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Effects of nutrient loading and mercury chemical speciation on the formation and degradation of methylmercury in estuarine sediment Van Liem-Nguyen, Sofi Jonsson, Ulf Skyllberg, Mats B. Nilsson, Agneta Andersson, Erik Lundberg, and Erik Björn Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01567 • Publication Date (Web): 03 Jun 2016 Downloaded from http://pubs.acs.org on June 7, 2016
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Environmental Science & Technology
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Effects of nutrient loading and mercury chemical speciation on the formation and degradation of methylmercury in estuarine sediment
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Van Liem-Nguyen1, Sofi Jonsson1,2,3, Ulf Skyllberg4, Mats B. Nilsson4, Agneta Andersson2,5, Erik Lundberg2, Erik Björn1,*
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Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden Umeå Marine Sciences Centre, Umeå University, SE-910 20 Hörnefors, Sweden 3 current address: Department of Marine Sciences, University of Connecticut, 1080 Shennecossett Road, Groton, CT06340, USA 4 Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden 5 Department of Ecology and Environmental Science, Umeå University, SE-901 87 Umeå, Sweden 2
AUTHOR INFORMATION
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Corresponding author
16
*e-mail:
[email protected]; phone +46 90 7865189
17 18
ABSTRACT
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Net formation of methylmercury (MeHg) in sediments is known to be affected by the availability of
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inorganic divalent mercury (HgII) and by the activities of HgII methylating and MeHg demethylating
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bacteria. Enhanced autochthonous organic matter deposition to the benthic zone, following
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increased loading of nutrients to the pelagic zone, has been suggested to increase the activity of HgII
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methylating bacteria and thus the rate of net methylation. However, the impact of increased nutrient
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loading on the biogeochemistry of mercury (Hg) is challenging to predict as different geochemical
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pools of Hg may respond differently to enhanced bacterial activities. Here, we investigate the
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combined effects of nutrient (N and P) supply to the pelagic zone and the chemical speciation of HgII
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and of MeHg on MeHg formation and degradation in a brackish sediment-water mesocosm model
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ecosystem. By use of Hg isotope tracers added in situ to the mesocosms or ex situ in incubation
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experiments, we show that the MeHg formation rate increased with nutrient loading only for HgII
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tracers with a high availability for methylation. Tracers with low availability did not respond
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significantly to nutrient loading. Thus, both microbial activity (stimulated indirectly through plankton
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biomass production by nutrient loading) and HgII chemical speciation were found to control the
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MeHg formation rate in marine sediments.
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INTRODUCTION
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Methylmercury (MeHg) is a neurotoxic compound which biomagnifies in aquatic food-webs causing
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adverse effects to wildlife and humans. Formation of MeHg from inorganic, divalent Hg (HgII) is
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mediated by phylogenetically diverse anaerobic microorganisms,1-5 carrying the hgcA and hgcB
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genes.6 Bacteria are also responsible for degradation of MeHg in environmental compartments with
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low sunlight exposure, such as in many sedimentary systems.7 Net formation of MeHg under such
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conditions is driven by multiple factors such as the availability of HgII and MeHg for uptake by
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methylating/demethylating bacteria8-13 and factors in control of the activity of these bacteria.2,14-15
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The current perception is that bacteria methylate HgII to MeHg in their cell, after up-take of aqueous
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forms such as HgII‒sulfide complexes,8 and low molecular mass HgII‒thiol complexes.11-12 The major
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pools of HgII and MeHg in sediments and soils however, are represented by different solid/adsorbed
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HgII and MeHg forms with different solubilities and dissolution/desorption rates. Under equilibrium
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or close to equilibrium conditions the chemical speciation of Hg in solid/adsorbed phases controls
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aqueous concentrations of HgII and MeHg, and will potentially limit their availabilities for bacterial
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uptake.13,16 Indeed, Jonsson et al.16 demonstrated that the MeHg concentration in estuarine
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sediments and biota is controlled by the bioavailability of different geochemical Hg pools, which in
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turn is largely controlled by the solid/adsorbed phase chemical speciation and vertical localization of
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Hg in the sediment.
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The rate of nutrient supply, e.g. via catchment runoff, as well as temperature and light conditions are
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principal factors controlling primary production in aquatic ecosystems and thus the rate of formation
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and deposition rate of autochthonous natural organic matter (NOM) to the sediment. The deposition
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of autochthonous NOM will in turn control the activity of saprotrophic microbial organisms,17-18
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including bacteria with the capability to methylate HgII. Observations made in the field have in
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several studies been interpreted as a direct or indirect link between autochthonous production of
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biomass and MeHg net formation in marine and estuarine ecosystems. Two hypotheses have
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emerged from these studies: 1) The sediment and pore water partitioning coefficient, Kd (L kg-1), for
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HgII (and MeHg) is controlled by, and increases with, the amount of NOM in the sediment.19-22 With
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increased Kd the pore water concentration of HgII decreases and thus the presumed forms available
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for uptake by methylating bacteria also decrease. The chemical speciation of HgII in solid/adsorbed
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phases was however not established in these studies and recent reports have highlighted that the
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type and concentration of dissolved organic matter strongly influences the relationship between
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sediment NOM content and Kd of Hg.22-24 It therefore remains to develop a comprehensive chemical
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speciation model that quantitatively describes the solubility of HgII (and MeHg) under or close to
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equilibrium conditions in marine sediments. 2) The second hypothesis proposes that the activity of
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benthic anaerobic bacteria, including those with the capacity to methylate HgII, is controlled by the
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availability of metabolic electron donors.10,23,25-26 These electron donors are largely constituted by
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short-chain fatty acids originating predominantly from autochthonous NOM deposited to the
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sediment. This hypothesis has been used to explain observed increases in HgII methylation rate or
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MeHg concentration with amount25 or the fraction of autochthonous algal NOM (manifested by a low
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C/N ratio10,26). These two hypotheses are not necessarily alternatives, as both processes may take
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place in parallel, yet different studies have suggested opposing net effects on MeHg formation from
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increases in pelagic biomass production and subsequent organic carbon content or C/N ratio in
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sediments.
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It is a challenge to experimentally investigate the combined effects of nutrient loading and chemical
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speciation of HgII on methylation rates. Mesocosm experiments, in which natural environmental
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conditions can be simulated and factors affecting MeHg formation controlled, have proven powerful
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in addressing these issues. Using mesocosms, Jonsson et al. [Jonsson et al. in prep] demonstrated
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that the net methylation of HgII bonded to thiol groups in NOM, or as metacinnabar (β-HgS), in an
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estuarine sediment was enhanced by 10‒40% in response to a twofold increase in pelagic biomass
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production caused by enhanced nutrient loading. Here, using a similar system, we further investigate
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how rates of methylation and demethylation of different Hg geochemical pools are affected by the
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nutrient loading (N and P) to the pelagic zone. We hypothesized that Hg pools with a low availability
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to methylating bacteria will show less of a response to nutrient loading in comparison to Hg pools
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with a higher availability. Sediment-brackish water mesocosm model ecosystems (n=6, 5 m high,
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0.75 m ⌀ cylinders) were constructed and rates of HgII methylation were compared for two
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principally different types of enriched Hg isotope tracers. One set of tracers were added in situ to the
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mesocoms, either as defined pre-equilibrated solid/adsorbed chemical forms where HgII and MeHg
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was bonded to sulfide and/or thiols, or as dissolved complexes which were equilibrated in the
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mesocoms’ water phase prior to deposition to the sediment. A second set of tracers were added ex
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situ to sediment sampled from the mesocosm sediments as non-equilibrated labile dissolved
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complexes. Nutrients were added to the water column at low to moderate concentrations during
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weeks 1‒2, and at high concentrations during weeks 2‒4 of the experiment to study the short-term
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effects of increased nutrient loading on MeHg formation/degradation for different chemical forms of
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Hg.
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EXPERIMENTAL SECTION
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Sampling of sediment and water. The study site, Öre river estuary (located in the Bothnian Sea at
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the Sweden east coast), has been described in detail in a previous study.16 Intact sediment cores 3
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were manually sampled from the estuary by divers at site 63° 33.905′ N, 19° 50.898′ E, at a water
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depth of 5‒7 m using custom-made sampling devices. The sampling cylinders (~0.2 m × 0.63 m ⌀)
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were made of high density polyethylene (HDPE) with a detachable bottom and lid. The cylinders
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were immersed into undisturbed sediment, and surrounding sediment was cleared to insert the
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bottom plate and lid of the core sampler. The intact sediment cores were stored (in the dark, at 15
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°C) in barrels filled with brackish water to avoid leakage and to avoid direct contact with air for up to
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2 months. The mesocosm cylinders were filled with 2000 L of unfiltered brackish water (salinity of 4
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Practical Salinity Units) 3 days before the start of the experiment. Water was collected using a
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peristaltic pump system with two pipes located 800 m from land and at a water depth of 2 and 8 m (a
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50:50 mixture of water from these two depths were used). To assure that all mesocosms had similar
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distribution of planktonic organisms, water was added to all tanks in parallel.
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Mesocosm system and setup. The experiment was conducted at a mesocosm facility located at the
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Umeå Marine Sciences Center. This facility includes 12 double-mantled HDPE tubes (5 m × 0.75 m ⌀)
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regularly used for studies of the pelagic food web ecology. For this experiment, 6 of the systems
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were utilized. Three of them included collected sediment cores and a 5 m high water column. The
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remaining three were used to study the deposition of organic matter from the pelagic zone, using
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sedimentation traps placed at the bottom of the mesocosm tanks, and included a 5 m water column.
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Temperature of the water column was maintained at 15 oC in the upper 3.5 m and 10 oC in the lower
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part (below 3.5 m) during the course of the experiment, via an outer glycol layer. The convection of
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the upper part of the water column was obtained by purging with ~20 mL s-1 air at a depth of 3.2 m
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from the water surface. 150 W metal halogen lamps (Master colour CDM-T 150w/942 G12 1CT) were
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used as light sources with 12/12 hour on/off cycles. When turned on, the lamps resulted in average
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photon flux through the water column of 410, 57 and 2.7 µmol s-1 m-2 at ~0.05 m, 1 m and 4 m depth,
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respectively. Further technical specifications of mesocosm system compartments are given in Table
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S1. The total experiment time was 30 days from 9th February (referred to as day 1) to 10th March (day
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30). Nutrients (nitrate (NO3-), phosphate (PO43-) and ammonium (NH4+) prepared from salts of NaNO3,
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NaH2PO4×H2O and NH4Cl) were added at moderate levels (corresponding to 19.5 µg L-1 NO3-, 3.49 µg
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L-1 NH4+ and 3.33 µg L-1 H2PO4- in the 2000 L mesocosm water phase) equivalent to 3-10% of the
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concentrations typical for winter conditions in the Bothnian Sea (600 µg L-1 NO3-, 30 µg L-1 NH4+ and
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70 µg L-1 H2PO4-)27 during the first two weeks of the experiments (additions were done at day 1, 8 and
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11). The nutrient concentrations were increased by a factor of five (corresponding to 97.5 µg L-1 NO3-,
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17.5 µg L-1 NH4+ and 16.7 µg L-1 H2PO4- in the 2000 L mesocosm water phase) during the last two
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weeks of the experiments to induce a plankton bloom effect (additions were done at day 15 and 22).
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The in situ net methylation and demethylation of Hg were measured using five HgII or MeHg tracers
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added to the water column (denoted
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The tracers were synthesized from HgII enriched
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(96.41%),
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Laboratory (TN, USA). β‒201HgSsed and isotopically enriched MeHg tracers were synthesized as
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described by Jonsson et al.13 and Snell et al.,28 respectively. The 200HgII‒NOMsed and Me198Hg‒NOMsed
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tracers were prepared by adding
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previously characterized by Skyllberg and Drott.29 The Hg/RSH molar ratio was kept in the range
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giving 1:2 HgII:RSH and 1:1 MeHg:RSH complex stoichiometry.30 A slurry mixture of β‒201HgSsed,
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200
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and injected at 1 cm depth into the ~0.2 m deep × 0.63 m ⌀ intact sediment cores one day prior to
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the start of the experiment using an electronic 12 channel pipette (VWR, for 10‒200 µL). The
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injections were made with help of a custom made grid system which was placed above the sediment
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surface. Totally 3384 injections were done per sediment core, resulting in 113 injections per dm2
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(Table S2) covering 95% of the sediment surface. Sediment sections not covered by addition of tracer
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were in the outer part (close to the wall) and no sediment sub-samples were taken from these areas
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during the experiment. The intact sediment cores were then placed into the water filled mesocosm
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with the lid retained during the placement to protect the sediment surface. This is denoted day 1 of
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the experiment. The average ambient HgII and MeHg concentrations in the sediment were 170 pmol
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g-1 and 2.9 pmol g-1 (dry weight), respectively. The added concentration of HgIIsed (sum of β‒201HgSsed
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and
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HgII and MeHg, respectively. Aqueous stock solutions of Me199Hgwt and
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extraction from toluene28 and dissolution of the oxide salt, respectively in 0.1 M HCl. A 20 mL mixture
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of Me199Hgwt and
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immediately added to the water column at day 1 and 15 with one aliquot of 13.3 mL at 1.5 m depth
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and 6.7 mL at 4 m depth from the water surface (Table S3). The two additions were made to sustain
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the concentration of Hg in the water phase, and were based on a previously observed average
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residence time of 10-15 days for Hg in the mesocosm water column.16
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Sampling and analysis. The sampling and analytical measurement procedures applied here have
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been previously described in detail by Jonsson et al.16 Briefly, water samples were collected during
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the experiment at specific depths using teflon tubing and peristaltic pumps. These samples were
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then used to measure photosynthetic primary and bacterial production rate (by incorporation
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experiments of NaH14CO3 and [3H-methyl]-thymidine, respectively), HgII methylation rate constants
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(km), MeHg demethylation rate constants (kd) and concentrations of chlorophyll a (Chl α), DOC,
201
Hg (98.11%) and
204
wt
tracers) or injected into the sediment (denoted sed tracers). 196
Hg (50%),
198
Hg (92.78%),
199
Hg (91.95%),
200
Hg
Hg (98.11%) (as HgO or HgCl2) purchased from Oak Ridge National
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HgII(aq) and Me198Hg(aq) to a homogenized humic soil extract
HgII‒NOMsed and Me198Hg‒NOMsed (2.59 µM, 3.89 µM and 0.25 µM, respectively) was prepared
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HgII‒NOMsed) and Me198Hg‒NOMsed tracers (Table S2) constituted 50% and 130% of ambient
204
204
HgIIwt were prepared by
HgIIwt (0.22 µM and 2.24 µM, respectively) was diluted with MQ water and
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nutrients and MeHg. Pressure, turbidity and O2 saturation were measured in situ in the water column
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using a Seaguard CTD with attached microelectrode sensors (Unisense), and light penetration was
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measured using a Spherical Quantum Sensor (LI-COR, 193-SA and LI-COR 1400 unit). Sediment sub-
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cores (diameter of 4.2 cm) were sampled during the experiment using a custom made sampler
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designed for the mesocosm system.16 Sediment was sampled for determination of km and kd, dry
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weight, concentrations of total C, total N, HgT and MeHg. The concentrations of MeHg in water and
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of HgT and MeHg in sediment were determined using
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isotope dilution analysis by inductively coupled plasma mass spectrometry (ICPMS) for HgT and Gas
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Chromatography‒ICPMS analysis for MeHg.31 Concentrations of HgT and MeHg for ambient Hg and
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tracers were then calculated from mass bias corrected signals using signal deconvolution.32
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Determination of km and kd (ex situ experiments). The
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the water column in situ were also added to sediment slurries incubated ex situ to determine the km
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and kd, respectively). The sediment slurries were prepared from the top 2 cm of sediment sub cores
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collected from the mesocosm during the experiment. The concentration of 204HgII and Me199Hg added
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ex situ were on average 7.0 and 0.60 pmol g-1 (d.w.), respectively, and exceeded the concentration of
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204
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(d.w.) for
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simultaneously, t0 and t48. The ex situ incubations were terminated by freezing directly after the
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addition of ex situ tracer for t0 samples and after incubating for 48 hours under a N2(g) atmosphere
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(using a glove box) and dark condition for t48 samples. The amount of MeHg from the 204HgIIwt tracers
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added in situ (which deposited from the water column and methylated in the sediments) in t0
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samples was subtracted from the amount formed from the tracer added ex situ in t48 samples. The kd
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and km were calculated using equations 1 and 2.
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km = ([Me204Hg]t48 - [Me204Hg]t0) / ([204HgII]added × time of incubation)
(d-1)
(1)
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kd = -1 × (ln[Me199Hg]t48 - ln[Me199Hg]t0) / time of incubation
(d-1)
(2)
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where subscripts “t48” and “t0” refer to measured MeHg concentrations at 48 h and 0 h of
196
incubation, respectively, and subscript “added” refers to the concentration of HgII added to sediment
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in the incubation assay.
196
204
HgII or Me196Hg as internal standard for
Hg(aq) and Me199Hg(aq) tracers added to
HgII and Me199Hg already present in the sediment from in situ added tracers (3.4 and 0.25 pmol g-1 204
HgII and Me199Hg, respectively). Two sets of sediment slurries were prepared
198 199
RESULTS AND DISCUSSION
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Pelagic productivity and sedimentation of organic matter to the benthic zone. Selected key
201
ecological and biogeochemical factors describing the model ecosystems are given in Table S4. At the
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start of the experiment, rates of photosynthetic primary biomass production and bacterial biomass
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production and the concentration of Chl α were 0.7 and 4.4 µmol C dm-2 h-1 and 1.1 µg dm-3,
204
respectively. These conditions corresponded well with those typically observed during mid-winter
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with ice cover in the Bothnian Sea.33 Both primary production and the concentration of Chl α in the
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mesocosm water phase (Figure 1a) responded quickly to the combined treatment of light exposure
207
and nutrient addition and reflected growth of phytoplankton. At day 22, the primary production and
208
the concentration of Chl α reached a maximum and corresponded to typical spring bloom conditions
209
in the Bothnian Sea (primary production of ~8 µmol C dm-2 h-1)27. After day 22 both variables
210
decreased despite a maintained nutrient loading, which may be explained by the growth rate of
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predatory zooplankton which commonly increases subsequent to phytoplankton growth, generating
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a predator-prey dynamic.34 The pelagic bacteria biomass production rate remained fairly constant at
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4.1 ± 1.1 (given as average ± CI throughout the text unless otherwise stated, p=0.05, n=12) µmol C
214
dm-2 h-1 throughout the experiment. The structure of the food web was thus shifted during the
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experiment, from a bacteria dominated (heterotrophic) to a photosynthetic primary production
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dominated (autotrophic) web, in response to the mesocosm light and nutrient addition treatment.
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Following the increases in primary production rate and Chl α concentration, the C/N molar ratio in
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the material collected in sedimentation traps (placed at the bottom of triplicate mesocosms without
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sediment) decreased from 10.7 ± 0.3 during weeks 0–2 to 8.6 ± 0.9 (p=0.05, n=6) during weeks 2–4 of
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the experiment. This decrease in C/N molar ratio suggested an increased fraction of autochthonous
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over allochthonous NOM in the deposited material during the second half of the experiment.35 As a
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comparison, the C/N molar ratio in the top 0‒5 cm of the sediment was 11.4 ± 0.3 (p=0.05, n=12).
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The total amount of deposited organic carbon during the 28 days of the experiment constituted 0.1%
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of the total organic carbon in the top 0-2 cm of the sediment. It is however expected that this
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fraction is important for controlling the activity of saprotrophic microbial organisms,17-18 including
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bacteria with the capability of methylating HgII. The redox conditions of the bulk sediment, below an
227
oxidized surface layer of 1‒2 mm (yellowish-brownish color by visual inspection), was in the
228
ferruginous to low sulfidic range with a concentration of dissolved sulfide in the pore water of < 0.05
229
to 1.0 µM. This is in good agreement with previous studies on sediment from the same area.13,16,25
230 231
Effect of nutrient loading on rates of methylation and demethylation of labile Hg tracers added ex
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situ. Benthic anaerobic bacteria primarily utilize metabolic electron donors originating from
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autochthonous rather than allochthonous NOM.17-18 In line with this, high MeHg formation rates
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have been related to low C/N molar ratios in estuarine sediment,10,26 suggesting that the activity of
235
mercury methylating bacteria could be driven by autochthonous NOM. As described above, the 7
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primary production and Chl α concentration increased during the second half of the experiment, and
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the C/N molar ratio was indicative of an increased proportion of planktonic material in the material
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deposited to the benthic zone in the last two weeks of the experiment. The HgII methylation rate
239
constant (km) for the tracer (dissolved labile Hg(OH)2(aq) complex) added ex situ to sediment
240
subsamples increased in parallel with increasing primary production and Chl α concentration (Figure
241
1b). The constant increased by a factor of four from day 9 to day 16 followed by a decrease after day
242
23. A significant correlation (R2=0.96) was found between the Chl α concentration and km. The MeHg
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demethylation rate constant (kd) in the sediment increased slightly towards the end of the
244
experiment, when primary production rates and Chl α concentrations decreased after day 23. Overall
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kd was only moderately affected by the treatments and varied less than 40% during the 21 days’ time
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period covered by measurements (day 9‒30), while km increased by a factor of 3.9. When HgII and
247
MeHg tracers are added as labile dissolved complexes to determine km and kd in short term
248
incubation experiments, they typically have a higher availability for transformation reactions as
249
compared to ambient Hg or the type of solid/adsorbed tracers added in situ to the mesocosms in this
250
study.13,16 The results in Figure 1 therefore suggest that the methylation of HgII chemical forms having
251
a high availability for methylation is rapid and largely responds to changes in pelagic primary
252
production rate and Chl α biomass following altered nutrient loading rate. These results also highlight
253
that although the amount of freshly deposited autochthonous organic carbon constituted only a
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small fraction (0.1%) of the total organic carbon in the surface sediment, it significantly enhanced
255
MeHg formation.
256 257
Effect of nutrient loading on net methylation of Hg tracers added in situ. Net methylation of tracers
258
added in situ (injected in the sediment at a depth of 1 cm; β‒201HgSsed,
259
NOMsed, and added to the water column; 204HgIIwt and Me199Hgwt) was determined as the MeHg/HgII
260
molar ratio16. The average net methylation was significantly different for the different HgII tracers
261
(ANOVA, p
