Proteobacteria, bacteria within the phylum Firmicutes (Clostridia), and in

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Methanomicrobia within the phylum Euryarchaeota. Hg methylators are thus represented in

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both Gram-negative and Gram-positive bacteria as well as in archaea, but seem to be most

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common within δ-Proteobacteria.10, 63, 64, 69-71 Yet, relatively few of the so far investigated

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anaerobes possess the hgcAB genes, even among δ-Proteobacteria.10, 63

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The phylogenetically irregular distribution of Hg methylation ability suggests that horizontal

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gene transfers (HGT) and/or gene deletions have occurred, and that the prevalence of hgcAB is

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significantly affected by the environment.10, 64, 72, 73 HGT can help microbial communities handle

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various stress factors, e.g. antibiotics74 and toxic elements such as arsenic.75 Hypothetically, Hg

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methylation could lead to decreased Hg exposure within microbial communities, provided that 11 ACS Paragon Plus Environment


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MeHg under the given conditions is less bioaccumulative and/or less toxic than the Hg substrate.

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It has been demonstrated in several laboratory incubations that Hg methylating anaerobes have

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the ability to quickly release MeHg after its formation.73, 76, 77 Also, most of the MeHg produced

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in bottles of anoxic lake water incubated in situ with an Hg(II) tracer passed 0.2 µm filters, as

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opposed to the remaining Hg(II) that was either cell-bound or precipitated as HgS(s).35 Efficient

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excretion and low uptake of MeHg by anaerobes is in contrast to the high accumulation rates of

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MeHg in higher organisms, notably fish, birds, and mammals, caused by active MeHg-cysteine

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uptake by L-amino acid transporters.78

236 237

The observation that MeHg levels can be high in relatively pristine inland waters and

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disproportionately low in Hg polluted systems (Table S1) is seemingly at odds with the

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hypothesis that Hg methylation is a detoxification mechanism. Possible reasons for low

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MeHg:THg ratios in Hg-polluted waters include high microbial demethylation rates and MeHg

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production being limited by Hg uptake and/or methylation capacity.19, 20 At least one study has

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shown that Hg pre-exposure did not increase MeHg production in two Hg methylating strains of

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D. desulfuricans.76 Moreover, there was not a clear increase in the expression of hgcA and hgcB

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in Desulfovibrio dechloracetivorans BerOc1 after exposure to 10 µg/L of Hg(II).79 Thus, it

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appears unlikely that hgcA, hgcB or any other gene involved in Hg methylation are induced

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simply by elevated Hg levels in the external medium.

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Genes involved in detoxification can be expected to increase in frequency when toxicant

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exposure increases.75 In a study of hgcA abundance along a soil THg gradient, a significant but

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weak correlation with THg was found, but of all variables tested (sulfate, OM, pH, NH4+, THg,

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and MeHg), OM was the one showing the strongest correlation with hgcA abundance, followed 12 ACS Paragon Plus Environment

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by MeHg and NH4+.80 These results may suggest that OM and anaerobic conditions rather than

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THg favored hgcA in the studied soils, although relative rather than absolute abundance in

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metagenomes may best reflect environmental selection pressures.64 In a study of Hg-polluted

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sediment from Romanian reservoirs, the activity of both merA (a well-known gene providing Hg

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resistance by encoding the reduction of Hg(II) to Hg(0)) and hgcA was measured. The number of

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transcripts of hgcA did not increase with Hg contamination level, but neither did that of merA. A

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conclusion drawn was that Hg speciation was a controlling factor in the different Hg

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transformations.81 Even though THg levels appear to have little effect on the expression and

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presence of hgcAB in microbial communities, increased Hg bioavailability could favor hgcAB.

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Poulain et al. (2007) found merA in Arctic microbial communities exposed to picomolar levels of

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Hg, suggesting that naturally low Hg levels could have detrimental effects on microbes lacking

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means of Hg depuration.82 In anoxic hypolimnia where MeHg is frequently a dominating Hg

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species as a result of in situ MeHg production,18, 22, 38 a large fraction of the inorganic Hg must be

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bioavailable and taken up by Hg metylating microbes, implying that the hgcAB gene pair is

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enriched in hypolimnetic microbial communities when the water turns anoxic. Future research

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may disclose whether or not enrichments of hgcAB are linked to increased Hg uptake.

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4

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The mechanism of methyl group transfer during microbial Hg methylation has implications for its

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evolutionary history and substrate specificity. The resemblance of HgcA and HgcB to proteins

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involved in the Wood-Ljungdahl (WL) carbon fixation pathway has led to the suggestion that

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HgcA and HgcB have evolved from WL proteins within methanogenic archaea,64 more

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specifically within the methanogenic lineage Methanomicrobia that reportedly diversified during

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the anoxic-oxic transition69 The evolutionary history of merB, a gene providing resistance to

The molecular mechanism of Hg methylation involving HgcA/B



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organic Hg compounds,83 perhaps could shed light on historic MeHg levels and thus on the

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environmental conditions that favored the evolution of the hgcAB genes. In the WL pathway,

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methyl groups are transferred from methyl-THF via a corrinoid iron-sulfur protein (CFeSP) to

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Ni(I) of acetyl-CoA synthase (ACS). Both of these methylation steps involve SN2-type reactions

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in which the strong nucleophiles Co(I) and Ni(I), respectively, attacks a methyl group and could

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be viewed formally as carbocation (CH3+) transfers. The methyl transfer between the corrinoid

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and Ni(I) thus helps keep the Co atom in the active Co(I) state.84, 85 The lower axial ligand of the

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cobamide cofactor may determine the nature and efficiency of the methyl group transfer. In the

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case of HgcA, this ligand is a highly conserved cysteine (Cys93)10, 67 and calculations have

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suggested that this configuration favors a carbanion transfer (CH3-) over a methyl radical transfer

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(CH3.). Carbocation transfers were not considered, because Hg(II) is not a nucleophile.65

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However, these calculations were recently challenged. Taking the relativistic and spin-orbit

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coupling effects on electronic structures of Hg into account, a computational study predicted

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formation of a transition state that is consistent with homolytic cleavage of the Co – C bond when

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Hg(SCH3)2 reacts with a corrinoid-based methyl donor. Notably, it was also predicted that the

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methyl radical transfer would be facilitated when cysteine is the axial base of cobalamin,86 in

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accordance with Cys93 being highly conserved in HgcA. As for CFeSP, HgcA receives methyl

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groups from methyl-THF.8, 68 Since there are reactions in bacterial cells other than Hg

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methylation involving methyl radical transfers,87 it cannot be excluded that Hg methylation

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involving HgcAB is accidental solely based on the fact that the Hg substrate is not a nucleophile.

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5

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The uptake of Hg by anaerobic methylators such as sulfate and iron reducing bacteria (SRB and

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FeRB) can occur by passive or active transport processes.28 To successfully model the

Hg uptake by methylating anaerobes


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environmental constraints on methylation rates, it is necessary to understand how these organisms

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acquire Hg substrate.

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5.1

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The passive transport of solutes into cells occurs via simple or facilitated diffusion, the difference

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being that facilitated transport involves specific transmembrane proteins. Both pathways involve

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solute movement down a concentration gradient, and neither requires energy expenditure by the

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cell.

Passive transport

305 306

Initially, Hg uptake in anoxic sulfidic environments was modelled to occur primarily by the

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passive diffusion of the neutrally charged complexes Hg(SH)2 and HgS(aq).88 This notion was

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supported by thermodynamic equilibrium speciation, octanol-water partitioning89 and by

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experiments and field studies showing that concentration of neutral sulfide species correlated

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more strongly than other dissolved Hg species with MeHg yields and ambient MeHg levels.90, 91

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However, more recent studies suggest that the presumptive HgSaq species was more likely HgS

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nanoparticles (HgSNP),92, 93 and experiments have shown that HgSNP are available for

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methylation both in pure cultures of bacteria,94-96 and in sediment samples.97, 98 Recent modelling

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also suggests that the passive diffusion of low molecular weight Hg-thiols through bacterial

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cytoplasmic membranes could be significant,99 but this has yet to be demonstrated

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experimentally.

317 318

The availability of HgSNP for methylation increases with decreasing aggregation and increasing

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disorder (irregular Hg – S coordination), as evidenced by relationships between MeHg yields,

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HgS particle size, Hg – S coordination numbers and bond lengths.94, 96, 100, 101 It has been shown 15 ACS Paragon Plus Environment


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that silver nanoparticles 10 %) of the total Hg in suboxic waters and in sediment,108, 109

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its passive uptake could potentially lead to significant MeHg production. High sulfide levels

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causing HgS(s) precipitation could further increase the relative importance of Hg(0) uptake,105 as

348

could reduction of Hg(II) to Hg(0) by FeS110 and by reduced OM.111

349

5.2

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Transport of solutes into cells against a concentration gradient requires direct consumption of

351

ATP or in the case of charged solutes (ions) an electrochemical gradient across the cell

352

membrane established by ATP-driven ion pumps. Low availability of essential metals due to low

353

dissolved concentrations can be overcome by release of extracellular chelators, outer membrane

354

receptors and inner membrane ABC (ATP Binding Cassette) transporters.

Active transport

355 356

Since the first discovery that cysteine enhances Hg methylation in pure cultures of Geobacter

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sulfurreducens PCA and Desulfovibrio desulfuricans ND132112, 113 several other studies have

358

confirmed this finding.73, 114, 115 This is also consistent with earlier observations of increased Hg

359

methylation upon cysteine additions in cultures of the Gram-positive firmicute Clostridium

360

cochlearium,116 and with more MeHg being produced from Hg(Cys)2 than from HgCl2 in

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estuarine sediment.117 Schaefer and Morel (2009) demonstrated that cysteine acts to facilitate

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intracellular Hg uptake.112 Furthermore, the uptake of Hg-cysteine complexes is energy-

363

dependent.113, 118

364 365

For mercury resistant Gram-negative bacteria with a mer operon, the active intracellular uptake

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of Hg(II) prior to its reduction is dependent on transfers between thiol groups of transport

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proteins.119 It has been suggested but never shown that thiol ligands such as cysteine act as a 17 ACS Paragon Plus Environment


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shuttle to membrane and/or periplasmic transporters also in Hg methylators.10, 66 However, a

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documented effect is that they increase Hg methylation by hindering the sorption of Hg to

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cellular sites not involved in the intracellular uptake.44 Yet, sorption to the cellular surface is

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likely an important step in bacterial uptake of Hg.46, 120 Sorbed Hg has been shown to be

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successively internalized,66 and this process is aided by thiols.44, 121

373 374

D. desulfuricans ND 132 was shown to methylate Hg complexed by glutathione (GSH),

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penicillamine (PEN) or cysteine at similar rates, whereas G. sulfurreducens PCA was only

376

capable of methylating the Hg-cysteine complex.113 In another study, Hg methylation was

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stimulated in the former organism and inhibited in the latter when Hg(II) was pre-equilibrated

378

with natural organic matter (NOM).122 Thus, of the two bacteria, D. desulfuricans ND 132

379

appears to have the strongest and/or the most flexible uptake system. Cysteine has been shown to

380

increase Hg uptake also in E. coli, which is incapable of Hg methylation,121, 123 suggesting that

381

Hg uptake follows similar routes among Gram-negative bacteria. Another example is the iron

382

reducer Shewanella oneidensis MR-1 which lacks both Hg methylation ability and the mer

383

operon.124 Moreover, methanogens possessing the hgcAB gene pair displayed enhanced Hg

384

methylation when cysteine was added,125 suggesting that cysteine complexation facilitates Hg

385

uptake also in archaea. In a study of Hg methylation by Desulfobulbus propionicus 1pr3, a 5-day

386

pre-equilibration of Hg with hydrophobic NOM led to higher MeHg yields than either a 4-hour or

387

a 30-day pre-equilibration period.126 Similar results were obtained in a study of Hg uptake by an

388

E.coli strain equipped with a mer-lux bioreporter.127 Taken together, these results suggest that Hg

389

uptake has organism-specific optima at certain Hg-complex stabilities (Figure 1). Charge, size,

390

and steric factors should also play a significant role in the uptake of different Hg-complexes. For


Page 19 of 71


391

instance, some ligands may form bulky polynuclear Hg complexes as a result of otherwise

392

unfavorable binding geometries (bond angles and/or lengths).128

393 394

395 396 397 398 399 400

Figure 1. Hypothetical curves drawn to illustrate that Hg(II) uptake rates first tend to increase with the stability of extracellular Hg(II) complexes and then to decrease at higher thermodynamic and/or kinetic stabilities because of decreasing exchange of Hg with putative membrane transporters. In the shown case, organism “B” has a stronger uptake system than organism “A”.

401

Field studies showing strong relationships between MeHg and thiol concentrations suggest that

402

thiols increase the availability of Hg for methylation also under natural conditions. In a study of

403

MeHg and Hg(II) complexes in natural waters, GSH but not cysteine showed pore water depth

404

profiles similar to those of MeHg.129 In the extracellular portions of lake periphytic biofilm (the

405

capsular fraction in close association with cells), both cysteine and GSH were significantly

406

correlated with MeHg.130 Strong correlations between km (Hg methylation rate) and the sum of

407

the concentrations of biogenic low molecular weight (LMW) thiols were recently demonstrated

408

for wetland soil and sediment, but there were no significant relationships between individual

409

thiols and km.131

410 411

Although evidence for LMW thiols playing an important role in the uptake and methylation of

412

Hg is compelling, the mechanisms involved are not yet clear. Notably, LMW thiols should be 19 ACS Paragon Plus Environment


Page 20 of 71

413

outcompeted by larger humic thiol ligands (NOM-RS) in the binding of Hg(II) simply because

414

the latter thiols show much higher concentrations in natural waters.132 There is even more

415

competition for Hg(II) when inorganic sulfide is present. For example, using available

416

thermodynamic constants, Liem-Nguyen et al. (2017) noted that Hg(SH)2(aq) would outcompete

417

Hg(NOM-RS)2(aq) at 1 µM total sulfide in wetland sediment pore water, despite a high NOM-RS

418

concentration (5.5 µM).132 Yet, other researchers measured Hg methylation rates of 10 % day-1 in

419

water from a dark-water Wisconsin lake having a sulfide concentration as high as 30 µM; and

420

ambient MeHg constituted 55 % of total Hg, suggesting that a high proportion of IHg was

421

available for methylation.22 Taken together, these results may imply that Hg(SH)2(aq) and/or

422

HgSNP are the main precursors of MeHg in sulfidic freshwater. By interfering with HgSNP

423

coagulation (allowing HgSNP passage through pores) and weakening Hg – S structures

424

(facilitating Hg exchange), NOM in brown-water lakes could increase microbial Hg uptake

425

(Section 5.1). Ticknor et al. (2015) studied MeHg in marine sediments and found that MeHg as a

426

fraction of THg (%MeHg) displayed a strong linear log-log relationship (r2 = 0.91, p < 0.0001)

427

with the fraction of THg extractable by GSH. 133 This finding is in line with the notion that there

428

is at least one extraction step prior to methylation involving thiols. GSH is generally not

429

produced by anaerobes, but they could produce other thiols having similar extraction

430

efficiencies.134, 135

431

5.3

432

The finding that the chirality of cysteine does not affect Hg uptake in D. desulfuricans ND 132

433

and G. sulfurreducens PCA militates against amino acid transporters being responsible for Hg

434

import, as does the finding that 2-MPA (not an amino acid) increased Hg uptake and methylation

435

in G. sulfurreducens PCA to almost the same extent as cysteine. Instead, it was proposed that

Evidence for essential metal transporters being involved in Hg uptake


Page 21 of 71


436

essential metal transporters are involved.113 This hypothesis is supported by the fact that metal

437

binding sites are seldom specific enough to exclude unwanted metal ions.136-138 Subsequently,

438

Zn(II) at realistic Zn(II):Hg(II) molar ratios was shown to significantly reduce both Hg(II) uptake

439

and methylation in Geobacter sulfurreducens PCA and Desulfovibrio desulfuricans ND 132.46, 118

440

Even in the absence of cysteine, Zn(II) reduced Hg methylation in Geobacter sulfurreducens

441

PCA (not tested for Desulfovibrio desulfuricans ND 132), but its inhibitory effect was stronger

442

(~3 times) when cysteine was present. Cd(II), known to have binding affinities similar to Zn, also

443

reduced Hg uptake and methylation. For Zn, toxicity was ruled out as a factor because the Zn

444

addition (50 µM) did not inhibit cell growth.118 In contrast, Fe(II), Co(II), and Ni(II) did not

445

inhibit Hg methylation (tested only in the presence of cysteine). Spheroplasts of Geobacter

446

sulfurreducens PCA (cells with the outer membrane removed) behaved similar to whole cells

447

with respect to Hg uptake and methylation, including the effects of Zn on these processes. In

448

addition to providing support for Hg methylation taking place in the cytoplasm, these findings

449

suggested that Zn(II) and Hg(II) were internalized by the same inner membrane transporters.118

450

Competition between Zn(II) and Hg(II) for the same transporters apparently takes place also in

451

non Hg methylators, because Zn decreased Hg uptake in an iron-reducer (Shewanella oneidensis

452

MR-1) incapable of Hg methylation,124 and in an E. coli mer-lux bioreporter equipped with a

453

merR gene but lacking mer transporter genes.120

454 455

Assuming that the transporters responsible for Hg uptake have evolved to acquire Zn, increased

456

Zn levels may decrease Hg uptake also as a result of downregulation of Zn transporters. It has

457

been demonstrated for a Gram-negative bacterium, Bradyrhizobium japonicum, that under

458

manganese limitation an outer membrane protein acting as an ion-specific channel is co-

459

expressed with an inner membrane high-affinity transporter of Mn(II).139 In order to compete 21 ACS Paragon Plus Environment


Page 22 of 71

460

with the host organism for Zn, pathogenic bacteria have evolved high-affinity outer membrane

461

Zn transporters as well as Zn-binding metallophores.140 Conceivably, low bioavailability of Zn as

462

a result of ZnS(s) precipitation may necessitate similar Zn uptake systems among microbes

463

inhabiting sulfidic environments, perhaps involving thiol release. In addition to acting as

464

metallophores, thiols can prevent the aggregation of ZnSNP.141, 142 However, this is unexplored

465

territory, as opposed to acquisition of sparingly soluble iron from oxic water involving

466

siderophore release.

467 468

Two studies suggest that the hgcAB genes may not only encode Hg methylation but perhaps also

469

affect Hg uptake rates. Mutants strains of G. sulfurreducens PCA and D. desulfuricans ND132

470

lacking these genes displayed lower thiol content of the cellular envelope and lower Hg uptake

471

rates than their respective wild type.66 In a proteome study of Geobacter sulfurreducens PCA,

472

deletion of hgcAB lowered the presence of ABC transporters known to be involved in active

473

metal ion uptake.68 Another mutant strain instead deficient in outer membrane cytochromes

474

suffered a similar loss of ABC transporter proteins; however, it displayed higher Hg methylation

475

rates than the wildtype,68 but only when cysteine was kept below 0.1 µM.114 Some of these

476

results cast doubt on the role of these transporters in Hg uptake. But some degree of control by

477

hgcAB on the expression of genes that encode essential metal acquisition systems would make

478

sense if Hg methylation serves to reduce intracellular Hg loads caused by essential metal

479

acquisition.

480 481

Alternatively, competitive uptake between Hg and Zn could potentially result from Zn(II) binding

482

to Hg transporters. For comparison, it is clear that the periplasmic and inner membrane

483

transporters encoded by the mer operon in Hg-resistant bacteria serve to perform controlled and 22 ACS Paragon Plus Environment

Page 23 of 71


484

efficient transfer of Hg(II) to the cytoplasm where it is reduced to volatile and unreactive Hg(0)

485

that diffuses out of the cell.143 However, the finding that Zn reduces Hg uptake also in bacteria

486

lacking both Hg methylation ability and mer transporters speaks in favor of Zn transporters.120, 124

487 488

It is feasible that Cu(I) reduces Hg(II) uptake, given similar coordination geometries and ligand

489

donor choices.136 In a study of Hg uptake by the aquatic plant, Elodea nuttallii, Cu(I) at

490

equimolar concentration (1 nM) suppressed Hg(II) uptake significantly, whereas Cu(II) did

491

not.144 However, prokaryotes and especially anaerobes tend to have low or no requirement for

492

Cu, a likely reason being low Cu availability due to the formation of Cu2S.145 Methane-oxidizing

493

bacteria (methanotrophs) host a membrane-associated Cu-dependent methane monooxygenase

494

(pMMO). These bacteria release a Cu-binding compound (methanobactin) which also has high

495

affinity for Hg.146 It has been demonstrated that methanobactin facilitates uptake and degradation

496

of MeHg in cultures of obligate aerobic methanotrophs. Furthermore MeHg degradation rates

497

were decreased by Cu additions.147 Conceivably, methanotrophs (some of which are anaerobes)

498

may also take up Hg(II). It has been suggested that shortage of bioavailable Cu as a result of

499

Cu2S formation favors methanotrophs expressing methanobactin.148 There is currently no

500

evidence that methanotrophs methylate Hg.

501

5.4

502

Although Hg methylation capacity is represented among diverse anaerobes using different

503

electron acceptors (Section 3), it is evident from field observations and field experiments that

504

sulfate-reducing bacteria play a key role. Several studies show that: 1) Hg methylation co-varies

505

strongly with sulfate reduction in time and space;62, 149, 150 2) blocking sulfate reduction typically

506

leads to dramatically lowered MeHg production;70, 151-153 and 3) sulfate addition to low sulfate

Effects of sulfate reduction on Hg uptake



Page 24 of 71

507

systems increases Hg methylation.70, 154-156 Strong relationships between sulfur stable isotope

508

composition (∂34S) and Hg in fish provide further evidence that dissimilatory sulfate reduction is

509

strongly associated with Hg methylation.157

510 511

In addition to the direct effects of SRB-generated sulfide on metal chemistry, such as the

512

formation of bioavailable HgS complexes and nanoparticles (Section 5.1), the sulfurization of

513

NOM (increased density of reduced sulfur groups) may facilitate Hg uptake by

514

microorganisms.158 The governing mechanism(s) are unclear but may be due to formation of Hg-

515

NOM complexes that easily exchange Hg with metal transporters; or it may reflect interference

516

of sulfurized NOM with HgSNP coagulation.158 Hypothetically, sulfidic conditions could enhance

517

Hg methylation also by favoring microbes with strong essential metal uptake systems.124

518

However, the formation of large HgS(s) particles would counteract and outweigh any stimulatory

519

effects of sulfide on Hg uptake.104 Figure 2 illustrates potential effects of sulfide and NOM and of

520

Zn uptake on Hg speciation and uptake by methylators.

521


Page 25 of 71


522 523 524 525 526 527 528 529 530

Figure 2. Hg(II) and Z(II) speciation in anoxic water and how it is controlled by natural organic matter (NOM) and sulfide. It is assumed that the Hg(II) and Zn(II) species within the bracket are available for uptake by Hg methylating anaerobes and that they compete for binding to metal transporters. Note that the role of metal transporters in Hg uptake is uncertain, and that there is currently no evidence that anaerobes release thiols in order to extract Zn from otherwise unavailable Zn species or to prevent the coagulation of ZnSNP. HgcAB: The Hg methylating apparatus associated with the cytoplasmic membrane. NOMs: Sulfurized NOM (NOM with an increased content of reduced sulfur groups).

531

The highest MeHg production in stratified lake water columns typically occurs just below the

532

oxic-anoxic (O/A) boundary,18, 35, 159 perhaps reflecting the migration of SRB to the sulfate source

533

as well as the dual role of sulfide in Hg methylation. In stratified water columns the proximity of

534

phototrophic sulfide bacteria (PSB) and SRB near the O/A boundary constitutes a syntrophic

535

relationship160 that reduces high sulfide levels and may thereby favor the formation of

536

bioavailable Hg(II) species over HgS(s) precipitation. Also, fresh OM produced by autotrophs is

537

likely to boost heterotrophic activity. Similar conditions may explain high rates of Hg

538

methylation in periphyton communities,130, 161 and in microbial communities of the

539

rhizosphere.162-165 In general, microbially-active redox transition zones seem particularly

540

conducive to Hg methylation, also including hyporheic zones166 and Sphagnum moss mats.57



Page 26 of 71

541

(Figure 3). Moreover, water level fluctuations leading to redox oscillations have been shown to

542

increase MeHg production, a likely reason being sulfide oxidation.167-169

543

544 545 546

Figure 3. Redox transition zones (shown in reddish color) with intense Hg methylation because of conditions that favor heterotrophic activity and cellular uptake of Hg.

547

6

548

As noted in Section 3, Hg methylating anaerobes have the ability to quickly release MeHg to the

549

external medium, suggesting that methylation facilitates the release of Hg from these organisms.

550

Organisms carrying the mer operon which reduce Hg(II) to Hg(0) can rely on simple diffusion of

551

Hg(0) as a means of Hg depuration. In contrast to Hg(0), MeHg is highly reactive and toxic to

552

microbes.119 In case Hg methylation has evolved to excrete Hg from the cell, one would expect

553

Hg methylators to possess a special apparatus for active MeHg efflux. In a study of Hg

554

partitioning in pure cultures of Desulfovibrio sp. BerOc1 (Hg methylator) and Desulfovibrio

555

desulfuricans G200 (non Hg methylator), the Me199Hg produced from 199HgCl2 was found almost

556

exclusively in the extracellular fraction. Furthermore, some of the Me199Hg was associated with

MeHg efflux in Hg methylating anaerobes


Page 27 of 71


557

biomolecules in the molecular weight range 17 – 70 kDa that were not seen in the Desulfovibrio

558

desulfuricans G200 cultures77 In another study, deletion of hgcAB in Geobacter sulfurreducens

559

PCA decreased its content of proteins belonging to the resistant-nodulation-cell division (RND)

560

protein family.68 These are integral membrane proteins known to be involved in the efflux of

561

metal ions137 and a large variety of other substances, e.g. toxic products produced by the

562

organism itself or by other organisms,170, 171 but evidence is lacking that RND efflux pumps

563

should be involved in MeHg efflux.

564 565

A build-up of MeHg in the external environment would seemingly create an energetically

566

unfavorable condition for efflux and a futile cycle. However, MeHg complexation could

567

ameliorate that condition, and studies have shown that a presence of MeHg-complexing agents

568

(thiols) in the external medium facilitates MeHg efflux.95, 172 Bisulfide ions (HS-) produced by

569

SRB should have a similar effect (preventing resorption), because they form strong water soluble

570

complexes with MeHg. 129, 173, 174 Sulfate reduction, in addition to creating conditions favoring

571

Hg(II) uptake (Section 5.4), may thus also facilitate MeHg efflux from cells. In contrast to the

572

studies referred to above,95, 172 Ndu et al. (2012) found that cysteine additions increased MeHg

573

uptake in an E. coli strain (non methylator) equipped with a mer-lux reporter but lacking Mer

574

transporters, and proposed that amino acid transporters were involved,123 but a later study by the

575

same group cast some doubt on this hypothesis.175

576 577

Absent sufficient MeHg efflux, demethylation may prevent a build-up of MeHg in some Hg

578

methylating microbes. In a study of the strong Hg methylator Geobacter bemidjiensis Bem,

579

additions of cysteine increased MeHg production partly because demethylation decreased as a

580

result of enhanced MeHg release. Although not entirely clear, Hg(0) rather than Hg(II) was the 27 ACS Paragon Plus Environment


Page 28 of 71

581

main product of demethylation, in line with the finding that this organism possesses the genes

582

merA and merB.115 It seems reasonable that strong Hg methylators can demethylate MeHg as a

583

safeguard against its buildup in the cell. In a study of an Hg-polluted freshwater system in

584

Romania, transcripts of merA were strongly associated with the presence of Geobacteraceae.81

585

Possibly, iron reducers inhabit redox zones where the MeHg complexing capacity is sometimes

586

too low to rely on Hg methylation as the sole means of Hg depuration. A broader conclusion is

587

that biogeochemical factors affecting the ability of Hg methylators to release MeHg may have a

588

strong impact on net MeHg production. This conclusion is supported by field observations of

589

positive relationships between total MeHg (MeHg in unfiltered water) and % filterable MeHg176,

590

177

591

7

592

There is as yet no evidence that Hg is methylated by aerobic freshwater microorganisms. One

593

study reported MeHg production in particles settling through an oxic lake water column, but it

594

was concluded that Hg methylation was confined to anaerobic micro zones.178 In coastal marine

595

waters, in the Black Sea, and in the brackish Baltic Sea where the bottom water gets fully anoxic,

596

MeHg depth profiles resemble those in stratified lakes.179-181 But in open ocean waters, Hg

597

methylation may be less dependent of anaerobic activity than in freshwaters.

598

7.1

599

At least two studies reported methylation of isotopically labeled Hg(II) added to open ocean

600

water samples in which dissolved oxygen was present.182, 183 Vertical concentration profiles of

601

methylated Hg species in the open ocean suggest that methylation takes place mainly in oxygen

602

deficient zones (ODZs), where settling particles are decomposed by heterotrophs.184-186 But in

and with similar results obtained in laboratory studies.95, 172

Hg methylation in marine waters

Hg methylation in open ocean water


Page 29 of 71


603

some cases, methylated Hg has displayed peaks coinciding with chlorophyll a maxima further up

604

in the water column.187, 188 In the euphotic zone, MeHg enrichment could be explained by algal

605

uptake of MeHg deposited via precipitation on the ocean surface, followed by release in the ODZ

606

from decomposing algal cells. However, significant in situ Hg methylation in the marine water

607

column has been inferred from budget calculations for both Pacific and Arctic waters,186, 189 and

608

from Hg isotope composition in marine fish.190, 191

609 610

Hg methylation in marine macro algae even after removal of epiphyton192 suggests that primary

611

producers are potential sources of MeHg. Moreover, a study of polar marine waters reported

612

similar Hg methylation rates for water samples from the depth of maximum chlorophyll a content

613

as for water samples from the oxycline.183 In the Ligurian Sea, North-western Mediterranean,

614

Heimbürger et al. (2010) found a local MeHg maximum in the euphotic zone above a larger peak

615

in the oxycline.188 They proposed that algal exudates stimulated heterotrophic activity responsible

616

for Hg methylation. A later study in the Arctic Ocean demonstrated peak levels of MeHg in high

617

oxygen waters (> 290 µmol dissolved oxygen) at depths as shallow as 150 – 250 m where

618

mineralization of sinking particles took place.193

619 620

There is no evidence that phytoplankton methylate Hg, but primary production is clearly

621

associated with the production of MeHg in ocean waters. In addition to fueling heterotrophic

622

microbial activity, settling algal matter transfers Hg from the euphotic zone to deeper layers

623

where heterotrophic activity dominates.194, 195 Hg is likely to become released as a result as of

624

OM decomposition, but it is unclear how marine heterotrophs internalize Hg. Based on the

625

finding that HgCl42- was readily taken up by a halotolerant E. coli strain, anion uptake, e.g. of

626

phosphate, may lead to inadvertent Hg uptake.120 Trace metals seem to concentrate in 29 ACS Paragon Plus Environment


Page 30 of 71

627

heterotrophs relative to autotrophs in the marine water column, a suggested reason being higher

628

demands for essential metals by heterotrophs.196 As already discussed, essential metal acquisition

629

may lead to inadvertent Hg uptake (Section 5.3 and 5.4).

630 631

A study of the global distribution of hgcA and hgcB orthologs reported that they were rarely

632

detected in open marine waters, including water layers displaying elevated heterotrophic activity

633

and depressed DO levels. Also, in the few hgcAB positive pelagic samples the orthologs were

634

distinct from those of known Hg methylators.64 These results, although based on metagenomes

635

not specifically collected or sequenced to look for these genes, corroborated an earlier study that

636

did not detect SRB and FeRB in either the euphotic zone or in deeper suboxic waters of the

637

Arabian sea.197 Thus, the reactions and processes leading to Hg methylation in marine waters

638

may be somewhat or entirely different from those in freshwaters. However, it seems premature to

639

assume that anaerobes possessing the hgcAB gene pair play an insignificant role in MeHg

640

production in open ocean waters. In settling particles referred to as “marine snow”, anoxic

641

conditions can develop even in fully oxygenated water if the particles are ≥ 1 mm.198 Sulfide

642

production in open oceans has been inferred from precipitation of Cd (CdS(s)) in ODZs,199 and

643

suggested to occur in micro zones of decomposing particles.196 For artificially produced marine

644

snow, Hg methylation was shown to occur in particles ranging from 8 – >300 µm.200 In upwelling

645

areas where ODZs typically develop, it is possible that sulfide and MeHg might emanate from

646

underlying sediments. ODZs are known to expand as a result of human impact on climate and

647

nutrient levels, potentially leading to a decrease in energy flow to upper trophic levels and

648

increased heterotrophic activity.201

649


Page 31 of 71


650

It has been suggested that Hg methylation in marine waters is partly an abiotic (non-enzymatic)

651

process.64 Both methyl donors and Hg could be released in conjunction with microbial activity.

652

Hg(0) formation is known to occur as a result of ligand-mediated photo reduction of Hg(II),202, 203

653

but has also been associated with primary production,204 and heterotrophic activity.205, 206 Because

654

Hg(0) can undergo oxidative methylation, methyl iodide (CH3I), known to be produced in sea

655

water,207 can act as a methyl donor.208 Bacteria which metabolize dimethylsulfoniopropionate

656

(DMSP), an osmolyte produced by marine algae, may be yet another potential source of

657

MeHg.209

658

7.2

659

In marine waters, (CH3)2Hg (Me2Hg) can reach concentrations similar to or even higher than

660

those of MeHg.210-212 Based on findings that Me2Hg displayed consistently higher concentrations

661

than MeHg in Mediterranean waters, Cossa et al. (2017) postulated that Me2Hg was a precursor

662

of MeHg rather than vice versa.195 In the South Atlantic outside Antarctica Me2Hg also displayed

663

higher concentrations than MeHg and closely followed the depth profile of chlorophyll a,

664

suggesting that primary producers somehow are involved in Me2Hg production.187 In contrast to

665

these findings of excess Me2Hg, MeHg:Me2Hg was consistently above one in water samples

666

from the northern Pacific and peaked at five in the oxygen minimum zone.185 Clearly, more

667

information is needed to apportion MeHg and Me2Hg in marine waters between different sources

668

and to mechanistically explain how these methylated Hg species are formed.

Dimethyl mercury

669 670

There are few reports of Me2Hg in freshwaters. Me2Hg was reported in River Elbe flood plain

671

soils and additions of sulfide to the soil samples increased Me2Hg production.213 Fagerström and

672

Jernelöv (1972) presented data for organic sediment suggesting that Me2Hg is the main species 31 ACS Paragon Plus Environment


Page 32 of 71

673

formed above pH 8 and that MeHg strongly dominates below pH 7.214 Such pH dependence, if

674

real, could result in lower Me2Hg concentrations in freshwaters than in ocean waters. Although

675

Me2Hg is subject to acidolysis,215 this reaction is insignificant even at pH 5 according to

676

experimental determinations of the rate constant and its pH dependence.216 It has been

677

demonstrated that MeHg adsorbed to reduced sulfur, including iron sulfide mineral particles and

678

thiol groups of algal cells, could form Me2Hg in surface-mediated reactions: 2MeHg → Me2Hg +

679

Hg(II).217 The reaction was insensitive to pH and ionic strength, suggesting that it could occur

680

also in freshwaters. Methodologically, sampling and sample storage can lead to losses of Me2Hg

681

that might explain some negative findings.215, 218

682

8

683

MeHg is a strong neurotoxin that biomagnifies in aquatic food webs to potentially harmful levels

684

even in pristine environments. To control and predict its presence in aquatic systems, we need to

685

understand the exogenous drivers and internal feedbacks that govern its net production.

Summary and outlook

686 687

The discovery of the Hg methylation genes genes hgcA and hgcB has paved the way toward a

688

better understanding of the evolutionary origin and the biochemical mechanisms that underlie Hg

689

methylation by anaerobes. In contrast to the mer operon which encodes conversion of toxic Hg

690

species to volatile and unreactive elemental Hg, the hgcAB genes do not appear to be induced or

691

favored by elevated levels of Hg(II) in the environment. Yet, they encode a methylating

692

apparatus that facilitates Hg efflux. Hg methylation may or may not be an accidental side reaction

693

where Hg enters the same methylation pathway as another substrate, but so far no such substrate

694

has been identified. A lack of clear phylogenetic relationships between organisms possessing the

695

hgcAB gene pair, the seemingly low representation of this gene pair in microbial genomes, and its 32 ACS Paragon Plus Environment

Page 33 of 71


696

seeming enrichment under certain conditions, suggest that specific environmental factors control

697

its presence in microbial communities.

698 699

The reason(s) that the hgcAB genes are restricted to anaerobes is not yet resolved, but it may be

700

related to their evolutionary origin, and/or to high bioavailability of Hg and the facilitated release

701

of MeHg in moderately sulfidic, anoxic water. It is also possible that lowered availability of

702

essential metals as a result of metal sulfide precipitation selects for strong metal uptake systems

703

that inadvertently bring Hg into the cell.

704 705

It is likely that future research will better define what conditions favor and disfavor the hgcAB

706

gene pair. There is a need for further knockout studies on cultured organisms incubated under

707

different conditions and in situ distributional studies of hgcAB, focusing on environments

708

displaying variable conditions, such as water columns of dimictic lakes. A key issue to resolve is

709

whether the hgcAB genes are favored by conditions that lead to increased Hg uptake by

710

anaerobes in pristine environments. Also, research at the cellular/molecular level is needed to

711

distinguish passive from active Hg uptake and MeHg efflux pathways, some of which may be

712

linked to the Hg methylation apparatus by coexistence/co-localization.

713 714

Based on this review of current information, we tentatively posit hypothetical connections

715

between the occurrence of the hgcAB gene pair and environmental conditions typically associated

716

with anaerobic microbial communities, emphasizing potential relationships between external and

717

endogenous factors, essential metal acquisition, Hg uptake, and MeHg export (Figure 4).

718



Page 34 of 71

719 720 721 722 723 724 725 726

Figure 4. A schematic drawing illustrating how cellular and extracellular processes occurring in a sulfide-generating microbial community may control Hg uptake, Hg methylation, and MeHg release, partly by favoring or disfavoring microbes possessing the gene pair hgcAB that is essential for Hg methylation. Unidirectional arrows indicate causeeffect relationships. Bidirectional arrows represent possible links between hgcAB and the indicated processes. Broken arrows represent uncertain cause-effect relationships or links that merit further research. Arrow length bears no meaning. DOM: Dissolved Organic Matter.

727

The lack of a strong relationship between redox conditions and Hg methylation in open ocean

728

waters suggests that factors controlling Hg methylation in marine environments can differ from

729

those indicated in Figure 4. The ubiquitous presence of Me2Hg in marine waters and the very few

730

reported detections of this Hg species in freshwaters is a further sign that fundamentally different

731

biogeochemical processes can lead to Hg methylation. Evidently, more research is needed to

732

better understand the pathways that lead to methylated Hg species in the oceans. Some of these

733

pathways may not involve the hgcAB gene pair.

734 735

As process-oriented research progresses, there remains a strong need to reduce anthropogenic Hg

736

emissions to the atmosphere, since they elevate methyl mercury production in the open sea and in

737

pristine inland waters.

738 34 ACS Paragon Plus Environment

Page 35 of 71


739

ASSOCIATED CONTENTS

740

A table with information on total Hg and MeHg levels in sediment and water from various

741

waters, and a figure illustrating effects of the methylation and demethylation rate constants on

742

MeHg concentrations and the time for MeHg to reach equilibrium, assuming first-order reactions.

743

(PDF)

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ACKNOWLEDGMENTS

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This work was financially supported by Sten K. Johnson`s Foundation and the Wisconsin

747

Department of Natural Resources. It is a joint contribution from the Biology Department at Lund

748

University and UW-Madison Trout Lake Research Station.

749 750

AUHOR INFORMATION

751

Corresponding author

752

*(O.R) Phone: +46462223781; e-mail: [email protected]

753

*(C.J.W.) Phone: +715-356-4892; fax: +715-356-6866; e-mail: [email protected]

754 755

ORCID

756

Olof Regnell: 0000-0002-1801-5695

757

Carl J. Watras: 0000-0002-2228-5882

758 759 760 761

REFERENCES (1)

WHO. Environmental Health Criteria 101 – Methylmercury. World Health Organization, Geneva. 1990. 35 ACS Paragon Plus Environment


762

(2)

Johnels, A. G.; Westermark, T.; Berg, W.; Persson, P. I.; Sjöstrand, B. Pike (Esox

763

Lucius L.) and some other aquatic organisms in Sweden as indicators of mercury

764

contamination in the environment. Oikos 1967, 18 (2), 323-333.

765

(3)

Westöö, G. Determination of methylmercury compounds in food stuffs. 1.

766

Methylmercury compounds in fish, identification and determination. Acta Chem.

767

Scand. 1966, 20, 2131-2137.

768

(4)

Bloom, N. S. Determination of picogram levels of methylmercury by aqueous phase

769

ethylation followed by cryogenic gas chromatography with cold vapor atomic

770

fluorescence detection. Can. J. Fish. Aquat. Sci. 1989, 46 (7), 1131-1140.

771

(5)

772 773

Page 36 of 71

Jensen, S.; Jernelöv, A. Biological methylation of mercury in aquatic organisms. Nature 1969, 223, 753-754.

(6)

Compeau, G. C.; Bartha, R. Sulfate-reducing bacteria: Principal methylators of

774

mercury in anoxic estuarine sediment. Appl. Environ. Microbiol. 1985, 50 (2), 498-

775

502.

776

(7)

777 778

Choi, S.-C.; Chase,T.; Bartha, R. Enzymatic catalysis of mercury methylation by Desulfovibrio desulfuricans LS. Appl. Environ. Microbiol. 1994, 60 (4), 1342-1346.

(8)

Choi, S.-C.; Chase,T.; Bartha, R. Metabolic pathways leading to mercury methylation

779

in Desulfovibrio desulfuricans LS. Appl. Environ. Microbiol. 1994, 60 (11), 4072-

780

4077.

781

(9)

Ekstrom, E. B.; Morel, F. M. M.; Benoit, J. M. Mercury methylation independent of

782

the acetyl-coenzyme A pathway in sulfate-reducing bacteria. Appl. Environ. Microbiol.

783

2003, 69 (9), 5414-5422.

784 785

(10) Parks, J. M.; Johs, A.; Podar; M.; Bridou, R.; Hurt R. A.; Smith S. D.; Tomanicek, S. J.; Qian, Y.; Brown, S. D.; Brandt; C. C.; Palumbo, A. V.; Smith, J. C.; Wall, J. D.; 36 ACS Paragon Plus Environment

Page 37 of 71


786

Elias, D. A.; Liang, L. The genetic basis for bacterial mercury methylation. Science

787

2013, 339 (6125), 1332-1335.

788

(11) Rapsomanikis, S.; Donard, O. X. F.; Weber, J. H. Speciation of lead and methyllead

789

ions in water chromatography/atomic absorption spectrometry after ethylation with

790

sodium tetraethylborate. Anal. Chem. 1985, 58 (1) 35-38.

791

(12) Horvat, M.; May, K.; Stoeppler, M.; Byrne, A. R. Comparative studies of

792

methylmercury determination in biological and environmental samples. Appl.

793

Organomet. Chem. 1988, 2 (6), 515-524.

794

(13) Bloom, N. Determination of picogram levels of methylmercury by aqueous phase

795

ethylation, followed by cryogenic gas chromatography with cold vapour atomic

796

fluorescence detection. Can. J. Fish. Aquat. Sci. 1989, 46 (7), 1131-1140.

797

(14) Patterson, C. C.; Settle, D. M. The reduction of orders of magnitude error in lead

798

analysis. In Accuracy in trace analysis: sampling, sample handling, analysis - Volume

799

1. NBS Special Publication 422; LaFleur, P. D., Ed.; US Government Printing Office:

800

1976; pp 321-353.

801

(15) Watras, C. J.; Bloom, N. S.; Hudson, R. J. M.; Gherini, S.; Munson, R.; Claas, S. A.;

802

Morrison, K. A.; Hurley, J.; Wiener, J. G.; Fitzgerald, W. F.; Mason, R.; Vandal, G.;

803

Powell, D.; Rada, R.; Rislov, L.; Winfrey, M.; Elder, J.; Krabbenhoft, D.; Andren, A.

804

W.; Babiarz, C.; Porcella, D. B.; Huckabee, J. W. Sources and fates of mercury and

805

methylmercury in Wisconsin lakes. In Mercury Pollution: Integration and Synthesis;

806

Watras, C. J., Huckabee, J. W., Eds.; Lewis Publishers: Boca Raton, FL 1994; pp 153-

807

177.

808 809

(16) Watras, C. J.; Morrison, K. A.; Back, R. C. Mass balance studies of mercury and methylmercury in small temperate/boreal lakes of the northern hemisphere. In Global 37 ACS Paragon Plus Environment


Page 38 of 71

810

and regional mercury cycles: sources, fluxes, and mass balances - NATO ASI Series

811

2: Environment Vol. 21; Bayens, R., Ebinghaus, R., Vasiliev, O., Eds.; Kluwer

812

Academic: Boston 1996; pp 329-358.

813

(17) Baeyens, W.; Leermakers, M. Particulate, dissolved and methylmercury budgets in the

814

Scheldt estuary. In Global and regional mercury cycles: sources, fluxes, and mass

815

balances - NATO ASI Series 2: Environment Vol. 21; Bayens, R., Ebinghaus, R.,

816

Vasiliev, O., Eds.; Kluwer Academic: Boston 1996; pp 285-302.

817

(18) Watras, C. J.; Bloom, N. S. The vertical distribution of mercury species in Wisconsin

818

lakes: Accumulation in plankton layers. In Mercury Pollution: Integration and

819

Synthesis; Watras, C. J., Huckabee, J. W., Eds.; Lewis Publishers: Boca Raton, FL

820

1994; pp 137-152.

821

(19) Schaefer, J. K.; Yagi, J.; Reinfelder, J. R.; Cardona, T.; Ellickson, K. M.; Tel-Or, S.;

822

Barkay, T. Role of bacterial organomercury lyase (MerB) in controlling

823

methylmercury accumulation in mercury-contaminated natural waters. Environ. Sci.

824

Technol. 2004, 38 (16), 4304-4311.

825

(20) Cossa, D.; Garnier, C.; Buscail, R.; Elbaz-Poulichet, F.; Mikac, N.; Patel-Sorrentino,

826

N.; Tessier, E.; Rigaud, S.; Lenoble, V.; Gobeil, C. A Michaelis-Menten type equation

827

for describing methylmercury dependence on inorganic mercury in aquatic sediments.

828

Biogeochem. 2014, 119 (1-3), 35-43.

829

(21) Hintelmann, H.; Evans, R. D.; Villeneuve, J. Y. Measurements of mercury methylation

830

in sediments by using stable mercury isotopes combined with methylmercury

831

determination by gas-chromatography inductively-coupled plasma-mass spectrometry.

832

J. Anal. At. Spectrom. 1995, 10, 619-624.


Page 39 of 71

833


(22) Eckley, C. S.; Watras, C. J.; Hintelmann, H.; Morrison, K.; Kent, A. D.; Regnell, O.

834

Mercury methylation in the hypolimnetic waters of lakes with and without connection

835

to wetlands in northern Wisconsin. Can. J. Fish. Aquat. Sci. 2005, 62 (2), 400-411.

836 837 838

(23) Jackson, T. A. Variations in the isotope composition of mercury in a freshwater sediment sequence and food web. Can. J. Fish. Aquat. Sci. 2001, 58 (1), 185-196. (24) Jackson, T. A.; Whittle, D. M.; Evans, M. S.; Muir, D. C. G. Evidence for mass-

839

independent and mass-dependent fractionation of the stable isotopes of mercury by

840

natural processes in aquatic systems. Appl. Geochem. 2008, 23 (3), 547-571.

841 842 843 844 845

(25) Bergquist, B. A.; Blum, J. D. Mass-dependent and mass-independent fractionation of Hg isotopes by photo-reduction in aquatic systems. Science 2007, 318 (5849), 417-420. (26) Morel, F. M. M.; Kraepiel, A. M. L.; Amyot, M. The chemical cycle and bioaccumulation of mercury. Annu. Rev. Ecol. Syst. 1998, 29, 543-566. (27) Ullrich, S. M.; Tanton, T. W.; Abdrashitova, S. A. Mercury in the aquatic environment:

846

A review of factors affecting mercury methylation. Crit. Rev. Environ. Sci. Technol.

847

2001, 31 (3), 241-293.

848

(28) Hsu-Kim, H.; Kucharzyk, K. H.; Zhang, T.; Deshusses, M. A. Mechanisms regulating

849

mercury bioavailability for methylating microorganisms in the aquatic environment: A

850

critical review. Environ. Sci. Technol. 2013, 47 (6), 2441-2456.

851 852 853

(29) Lehnherr, I. Methylmercury biogeochemistry: a review with special reference to Arctic ecosystems. Environ. Rev. 2014, 22 (3), 229-243. (30) Drott, A.; Lambertsson, L.; Björn, E.; Skyllberg, U. Do potential methylation rates

854

reflect accumulated methyl mercury in sediments? Environ. Sci. Technol. 2008, 42 (1),

855

153-158.



856

Page 40 of 71

(31) Tjerngren, I.; Karlsson, T.; Björn, E.; Skyllberg U. Potential Hg methylation and

857

MeHg demethylation rates related to the nutrient status of different boreal wetlands.

858

Biogeochem. 2012. 108 (1). 335-.350.

859

(32) Fleck, J. A.; Marvin-DiPasquale, M.; Eagles-Smith, C. A.; Ackerman, J. T.; Lutz, M.

860

A.; Tate, M.; Alpers, C. N.; Hall, B. D.; Krabbenhoft, D. P.; Eckley, C. S. Mercury and

861

methylmercury in aquatic sediment across western North America. Sci. Tot. Environ.

862

2016, 568, 727-738.

863

(33) Oremland, R. S.; Culbertson, C. W.; Winfrey, M. R. Methylmercury decomposition in

864

sediments and bacterial cultures: Involvement of methanogens and sulfate reducers in

865

oxidative demethylation. Appl. Environ. Microbiol. 1991, 57 (1), 130-137.

866

(34)

Sellers, P.; Kelly C. A.; Rudd, J. W. M. Fluxes of methylmercury to the water column

867

of a drainage lake: The relative importance of internal and external sources. Limnol.

868

Oceanogr. 2001, 46 (3), 623-631.

869

(35)

Regnell, O.; Ewald, G.; Lord, E. Factors controlling variation in methyl mercury

870

levels in sediment and water in a seasonally stratified lake. Limnol. Oceanogr. 1997,

871

42 (8), 1784-1795.

872

(36) Merrit, K.A.; Amirbahman, A. Methylmercury cycling in estuarine sediment pore

873

waters (Penobscot River estuary, Maine, USA). Limnol. Oceanogr. 2008, 53 (3), 1064-

874

1075.

875

(37) Bailey, L. T.; Mitchell, C. P. J.; Engstrom, D. R.; Berndt, M. E.; Wasik, J. K. C.;

876

Johnson, N. W. Influence of porewater sulfide on methylmercury production and

877

partitioning in sulfate-impacted sediments. Sci. Tot. Environ. 2017, 580, 1197-1204.


Page 41 of 71

878


(38) Watras, C. J.; Morrison, K. A.; Kent, A.; Price, N.; Regnell, O.; Eckley, C.;

879

Hintelmann, H.; Hubacher, T. Sources of methylmercury to a wetland-dominated lake

880

in northern Wisconsin. Environ. Sci. Technol. 2005, 39 (13), 4747-4758.

881

(39)

Hintelmann, H.; Evans, R. D. Application of stable isotopes in environmental studies –

882

measurement of monomethylmercury (CH3Hg+) by isotope dilution ICP-MS and

883

detection of species transformation. Fresenius J. Anal. Chem. 1997, 358 (3), 378-385.

884

(40) Bridou, R.; Monperrus, M.; Rodriguez-Gonzales, P.; Guyoneaud, R.; Amouroux, D.

885

Simultaneous determination of mercury methylation and demethylation capacities of

886

various sulfate-reducing bacteria using species-specific isotopic tracers. Environ.

887

Toxicol. Chem. 2011, 30 (2), 337-344.

888

(41) Benoit, J. M.; Gilmour, C. C.; Mason R. P. Aspects of bioavailability of mercury for

889

methylation in pure cultures of Desulfobulbus propionicus (1pr3). Appl. Environ.

890

Microbiol. 2001, 67 (1), 51-58.

891

(42) Jonsson, S.; Skyllberg, U.; Nilsson, M. B.; Westlund, P.-O.; Shchukarev, A.;

892

Lundberg, E.; Björn, E. Mercury methylation rates for geochemically relevant HgII

893

species in sediments. Environ. Sci. Technol. 2012, 46 (21), 11653-11659.

894

(43) Jiskra, M.; Saile, D.; Wiederhold, J. G.; Bourdon, B.; Björn, E.; Kretzschmar, R.

895

Kinetic exchange between organic ligands, goethite and natural organic matter studied

896

with an enriched stable isotope approach. Environ. Sci. Technol. 2014, 48 (22), 4366-

897

4373.

898

(44) Liu, Y.-R.; Lu, X.; Zhao, L.; An, J.; He, J.-Z.; Pierce, E. M.; Johs, A.; Gu, B. Effects of

899

cellular sorption on mercury bioavailability and methylmercury production by

900

Desulfovibrio desulfuricans ND132. Environ. Sci. Technol. 2016, 50 (24), 13335-

901

13341. 41 ACS Paragon Plus Environment


Page 42 of 71

902

(45) Hu, H.; Lin, H.; Zheng, W.; Rao, B.; Feng, X.; Liang, L.; Elias, D. A.; Gu, B. Mercury

903

reduction and cell-surface adsorption by Geobacter sulfurreducens PCA. Environ. Sci.

904

Technol. 2013, 47 (19), 10922-10930.

905

(46) Lin, H.; Morell-Falvey, J. L.; Rao, B.; Liang; L.; Gu, B. Coupled mercury-cell

906

sorption, reduction, and oxidation on methylmercury production by Geobacter

907

sulfurreducens PCA. Environ. Sci. Technol. 2014, 48 (20), 11969-111976.

908

(47) Achá; D.; Hintelmann, H.; Pabón, C. A. Sulfate reducing bacteria and mercury

909

methylation in the water column of the Lake 658 of the Experimental Lake Area.

910

Geomicrobiol. J. 2012, 29 (7), 667-674.

911

(48) Hintelmann, H.; Keppel-Jones, K.; Evans, R. D. Constants of mercury methylation and

912

demethylation rates in sediments and comparison of tracer and ambient mercury.

913

Environ. Toxicol. Chem. 2000, 19 (9), 2204-2211.

914

(49) Harris, R. C.; Rudd, J. W. M.; Amyot, M.; Babiarz, C. L.; Beaty, K. G.; Blanchfield, P.

915

J.; Bodaly, R. A.; Branfireun, B. A.; Gilmour, C. C.; Graydon, J.A.; Heyes, A.;

916

Hintelmann, H.; Hurley, J. P.; Kelly, C. A.; Krabbenhoft, D. P.; Lindberg, S. E.;

917

Mason, R. P.; Paterson, M. J.; Podemski, C. L.; Robinson, A.; Sandlands, K. A.;

918

Southworth, G. R.; St. Louis, V. L.; Tate, M. T. Whole-ecosystem study shows rapid

919

fish-mercury response to changes in mercury deposition. Proc. Natl. Acad. Sci. 2007,

920

104 (42), 16586-16591.

921

(50) Gilmour, C. C.; Riedel, G. S. Measurement of Hg methylation in sediments using high

922

specific-activity 203Hg and ambient incubation. Water Air Soil Pollut. 1995, 80 (1-4),

923

747-756.


Page 43 of 71


924

(51) Jonsson, S.; Skyllberg, U.; Nilsson, M. B.; Lundberg, E.; Andersson, A.; Björn E.

925

Differentiated availability of geochemical mercury pools controls methylmercury

926

levels in estuarine sediment and biota. Nat. Commun. 2014, 5, 4624.

927

(52) Zhu, W.; Song, Y.; Adediran, G. A.; Jiang T.; Reis, A. T.; Pereira, E.; Skyllberg, U.;

928

Björn E. Mercury transformations in resuspended contaminated sediment controlled by

929

redox conditions, chemical speciation and sources of organic matter. Geochim.

930

Cosmochim. Acta 2018, 220, 158-179.

931

(53) Liem-Nguyen, V.; Jonsson, S.; Skyllberg, U.; Nilsson M. B.; Andersson, A.;

932

Lundberg, E.; Björn, E. Effects of nutrient loading and mercury chemical speciation on

933

the formation and degradation of methylmercury in estuarine sediment. Environ. Sci.

934

Technol. 2016, 50 (13). 6983-6990.

935

(54) Bravo, A. G.; Bouchet, S.; Tolu, J.; Björn, E.; Mateos-Rivera, A.; Bertilsson, S.

936

Molecular composition of organic matter controls methylmercury formation in boreal

937

lakes. Nat. Commun. 2017, 8, 14255.

938

(55) Kellerman, A. M.; Guillemette, F.; Podgorski, D. C.; Aiken, G. R.; Butler, K. D.;

939

Spencer, R. G. M. Unifying concepts linking organic matter composition to persistence

940

in aquatic ecosystems. Environ. Sci. Technol. 2018, 52 (5), 2538-2548.

941

(56) St. Louis, V. L.; Rudd, J. V. M.; Kelly, C. A.; Beaty, K. G.; Bloom, N. S.; Flett, R. J.

942

Importance of wetlands as sources of methyl mercury to boreal forest ecosystems. Can.

943

J. Fish. Aquat. Sci. 1994, 51 (5), 1065-1076.

944

(57) Yu, R.-Q.; Adatto, I.; Montesdeoca, M. R.; Driscoll, C. T.; Hines, M. E.; Barkay, T.

945

Mercury methylation in Sphagnum moss mats and its association with sulfate-reducing

946

bacteria in an acidic Adirondack forest wetland. FEMS Microbiol. Ecol. 2010, 74 (3),

947

655-668. 43 ACS Paragon Plus Environment


948

Page 44 of 71

(58) Elser, J. J.; Fagan, W. F.; Denno, R. F.; Dobberfuhl, D. R.; Folarin, A.; Huberty, A.;

949

Interlandl, S.; Kilham, S. S.; McCauley, E.; Schulz, K. L.; Siemann, E. H.; Sterner, R.

950

W. Nutritional constraints in terrestrial and freshwater food webs. Nature 2000, 408

951

(6812), 578-580.

952

(59) Wright, A. L.; Reddy, K. R. Substrate-induced respiration for phosphorus-enriched an

953

oligotrophic peat soils in an Everglades wetland. Soil. Sci. Soc. Am. J. 2007, 71 (5),

954

1579-1583.

955

(60) Fontaine, S.; Barot, S.; Barré, P.; Bdioui, N.; Mary, B.; Rumpel, C. Stability of

956

organic carbon in deep soil layers controlled by fresh carbon supply. Nature 2007, 450,

957

277-280.

958

(61) Mitchell, C. P. J.; Branfireun, B. A.; Kolka, R. K. Spatial characteristics of net

959

methylmercury production hot spots in peatlands. Environ. Sci. Technol. 2008, 42 (4),

960

1010-1016.

961

(62) Regnell, O.; Elert, M.; Höglund, L. O.; Falk, A. H.; Svensson, A. Linking cellulose

962

fiber sediment methyl mercury levels to organic matter decay and major element

963

composition. Ambio 2014, 43 (7), 878-890.

964

(63) Gilmour, C. C.; Podar, M.; Bullock, A. B.; Graham, A. W.; Brown, S. D.;

965

Somenahally, A. C.; Johs, A.; Hurt, R. A.; Bailey, K. L.; Elias, D. A. Mercury

966

methylation by novel microorganisms from new environments. Environ. Sci. Technol.

967

2013, 47 (20), 11810-11820.

968

(64) Podar, M.; Gilmour, C. C.; Brandt, C. C.; Soren, A.; Brown, S. D.; Crable, B. R.;

969

Palumbo, A. V.; Somenahally, A. C.; Elias. D. A. Global prevalence and distribution of

970

genes and microorganisms involved in mercury methylation. Sci. Adv. 2015, 1 (9),

971

e1500675. 44 ACS Paragon Plus Environment

Page 45 of 71

972


(65) Zhou, J.; Riccardi, D.; Beste, A.; Smith, J. C.; Parks, J. M. Mercury methylation by

973

HgcA: Theory supports carbanion transfer to Hg(II). Inorg. Chem. 2014, 53 (2), 772-

974

777.

975

(66) Lin, H.; Hurt, R. A.; Johs, A.; Parks, J. M.; Morell-Falvey, J. L.; Liang, L.; Elias, D.

976

A.; Gu, B. Unexpected effects of gene deletion on interactions of mercury with the

977

methylation-deficient mutant ΔhgcAB. Environ. Sci. Technol. Lett. 2014, 1 (5), 271-

978

276.

979

(67) Smith, S. D.; Bridou, R.; Johs, A.; Parks, J. M.; Elias, D. A.; Hurt, R. A.; Brown, S. D.;

980

Podar, M.; Wall, J. D. Site-directed mutagenesis of HgcA and HgcB reveals amino

981

acid residues important for mercury methylation. Appl. Environ. Microbiol. 2015, 81

982

(9), 3205-3217.

983

(68) Qian, C.; Johs, A.; Chen, H.; Mann, B. F.; Lu, X.; Abraham, P. E.; Hettich, R. L.; Gu,

984

B. Global proteome response to deletion of genes related to mercury methylation and

985

dissimilatory metal reduction reveals changes in respiratory metabolism in Geobacter

986

sulfurreducens PCA. J. Proteome Res. 2016, 15 (10), 3540-3549.

987

(69) Yu, R.-Q.; Reinfelder, J. R.; Hines, M. A.; Barkay, T. Mercury methylation by the

988

methanogen Methanospirillum hungatei. Appl. Environ. Microbiol. 2013, 79 (20),

989

6325-6330.

990

(70) Bae, H.-S.; Dierberg, F. E.; Ogram, A. Syntrophs dominate sequences associated with

991

the mercury methylating-related gene hgcA in the water conservation areas of the

992

Florida Everglades. Appl. Environ. Microbiol. 2014, 80 (20), 6517-6526.

993

(71) Christensen, G. A.; Wymore, A. M.; King, A. J.; Podar; M.; Hurt, R. A.; Santillan, E.

994

U.; Soren, A.; Brandt, C. C.; Brown, S. D,; Palumbo, A. V.; Wall, J. D.; Gilmour, C.

995

C.; Elias D. A. Development and validation of broad-range qualitative and clade45 ACS Paragon Plus Environment


996

specific quantitative specific probes for assessing mercury methylation in the

997

environment. Appl. Environ. Microbiol. 2016, 82 (19), 6068-6078.

998 999

(72) Ranchou-Peyruse, M.; Monperrus, M.; Bridou, R.; Duran, R.; Amouroux, D.; Salvado, J. C.; Guyoneaud, R. Overview of mercury methylation capacities among anaerobic

1000

bacteria including representatives of the sulphate-reducers: Implications for

1001

environmental studies. Geomicrobiol. J. 2009, 26 (1), 1-8.

1002

Page 46 of 71

(73) Graham, A. M.; Bullock, A. L.; Maizel, A. C.; Elias, D. A.; Gilmour, C. C. Detailed

1003

assessment of the kinetics of Hg-cell association, Hg methylation, and methylmercury

1004

degradation in several Desulfovibrio species. Appl. Environ. Microbiol. 2012, 78 (20),

1005

7337-7346.

1006

(74) Dimitriu, T.; Lotton, C.; Bénard-Capelle, J.; Misevic, D.; Brown, S. P.; Lindner, A. B.;

1007

Taddei, F. Genetic information transfer promotes cooperation in bacteria. Proc. Natl.

1008

Acad. Sci. 2014, 111 (30), 11103-11108.

1009

(75) Cai, L.; Liu, G.; Rensing, C.; Wang, G. Genes involved in arsenic transformation and

1010

resistance associated with different levels of arsenic-contaminated soils. BMC

1011

Microbiol. 2009, 9, 4.

1012

(76) Gilmour, C. C.; Elias, D. A.; Kucken, A. M.; Brown, S. D.; Palumbo, A. V.; Schadt, C.

1013

W.; Wall, J. D. Sulfate-reducing bacterium Desulfovibrio desulfuricans ND132 as a

1014

model for understanding bacterial mercury methylation. Appl. Environ. Microbiol.

1015

2011, 77 (12), 3938-3951.

1016

(77) Pedrero, Z.; Bridou, R.; Mounicou, S.; Guyoneaud, R.; Monperrus, M.; Amouroux, D.

1017

Transformation, localization, and biomolecular binding of Hg species at subcellular

1018

level in methylating and and nonmethylating sulfate-reducing bacteria. Environ. Sci.

1019

Technol. 2012, 46 (21), 11744-11751. 46 ACS Paragon Plus Environment

Page 47 of 71

1020


(78) Yin, Z.; Jiang, H.; Syversen, T.; Rocha, J. B. T.; Farina, M.; Aschner, M. The

1021

methylmercury-L-cysteine conjugate is a substrate for the L-type large neutral amino

1022

acid transporter, LAT1. J. Neurochem. 2008, 107 (4), 1083-1090.

1023

(79) Gõni-Urriza, M.; Corsellis, Y.; Lanceleur, L.; Tessier, E.; Gury, J.; Monperrus, M.;

1024

Guyoneaud, R. Relationships between energetic metabolism, mercury methylation

1025

potential, and hgcA/hgcB gene expression in Desulfovibrio dechloroacetivorans

1026

BerOc1. Environ. Sci. Pollut. Res. 2015, 22 (18), 13764-13771.

1027

(80) Liu, Y.-R.; Yu, R.-Q.; Zheng, Y.-M.; He, J.-Z. Analyses of microbial community

1028

structure by monitoring an Hg methylating gene (hgcA) in paddy soils along an Hg

1029

gradient. Appl. Environ. Microbiol. 2014, 80 (9), 2874-2879.

1030

(81) Bravo, A. G.; Loizeau, J.-L.; Dranguet, P.; Makri, S.; Björn, E.; Ungureanum, V. G.;

1031

Slaveykova, V. I.; Cosio, C. Persistent Hg contamination and occurrence of Hg-

1032

methylating transcript (hgcA), downstream of a chlor-alkali plant in the Olt River

1033

(Romania). Environ. Sci. Pollut. Res. 2016, 23 (11), 10529-10541.

1034

(82) Poulain, A. J.; Ni Chadhain, S. M.; Ariya, P. A.; Amyot, M.; Garcia, E.; Campbell, P.

1035

G. C.; Zylstra, G. J.; Barkay, T. Potential for mercury reduction by microbes in the

1036

high Arctic. Appl. Environ. Microbiol. 2007, 73 (7), 2230-2238.

1037 1038 1039 1040 1041 1042

(83) Boyd, E. S.; Barkay, T. The mercury resistance operon: from an origin in a geothermal environment to an efficient detoxification machine. Front. Microbiol. 2012, 3, 349. (84) Banerjee, R.; Ragsdale, S. W. The many faces of vitamin B12: catalysis by cobalamindependent enzymes. Annu. Rev. Biochem. 2003, 72, 209-247. (85) Ragsdale, S. W. Catalysis of methyl group transfers involving tetrahydrofolate and B12. Vitam. Horm. 2008, 79, 293-324.



1043

(86)

Page 48 of 71

Demissie, T. B.; Garabato, B. D.; Ruud, K.; Kozlowski, P. M. Mercury methylation by

1044

cobalt corrinoids: Relativistic effects dictate the reaction mechanism. Angew. Chem.

1045

Int. Ed. 2016, 55 (38), 11503-11506.

1046 1047 1048

(87) Fujimori, D. G. Radical SAM-mediated methylation reactions. Curr. Opin. Chem. Biol. 2013, 17, 597-604. (88) Hudson, R. J. M.; Gherini, S.; Watras, C. J.; Porcella, D. B. Modelling the

1049

biogeochemical cycle of mercury in lakes: The mercury cycling model (MCM) and its

1050

application to the MTL study lakes. In Mercury Pollution: Integration and Synthesis;

1051

Watras, C. J., Huckabee, J. W., Eds.; Lewis Publishers: Boca Raton, FL, 1994; pp 137-

1052

152.

1053

(89) Benoit, J. M.; Mason, R. P.; Gilmour, C. C. 1999. Estimation of mercury-sulfide

1054

speciation in sediment pore waters using octanol-water partitioning and implications

1055

for bioavailability to methylating bacteria. Environ. Toxicol. Chem. 1999, 18 (10),

1056

2138-2141.

1057

(90) Benoit, J. M.; Gilmour, C. C.; Mason R. P. The influence of sulfide on solid-phase

1058

mercury bioavailability for methylation by pure cultures of Desulfobulbus propionicus

1059

(1pr3). Environ. Sci. Technol. 2001, 35 (1), 127-132.

1060

(91) Drott, A.; Lambertsson, L.; Björn, E.; Skyllberg, U. Importance of dissolved neutral

1061

mercury sulfides for methyl mercury production in contaminated sediments. Environ.

1062

Sci. Technol. 2007, 41 (7), 2270-2276.

1063

(92) Deonarine, A.; Hsu-Kim, H. Precipitation of mercuric sulfide nanoparticles in NOM-

1064

containing water: Implications for the natural environment. Environ. Sci. Technol.

1065

2009, 43 (7), 2368-2373.


Page 49 of 71

1066


(93) Drott, A.; Björn, E.; Bouchet, S.; Skyllberg, U. Refining thermodynamic constants for

1067

mercury(II)-sulfides in equilibrium with metacinnabar at sub-micromolar aqueous

1068

sulfide concentrations. Environ. Sci. Technol. 2013, 47 (9), 4197-4203.

1069

(94) Zhang, T.; Kim, B.; Levard, C.; Reinsch, B. C.; Lowry, G. V.; Deshusses, M. A.; Hsu-

1070

Kim, H. Methylation of mercury by bacteria exposed to dissolved, nanoparticulate and

1071

microparticulate mercuric sulfides. Environ. Sci. Technol. 2012, 46 (13), 6950-6958.

1072

(95) Graham, A. M.; Aiken, G. R.; Gilmour, C. C. Effects of dissolved organic matter

1073

source and character on microbial Hg methylation in Hg-S-DOM solutions. Environ.

1074

Sci. Technol. 2013, 47 (11), 5746-5754.

1075

(96) Pham, A. L.-T.; Morris, A.; Zhang, T.; Ticknor, J.; Levard, C.; Hsu-Kim, H.

1076

Precipitation of nanoscale mercuric sulfides in the presence of natural organic matter:

1077

Structural properties, aggregation, and biotransformation. Geochim. Cosmochim. Acta

1078

2014, 133, 204-215.

1079

(97) Zhang, T.; Kucharzyk, K. H.; Kim, B.; Deshusses, M. A.; Hsu-Kim, H. Net

1080

methylation of mercury in estuarine sediment microcosms amended with dissolved,

1081

nanoparticulate, and microparticulate mercuric sulfides. Environ. Sci. Technol. 2014,

1082

48 (16), 9133-9141.

1083

(98) Mazrui, N. M.; Jonsson, S.; Thota S.; Zhao, J.; Mason, R. P. Enhanced availability of

1084

mercury bound to dissolved organic matter for methylation in marine sediments.

1085

Geochim. Cosmochim. Acta 2016, 194, 153-162.

1086

(99) Zhou, J.; Smith, M. D.; Cooper, S. J.; Cheng, X.; Smith J. C.; Parks, J. M. Modeling of

1087

the passive permeation of mercury and methylmercury complexes through a bacterial

1088

cytoplasmic membrane. Environ. Sci. Technol. 2017, 51 (18), 10596-10604.



1089

Page 50 of 71

(100) Gerbig, C. A.; Kim, C. S.; Stegemeier, J. P.; Ryan, J. N.; Aiken, G. R. Formation of

1090

nanocolloidal metacinnabar in mercury-DOM-sulfide systems. Environ. Sci. Technol.

1091

2011, 45 (21), 9180-9187.

1092

(101) Graham, A. M.; Aiken, G. R.; Gilmour, C. C. Dissolved organic matter enhances

1093

microbial mercury methylation under sulfidic conditions. Environ. Sci. Technol. 2012,

1094

46 (5), 2715-2723.

1095

(102) Mukha, I. P.; Eremenko, A. M.; Smirnova, N. P.; Mikhienkova, A. I.; Korchak, G. I.;

1096

Gorchev, V. F.; Chunikhin. A. Y. Antimicrobial activity of stable silver nanoparticles

1097

of a certain size. Appl. Biochem. Microbiol. 2013, 49 (2), 199-206.

1098

(103) Ravichandran, M.; Aiken, G.; Reddy, M. M.; Ryan, J. N. Enhanced dissolution of

1099

cinnabar (mercuric sulfide) by dissolved organic matter isolated from the Florida

1100

Everglades. Environ. Sci. Technol. 1998, 32 (21), 3305-3311.

1101

(104) Poulain, B. A.; Gerbig, C. A.; Kim, C. S.; Stegemeier, J. P.; Ryan, J. N.; Aiken G. R.

1102

Effects of sulfide concentration and dissolved organic matter characteristics on the

1103

structure of nanocolloidal metacinnabar. Environ. Sci. Technol. 2017, 51 (22), 13133-

1104

13142.

1105

(105) Wang, Y.; Schaefer, J. K.; Mishra, B.; Yee, N. Intracellular Hg(0) oxidation in

1106

Desulfovibrio desulfuricans ND132. Environ. Sci. Technol. 2016, 50 (20), 11049-

1107

11056.

1108

(106) Colombo, M. J.; Ha, J.; Reinfelder, J. R.; Barkay, T.; Yee, N. Anaerobic oxidation of

1109

Hg(0) and methylmercury formation by Desulfovibrio desulfuricans ND 132.

1110

Geochim. Cosmochim. Acta 2013, 112, 166-177.


Page 51 of 71

1111


(107) Hu, H.; Lin, H.; Zheng, W.; Tomanicek, S. J.; Johs, A.; Feng, X.; Elias, D. A.; Liang,

1112

L.; Gu, B. Oxidation and methylation of dissolved elemental mercury by anaerobic

1113

bacteria. Nature Geosci. 2013, 6, 751-754.

1114 1115 1116 1117 1118 1119 1120

(108) Iverfeldt, Å. Mercury in the Norwegian fjord Framvaren. Mar. Chem. 1988, 23 (3-4) 441-456. (109) Bouffard, A.; Amyot, M. Importance of elemental mercury in lake sediment. Chemosphere 2009, 74 (8), 1098-1103. (110) Bone, S. E.; Bargar, J. R.; Sposito, G. Mackinawite (FeS) reduces mercury(II) under sulfidic conditions. Environ. Sci. Technol. 2014, 48 (18), 10681-10689. (111) Zheng, W.; Liang, L.; Gu, B. Mercury reduction and oxidation by reduced natural

1121

organic matter in anoxic environments. Environ. Sci. Technol. 2012, 46 (1), 292-299.

1122

(112) Schaefer, J. K.; Morel, F. M. M. High methylation rates of mercury bound to cysteine

1123 1124

by Geobacter sulfurreducens. Nature Geosci. 2009, 2, 123-126. (113) Schaefer, J. K.; Rocks, S. S.; Zheng, W.; Liang, L.; Gu, B.; Morel, F. M. M. Active

1125

transport, substrate specificity, and methylation of Hg(II) in anaerobic bacteria. Proc.

1126

Natl. Acad. Sci. 2011, 108 (21), 8714-8719.

1127

(114) Lin, H.; Lu, X.; Liang, L.; Gu, B. Cysteine inhibits mercury methylation by Geobacter

1128

sulfurreducens PCA mutant ΔomcBESTZ. Environ. Sci. Technol. Lett. 2015, 2 (5),

1129

144-148.

1130

(115) Lu, X.; Liu, Y.; Johs, A.; Zhao, L.; Wang, T.; Yang, Z.; Lin, H.; Elias, D. A.; Pierce, E.

1131

M.; Liang; L.; Barkay, T.; Gu, B. Anaerobic mercury methylation and demethylation

1132

by Geobacter bemidjiensis BEM. Environ. Sci. Technol. 2016, 50 (8), 13207-13217.

1133

(116) Yamada, M.; Tonomura, K. Formation of methylmercury compounds from inorganic

1134

mercury by Clostridium cochlearium. J. Ferment. Technol. 1972, 50, 159-166. 51 ACS Paragon Plus Environment


1135 1136 1137

Page 52 of 71

(117) Craig, P. J.; Moreton P. A. The rôle of speciation in mercury methylation in sediment and water. Environ. Pollut. Ser. B Chem. Phys. 1985, 10 (2), 141-158. (118) Schaefer, J. K.; Szczuka, A.; Morel, F. M. M. Effect of divalent metals on Hg(II)

1138

uptake and methylation by bacteria. Environ. Sci. Technol. 2014, 48 (5), 3007-3013.

1139

(119) Silver, S.; Phung, L. T. A bacterial view of the periodic table: genes and proteins for

1140

toxic inorganic ions. J. Ind. Microbiol. Biotechnol. 2005, 32 (11-12), 587-605.

1141

(120) Stenzler, B.; Hinz, A.; Ruuskanen, M.; Poulain, A. J. Ionic strength differently affects

1142

the bioavailability of neutral and negatively charged inorganic Hg complexes. Environ.

1143

Sci. Technol. 2017, 51 (17), 9653-9662.

1144

(121) Thomas, S. A.; Tong; T.; Gaillard, J.-F. Hg(II) bacterial biouptake: the role of

1145

anthropogenic and biogenic ligands present in solution and spectroscopic evidence of

1146

ligand exchange reactions at the cell surface. Metallomics 2014, 6, 2213-2222.

1147

(122) Zhao,, L.; Chen, H.; Lu, X.; Lin, H.; Christensen, G. A.; Pierce, E. M.; Gu, B.

1148

Contrasting effects of dissolved organic matter on mercury methylation by Geobacter

1149

sulfurreducens PCA and Desulfovibrio desulfuricans ND132. Environ. Sci. Technol.

1150

2017, 51 (18), 10468-10475.

1151

(123) Ndu, U.; Mason, R. P.; Zhang, H.; Lin, S.; Visscher, P. T. Effect of inorganic and

1152

organic ligands on the bioavailability of methylmercury as determined by using a mer-

1153

lux bioreporter. Appl. Environ. Microbiol. 2012, 78 (20), 7276-7282.

1154

(124) Szczuka, A.; Morel, F. M. M.; Schaefer, J. K. Effects of thiols, zinc, and redox

1155

conditions on Hg uptake in Shewanella oneidensis. Environ. Sci. Technol. 2015, 49

1156

(12), 7432-7438.

1157 1158

(125) Gilmour, C. C.; Bullock, A. L.; McBurney, A.; Podar, M.; Elias, D. A. Robust mercury methylation across diverse methanogenic Achaea. mBio 2018, 9:e02403-17. 52 ACS Paragon Plus Environment

Page 53 of 71

1159


(126) Moreau, J. W.; Gionfriddo, C. M.; Krabbenhoft, D. P.; Ogorek, J. M.; DeWild, J. F.;

1160

Aiken, G. R.; Roden, E. E. The effect of natural organic matter on mercury methylation

1161

by Desulfobulbus propionicus 1pr3. Front. Microbiol. 2015, 6, 1389.

1162

(127) Chiasson-Gould; S. A.; Blais, J. M.; Poulain, A. J. Dissolved organic matter kinetically

1163

controls mercury bioavailability to bacteria. Environ. Sci. Technol. 2014, 48 (6), 3153-

1164

3161.

1165

(128) George, G. N.; Prince, R. C.; Gailer, J.; Buttigieg, G. A.; Denton, M. B.; Harris, H. H.;

1166

Pickering I. J. Mercury binding to the chelation therapy agents DMSA and DMPS and

1167

the rational design of custom chelators for mercury. Chem. Res. Toxicol. 2004, 17 (8),

1168

999- 1006.

1169

(129) Zhang, J.; Wang, F.; House, J. D.; Page, B. Thiols in wetland interstitial waters and

1170

their role in mercury and methylmercury speciation. Limnol. Oceanogr. 2004, 49 (6),

1171

2276-2286.

1172

(130) Leclerc, M.; Planas, D.; Amyot, M. Relationship between extracellular low-molecular-

1173

weight thiols and mercury species in natural lake periphytic biofilms. Environ. Sci.

1174

Technol. 2015, 49 (13), 7709-7716.

1175

(131) Liem-Nguyen, V. Determination of mercury chemical speciation in the presence of

1176

low molecular mass thiols and its importance for mercury methylation. Ph. D. thesis.

1177

Umeå University, 2016.

1178

(132) Liem-Nguyen, V.; Skyllberg, U.; Björn, E. Thermodynamic modeling of the solubility

1179

and chemical speciation of mercury and methylmercury driven by organic thiols and

1180

micromolar sulfide concentrations in boreal wetland soils. Environ. Sci. Technol. 2017,

1181

51 (7), 3678-3686.



1182

Page 54 of 71

(133) Ticknor, J. L.; Kucharzyk, K. H.; Porter, K. A.; Deshusses, M. A.; Hsu-Kim, H.

1183

Thiol-based selective extraction assay to comparatively assess bioavailable mercury in

1184

sediments. Environ. Eng. Sci. 2015, 32 (7), 564-573.

1185

(134) Fahey, R. C. Novel thiols in prokaryotes. Annu. Rev. Microbiol. 2001, 55, 333-356.

1186

(135) Masip, L.; Veeravalli, K.; Georgiou, G. The many faces of glutathione in bacteria.

1187 1188 1189 1190 1191 1192

Antioxid. Redox. Signal. 2006, 8 (5-6), 753-762. (136) Haas, K. L.; Franz, K. J. Application of metal coordination chemistry to explore and manipulate cell biology. Chem. Rev. 2009, 109 (10), 4921-4960. (137) Ma, Z.; Jacobsen; F. E.; Giedroc, D. P. Coordination chemistry of bacterial metal transport and sensing. Chem. Rev. 2009, 109 (10), 4644-4681. (138) Argüello, J. M.; Raimunda, D.; Gonzalés-Guerrero, M. Metal transport across

1193

biomembranes: emerging models for a distinct chemistry. J. Biol. Chem. 2012, 287

1194

(17), 13510-13517.

1195

(139) Hohle T. H.; Franck, W. L.; Stacey, G.; O’Brian, M. R. Bacterial outer membrane

1196

channel for divalent metal ion acquisition. Proc. Natl. Acad. Sci. 2011, 108 (37),

1197

15390–15395.

1198

(140) Mastropasqua, M. C.; D’Orazio, M.; Cerasi, M.; Pacello, F.; Gismondi, A.; Canini, A.;

1199

Canuti, L.; Consalvo, A.; Ciavardelli, D.; Chirullo, B.; Pasquali, P.; Battistoni, A.

1200

Growth of Pseudomonas aeruginosa in zinc poor environments is promoted by a

1201

nicotianamin-related metallophore. Mol. Microbiol. 2017, 106 (4), 543-561.

1202

(141) Lau, B. L. T.; Hsu-Kim, H. Precipitation and growth of zinc nanoparticles in the

1203

presence of thiol-containing natural organic ligands. Environ. Sci. Technol. 2008, 42

1204

(19), 7236-7241.


Page 55 of 71

1205


(142) Gondikas, A. P.; Maison, A.; Auffan, M.; Lau, B. L. T.; Hsu-Kim, H. Early-stage

1206

precipitation kinetics of sulfide nanoclusters forming in the presence of cysteine.

1207

Chem. Geol. 2012, 329, 10-17.

1208

(143) Hamlet, N. V.; Landale, E. C.; Davis, B. H.; Summers, A. O. Roles of the Tn21 merT,

1209

merP, and merC gene products in mercury resistance and mercury binding. J.

1210

Bacteriol. 1992, 174 (20), 6377-6385.

1211

(144) Regler, N.; Larras, F.; Bravo, A. G.; Ungereanu, V. G.; Amouroux, D.; Cosio. C. Hg

1212

accumulation in the macrophyte Elodea Nuttallii in the field and in microcosm: Hg in

1213

shoots accumulated from the water might involve copper transporters. Chemosphere

1214

2012, 90 (2), 595-602.

1215 1216 1217

(145) Festa, R. A.; Thiele, D. J. Copper: an essential metal in biology. Curr. Biol. 2011, 21 (21), R877-R883. (146) DiSpirito, A. A.; Semrau, J. D.; Murrel, J. C.; Gallagher, W. H.; Dennison, C.;

1218

Vuilleumier, S. Methanobactin and the link between copper and bacterial methane

1219

oxidation. Microbiol. Mol. Biol. Rev. 2016, 80 (2), 387-409.

1220

(147) Lu, X.; Gu, W.; Zhao, L.; Farhan, M.; Haque, L.; DiSpirito, A. A.; Semrau, J. D.; Gu,

1221

B. Methylmercury degradation and uptake by methanotrophs. Sci. Adv. 2017, 3,

1222

e170001.

1223 1224

(148) Semrau, J. D.; DiSpirito, A. A.; Yoon, S. Methanotrophs and copper. FEMS Microbiol. Rev. 2010, 34 (4), 496-531.

1225

(149) King, J. K.; Saunders, F. M.; Lee, R. F.; Jahnke, R. A. Coupling mercury methylation

1226

rates with sulfate reduction rates in marine sediments. Environ. Toxicol. Chem. 1999,

1227

18 (7), 1362-1369.



1228

Page 56 of 71

(150) Shih, R.; Robertson, W. D.; Schiff, S. L.; Rudolf, D. L. Nitrate controls methyl

1229

mercury production in a streambed bioreactor. J. Environ. Qual. 2011, 40 (5), 1586-

1230

1592.

1231

(151) Todorova, S. G.; Driscoll, Jr., C. T.; Matthews, D. A.; Effler, S. W.; Hines, M. E.;

1232

Henry, E. A. Evidence for regulation of monomethyl mercury by nitrate in a seasonally

1233

stratified, eutrophic lake. Environ. Sci. Technol. 2009, 43 (17), 6572-6578.

1234

(152) Rask, M.; Verta, M.; Korhonen, M.; Salo, S.; Forsius, M.; Arvola, L.; Jones, R. I.;

1235

Kiljunen, M. Does lake thermocline depth affect methyl mercury concentrations in

1236

fish? Biogeochem. 2010, 101 (1), 311-322.

1237

(153) Austin, D.; Scharf, R.; Carrol, J.; Enochs, M. Suppression of hypolimnetic

1238

methylmercury accumulation by liquid calcium nitrate amendments: redox dynamics

1239

and fate of nitrate. Lake Reserv. Manage. 2016, 32 (1), 61-73.

1240 1241 1242

(154) Gilmour, C. C.; Mitchell, R. Sulfate stimulation of mercury methylation in freshwater sediments. Environ. Sci. Technol. 1992, 26 (11), 2281-2287. (155) Jeremiason, J. D.; Engstrom, D. R.; Swain, E. B.; Nater, E. A.; Johnson, B. M.;

1243

Almendinger, J. E.; Monson, B. A.; Kolka, R. K. Sulfate addition increases

1244

methylmercury production in an experimental wetland. Environ. Sci. Technol. 2006, 40

1245

(12), 3800-3806.

1246

(156) Watras, C. J.; Morrison, K. A.; Regnell, O.; Kratz, T. K. The methylmercury cycle in

1247

Little Rock Lake during experimental acidification and recovery. Limnol. Oceanogr.

1248

2006, 51 (1), 257-270.

1249

(157) Willacker, J. J.; Eagles-Smith, C. A.; Ackerman, J. T. Mercury bioaccumulation in

1250

estuarine fishes: Novel insights from stable sulfur isotopes. Environ. Sci. Technol.

1251

2017, 51 (4), 2131-2139. 56 ACS Paragon Plus Environment

Page 57 of 71

1252


(158) Graham, A. M.; Cameron-Burr, K. T.; Hajic, H. A.; Lee, C.; Msekela, D.; Gilmour, C.

1253

C. Sulfurization of dissolved organic matter increases Hg-sulfide-dissolved organic

1254

matter bioavailability to Hg-methylating bacterium. Environ. Sci. Technol. 2017, 51

1255

(16), 9080-9088.

1256

(159) Eckley, C. S.; Hintelmann, H. Determination of mercury methylation potentials in the

1257

water column of lakes across Canada. Sci. Tot. Environ. 2006, 368 (1), 111-125.

1258

(160) Madigan, M. T. Microbiology, physiology and ecology of phototrophic bacteria. In

1259

Biology of anaerobic microorganisms. Zender, A. J. B, Ed.; Wiley: New York 1988;

1260

pp 39-111.

1261

(161) Guimarães, J. R. D.; Mauro, J. B. N.; Meili, M.; Sundbom, M.; Haglund, A. L.;

1262

Coelho-Souza, S. A.; Hylander, L. D. Simultaneous radioassays of bacterial production

1263

and mercury methylation in the periphyton of a tropical and a temperate wetland. J.

1264

Environ. Manage. 2006, 81 (2), 95-100.

1265

(162) Guimarães, J. R. D.; Meili, M.; Hylander, L. D.; de Castro e Silva, E.; Roulet, E.;

1266

Mauro, J. B. N.; deLemos, R. A. Mercury net methylation in five tropical flood plain

1267

regions of Brazil: high in the root zone of floating macrophyte roots but low in surface

1268

sediment and flooded soils. Sci. Tot. Environ. 2000, 261, 99-107.

1269

(163) Marvin-DiPasquale, M. C.; Agee, J. L.; Bouse, R. M.; Jaffe, B. E. Microbial cycling of

1270

mercury in contaminated pelagic and wetland sediments of San Pablo Bay, California.

1271

Environ. Geol. 2003, 43 (3), 260-267.

1272

(164) Windham-Myers, L.; Marvin-Dipasquale, M.; Krabbenhoft, D. P.; Agee, J. L; Cox, M.

1273

H.; Heredia-Middleton, P.; Coates, C.; Kakouros, E. Experimental removal of wetland

1274

emergent vegetation leads to decreased methylmercury production in surface sediment.

1275

J. Geophys. Res. 2009, 114 (G2), G00C05. 57 ACS Paragon Plus Environment


1276

Page 58 of 71

(165) Regler, N.; Frey, B.; Converse, B.; Roden, E.; Grosse-Honebrink, A.; Bravo, A. G.;

1277

Cosio, C. Effect of Elodea nuttallii roots on bacterial communities and MMHg

1278

proportion in a Hg polluted sediment. PLoS One 2012, 7 (9), e45565.

1279

(166) Creswell, J. E.; Kerr, S. C.; Meyer, M. H.; Babiarz, C. L.; Shafer, M. M.; Armstrong,

1280

D. E.; Roden, E. E. Factors controlling temporal and spatial distribution of total

1281

mercury and methylmercury in hyporheic sediments of the Allequash Creek wetland,

1282

northern Wisconsin. J. Geophys. Res. 2008, 113, G00C02.

1283

(167) Sorensen, J. A.; Kallemeyn, L. W.; Sydor, M. Relationship between mercury in young-

1284

of-the-year yellow perch and water level fluctuations. Environ. Sci. Technol. 2005, 39

1285

(23), 9237-9243.

1286

(168) Watras, C. J.; Morrison, K. A. The response of two remote, temperate lakes to changes

1287

in atmospheric mercury deposition, sulfate, and the water cycle. Can. J. Fish. Aquat.

1288

Sci. 2008, 65 (1), 100-116.

1289

(169) Eckley, C. S.; Luxton, T. P.; McKernan, J. L; Goetz, J.; Goulet, J. Influence of

1290

reservoir water level fluctuations on sediment methylmercury concentrations

1291

downstream of the historical Black Butte mercury mine, OR. Appl. Geochem. 2015, 61,

1292

284-293.

1293

(170) Amaral, L.; Martins, A.; Spengler, G.; Molnar, J. Efflux pumps of Gram-negative

1294

bacteria: what they do, how they do it, with what and how to deal with them. Front.

1295

Pharmacol. 2014, 4, 168.

1296 1297

(171) Nikaido, H. Preventing drug access to targets: cell surface permeability barriers and active efflux in bacteria. Semin. Cell. Dev. Biol. 2001, 12 (3), 215-223.


Page 59 of 71

1298


(172) Lin, H.; Lu, X.; Liang, L.; Gu, B. Thiol-facilitated cell export and desorption of

1299

methylmercury by anaerobic bacteria. Environ. Sci. Technol. Lett. 2015, 2 (10), 292-

1300

296.

1301

(173) Hintelmann, H.; Welbourn, P. M.; Evans, R. D. Measurements of complexation of

1302

methylmercury(II) compounds by freshwater humic substances using equilibrium

1303

dialysis. Environ. Sci. Technol. 1997, 31 (2), 489-495.

1304

(174) Skyllberg, U. Competition among thiols and inorganic sulfides and polysulfides for Hg

1305

and MeHg in wetland soils and sediments under suboxic conditions: Illumination of

1306

controversies and implication for MeHg net production. J. Geophys. Res. 2008, 113

1307

(G2), G00C03.

1308

(175) Ndu, U.; Barkay, T.; Schartup, A. T.; Mason, R. P.; Reinfelder, J. R. The effect of

1309

aqueous speciation and cellular ligand binding on the biotransformation and

1310

bioavailability of methylmercury in mercury-resistant bacteria. Biodegradation 2016,

1311

27 (1), 29-36.

1312

(176) Regnell, O.; Hammar, T. Coupling of methyl and total mercury in a minerotrophic

1313

peat bog in southeastern Sweden. Can. J. Fish. Aquat. Sci. 2004, 61 (10), 2014-2023.

1314

(177) Waldron, M. C.; Colman, J. A.; Breault, R. F. Distribution, hydrologic transport, and

1315

cycling of total mercury and methyl mercury in a contaminated river-reservoir-wetland

1316

system (Sudbury River, eastern Massachusetts). Can. J. Fish. Aquat. Sci. 2000, 57 (5),

1317

1080-1091.

1318

(178) Gascón Diez, E.; Loizeau, J.-L.; Cosio, C.; Bouchet, S.; Adatte, T.; Amouroux, D.;

1319

Bravo, A. G. Role of settling particles on mercury methylation in the oxic water

1320

column of freshwater systems. Environ. Sci. Technol. 2016, 50 (21), 11672-11679.



1321

Page 60 of 71

(179) Pakhomova, S.; Veiteberg Braaten, H. F.; Yakushev, E.; Skei, J. Biogeochemical

1322

consequences of an oxygenated intrusion into an anoxic fjord. Geochem. T. 2014, 5,

1323

15.

1324

(180) Rosati, G.; Heimbürger, L. E.; Melaku Canu, D.; Lagane, C.; Laffont, L.; Rijkenberg,

1325

M. J. A.; Gerringa, L. J. A.; Solidoro, C.; Gencarelli, C. N.; Hedgecock, I. M.; De

1326

Baar, H. J. W.; Sonke, J. E. Mercury in the Black Sea: New insights from

1327

measurements and numerical modeling. Global Biogeochem. Cy. 2018, 32 (4),

1328

GB005700.

1329

(181) Kuss, J.; Cordes, F.; Mohrholz, V.; Nausch, G.; Naumann, M.; Krüger, S.; Schulz-Bull,

1330

D. E. The impact of the major Baltic inflow of December 2014 on the mercury species

1331

distribution in the Baltic Sea. Environ. Sci. Technol. 2017, 51 (20), 11692-11700.

1332

(182) Monperrus, M.; Tessier, E.; Amouroux, D.; Leynaert, A.; Huonnic, P.; Donard, O. F.

1333

X. Mercury methylation, demethylation and reduction rates in coastal and marine

1334

surface waters of the Mediterranean Sea. Mar. Chem. 2007, 107 (1), 49-63.

1335

(183) Lehnherr, I.; St.Louis, V. L.; Hintelmann, H.; Kirk, J. L. Methylation of inorganic

1336 1337 1338 1339 1340 1341

mercury in polar waters. Nature Geosci. 2011, 4, 298-302. (184) Cossa, D.; Averty, B.; Pirrone, N. The origin of methylmercury in open Mediterranean waters. Limnol. Oceanogr. 2009, 54 (3), 837-844. (185) Hammerschmidt, C. R.; Bowman, K. L. Vertical methylmercury distribution in the subtropical North Pacific Ocean. Mar. Chem. 2012, 132-133, 77-82. (186) Kim, H.; Soerensen, A. L.; Hur, J.; Heimbürger, L. E.; Hahm, D.; Rhee, T. S.; Noh, S.;

1342

Han, S. Methylmercury mass budgets and distribution characteristics in the Western

1343

Pacific Ocean. Environ. Sci. Technol. 2017, 51 (3), 1186-1194.


Page 61 of 71

1344


(187) Pongratz, R.; Heumann, K. G. Production of methylated mercury, lead, and cadmium

1345

by marine bacteria as a significant atmospheric source for atmospheric heavy metals in

1346

polar regions, Chemosphere 1999, 39 (1), 89-102.

1347

(188) Heimbürger, L.-E.; Cossa, D.; Marty, J.-C.; Migon, C.; Averty, B.; Dufour, A.; Ras, J.

1348

Methyl mercury distributions in relation to the presence of nano- and

1349

picophytoplankton in an oceanic water column (Ligurian Sea, North-western

1350

Mediterranean), Geochim. Cosmochim. Acta 2010, 74 (19), 5549-5559.

1351

(189) Soerensen, A. L,; Jacob, D. J.; Schartup, A. T.; Fisher, J. A.; Lehnherr, I,; St. Louis, V.

1352

L.; Heimbürger, L.-E.; Sonke, J. E.; Krabbenhoft, D. P.; Sunderland, E. M. A mass

1353

budget for mercury and methylmercury in the Arctic Ocean. Global. Biogeochem. Cy.

1354

2016, 30 (4), 560-575.

1355

(190) Blum, J. D.; Popp, B. N.; Drazen, J. C.; Choy, A.; Johnson, M. W. Methylmercury

1356

production below the mixed layer in the North Pacific Ocean. Nature Geosci. 2013, 6,

1357

879-884.

1358

(191) Li, M.; Schartup, A. T.; Valberg, A. P.; Ewald, J. D.; Krabbenhoft, D. P.; Yin, R.;

1359

Balcom, P. H.; Sunderland, E. M. Environmental origins of methylmercury

1360

accumulated in subarctic fish indicated by mercury stable isotopes. Environ. Sci.

1361

Technol. 2016, 50 (21), 11559-11568.

1362

(192) Pongratz; R.; Heumann, K. G. Production of methylated mercury and lead by polar

1363

macroalgae – A significant source for atmospheric heavy metals in clean room

1364

departments. Chemosphere 1998, 36 (9), 1935-1946.

1365

(193) Heimbürger, L.-E.; Sonke, J. E.; Cossa, D.: Point, D.; Lagane, C.; Laffont, L.; Galfond,

1366

B. T.; Nicolaus, M.; Rabe, B.; Rutgers van der Loeff, M. Shallow methylmercury



Page 62 of 71

1367

production in the marginal sea ice zone of the central Arctic Ocean. Sci. Rep. 2015, 5,

1368

10318.

1369

(194) Sunderland, E. M.; Krabbenhoft, D. P.; Moreau, J. W.; Strode, S. A.; Landing, W. M.

1370

Mercury sources, distribution, and bioavailability in the North Pacific Ocean; Insights

1371

from data and models. Global Biogeochem. Cy. 2009, 23 (2), GB2010.

1372

(195) Cossa, D.; Durrieu de Madron, X.; Schäfer, J.; Lanceleur, L.; Guédron, S.; Buscail, R.;

1373

Thomas, B.; Castelle, S.; Naudin, J.-J. The open sea as the main source of

1374

methylmercury in the water column of the Golf of Lyon (Northwestern Mediterranean

1375

margin). Geochim. Cosmochim. Acta 2017, 199, 222-237.

1376

(196) Ohnemus. D. C.; Rauschenberg, S.; Cutter, G. A.; Fitzsimmons, J. N.; Sherrell, R. M.;

1377

Twining, B. S. Elevated trace metal content of prokaryotic communities associated

1378

with marine oxygen deficient zones. Limnol. Oceanogr. 2017, 62 (1), 3-25.

1379

(197) Malcolm, E. G.; Schaefer, J. K.; Ekstrom, E. B.; Tuit, C. B.; Jayakumar, A.; Park, H.;

1380

Ward, B. B.; Morel, F. M. M. Mercury methylation in oxygen deficient zones of the

1381

oceans: No evidence for the predominance of anaerobes. Mar. Chem. 2010, 122 (1-4),

1382

11-19.

1383

(198) Klawonn, I.; Bonaglia, S.; Brüchert, V.; Ploug, H. Aerobic and anaerobic nitrogen

1384

transformation processes in N2-fixing cyanobacterial aggregates. ISME J. 2015, 9 (6),

1385

1456-1466.

1386

(199) Janssen, D. J.; Conway, T. M.; John, S. G.; Christian, J. R.; Kramer, D. I.; Pedersen, T.

1387

F.; Cullen, J. T. Undocumented water column sink for cadmium in open ocean oxygen-

1388

deficient zones. Proc. Natl. Acad. Sci. 2014, 111 (19), 6888-6893.


Page 63 of 71

1389


(200) Ortiz, V. L.; Mason, R. P.; Ward, J. E. An examination of the factors influencing

1390

mercury and methylmercury particulate distributions, methylation and demethylation

1391

rates in laboratory-generated marine snow. Mar. Chem. 2015, 177 (5), 753-762.

1392 1393 1394

(201) Wright, J. J.; Konwar, K. M.; Hallam, S. J. Microbial ecology of expanding oxygen minimum zones. Nat. Rev. Microbiol. 2012, 10 (6), 381-394. (202) Jeremiason, J. D.; Portner, J. C.; Aiken, G. R.; Hiranaka, A. J.; Dvorak, M. T.; Tran, K.

1395

T.; Latch, D. E. Photoreduction of Hg(II) and photodemethylation of methylmercury:

1396

the key role of thiol sites on dissolved organic matter. Environ. Sci.: Processes and

1397

Impacts 2015, 17 (1), 1892-1903.

1398

(203) Luo, H.-W.; Yin, X.; Jubb, A. M.; Chen, H.; Lu, X.; Zhang, W.; Lin, H.; Yu, H.-Q.;

1399

Liang, L.; Sheng, G.-P.; Gu, B. Photochemical reactions between mercury (Hg) and

1400

dissolved organic matter decrease Hg bioavailability and methylation. Environ. Pollut.

1401

2017, 220 (Pt B), 1359-1365.

1402

(204) Poulain, A. J.; Amyot, M.; Findlay, M.; Telor, S.; Barkay, T.; Hintelmann, H.

1403

Biological and photochemical production of dissolved gaseous mercury in a boreal

1404

lake. Limnol. Oceanogr. 2004, 49 (6), 2265-2275.

1405

(205) Mason, R. P.; Morel, F. M. M.; Hemond, H. F. The role of microorganisms in

1406

elemental mercury formation in natural waters. Water Air Soil Pollut. 1995, 80 (1-4),

1407

775-787.

1408

(206) Poulain, A. J.; Ní Chadhain, S. M.; Ariya, P. A.; Amyot, M.; Garcia, E.; Campbell, P.

1409

G. C.; Zylstra, G. J.; Barkay, T, Potential for mercury reduction by microbes in the

1410

high Arctic. Appl. Environ. Microbiol. 2007, 73 (1), 2230-2238.

1411 1412

(207) Stemmler, I.; Hense, I.; Quack, B.; Maier-Reimer, E. Methyl iodide production in the open ocean. Biogeosciences 2014, 11 (16), 4459-4476. 63 ACS Paragon Plus Environment


1413

(208) Yin, Y.; Li, Y.; Tai, C.; Cai, Y.; Jiang, G. Fumigant methyl iodide can methylate

1414

inorganic mercury species in natural waters. Nat. Commun. 2014, 5, 4633.

Page 64 of 71

1415

(209) Larose, C.; Dommergue, A.; De Angelis, M.; Cossa, D.; Averty, B.; Marusczak, N.;

1416

Soumis, N.; Schneider, D.; Ferrari, C. Springtime changes in snow chemistry lead to

1417

new insights into mercury methylation in the Arctic. Geochim. Cosmochim. Acta 2010,

1418

74 (22), 6263-6275.

1419 1420 1421

(210) Mason, R. P.; Fitzgerald, W. F. The distribution and biogeochemical cycling of mercury in the equatorial Pacific Ocean. Deep-Sea Res. I 1993, 40 (9), 1897-1924. (211) Kirk, J. L.; St. Louis, V. L.; Hintelmann, H.; Lehnherr, I.; Else, B.; Poissant, L.

1422

Methylated mercury species in marine waters of the Canadian high and sub Arctic.

1423

Environ. Sci. Technol. 2008, 42 (22), 8367-8373.

1424

(212) Bowman, K. L.; Hammerschmidt, C. R.; Lamborg, C. H.; Swarr, G. Mercury in the

1425

North Atlantic Ocean: The U.S. GEOTRACES zonal and meridional sections. Deep-

1426

Sea Res. II 2015, 116, 251-261.

1427

(213) Wallschläger, D.; Hintelmann, H.; Evans, R. D.; Wilken, R.-D. Volatilization of

1428

dimethylmercury and elemental mercury from River Elbe floodplain soils. Water Air

1429

Soil Pollut. 1995, 80 (1-4), 1325-1329.

1430 1431

(214) Fagerström, T.; Jernelöv, A. Some aspects of the quantitative ecology of mercury. Water Res. 1972, 6 (10), 1193-1202.

1432

(215) Black, F. J.; Connaway, C. H.; Flegal, A. R. Stability of dimethyl mercury in sea water

1433

and its conversion to monomethyl mercury. Environ. Sci. Technol. 2009, 43 (11),

1434

4056-4052.

1435 1436

(216) Wolfe, N. L; Zepp, R. G.; Gordon, J. A.; Baughman, G. L. Chemistry of methyl mercurials in aqueous solutions. Chemosphere 1973, 2 (4), 147-152. 64 ACS Paragon Plus Environment

Page 65 of 71

1437 1438 1439 1440


(217) Jonsson, S.; Mazrui, N. M.; Mason, R. P. Dimethylmercury formation mediated by inorganic and organic reduced sulfur surfaces. Sci. Rep. 2016, 6, 27958. (218) Parker, J. L.; Bloom, N. S. Preservation and storage techniques for low-level aqueous mercury speciation. Sci. Total. Environ. 2005, 337 (1-3), 253-263.



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1441

FIGURE LEGENDS

1442

Figure 1.

1443

Hypothetical curves drawn to illustrate that Hg(II) uptake rates first tend to increase with the

1444

stability of extracellular Hg(II) complexes and then to decrease at higher thermodynamic and/or

1445

kinetic stabilities because of decreasing exchange of Hg with putative membrane transporters. In

1446

the shown case, organism “B” has a stronger uptake system than organism “A”.

1447 1448

Figure 2.

1449

Hg(II) and Z(II) speciation in anoxic water and how it is controlled by natural organic matter

1450

(NOM) and sulfide. It is assumed that the Hg(II) and Zn(II) species within the bracket are

1451

available for uptake by Hg methylating anaerobes and that they compete for binding to metal

1452

transporters. Note that the role of metal transporters in Hg uptake is uncertain, and that there is

1453

presently no evidence that anaerobes release thiols in order to extract Zn from otherwise

1454

unavailable Zn species or to prevent the coagulation of ZnSNP. HgcAB: The Hg methylating

1455

apparatus associated with the cytoplasmic membrane. NOMs: Sulfurized NOM (NOM with an

1456

increased content of reduced sulfur groups).

1457 1458

Figure 3.

1459

Redox transition zones (shown in reddish color) with intense Hg methylation because of

1460

conditions that favor heterotrophic activity and cellular uptake of Hg.

1461 1462

Figure 4.

1463

A schematic drawing illustrating how cellular and extracellular processes occurring in a sulfide-

1464

generating microbial community may control Hg uptake, Hg methylation, and MeHg release, 66 ACS Paragon Plus Environment

Page 67 of 71


1465

partly by favoring or disfavoring microbes possessing the gene pair hgcAB that is essential for Hg

1466

methylation. Unidirectional arrows indicate cause-effect relationships. Bidirectional arrows

1467

represent possible links between hgcAB and the indicated processes. Broken arrows represent

1468

uncertain cause-effect relationships or links that merit further research. Arrow length bears no

1469

meaning. DOM: Dissolved Organic Matter.

1470 1471



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1473 1474 1475

FIGURE 1

1476


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1477

1478 1479 1480

FIGURE 2

1481 1482



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1483

FIGURE 3

1484 1485


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1487 1488 1489

FIGURE 4


Microbial Mercury Methylation in Aquatic Environments: A Critical

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