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Contrasting effects of marine and terrestrially derived dissolved organic matter on mercury speciation and bioavailability in seawater Amina Traore Schartup, Udonna Chinedu Ndu, Prentiss H Balcom, Robert P. Mason, and Elsie M. Sunderland Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es506274x • Publication Date (Web): 16 Apr 2015 Downloaded from http://pubs.acs.org on April 20, 2015
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Environmental Science & Technology
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Contrasting effects of marine and terrestrially
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derived dissolved organic matter on mercury
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speciation and bioavailability in seawater
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Amina T. Schartup1*, Udonna C. Ndu2, Prentiss H. Balcom3,4, Robert P. Mason3 and Elsie M.
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Sunderland1,4
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United States
Department of Environmental Health, Harvard School of Public Health, Boston MA 02215,
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Brunswick NJ 08901, United States
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Department of Environmental Science, Rutgers University, 14 College Farm Road, New
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Department of Marine Sciences, 1080 Shennecossett Rd Groton CT 06340, United States
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4School
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*Corresponding author
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[email protected] of Engineering and Applied Sciences, Harvard University, Cambridge MA, 02138
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Abstract
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Methylmercury (MeHg) is the only species of mercury (Hg) to biomagnify in aquatic
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food-webs to levels that are a widespread concern for human and ecological health. Here we
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investigate the association between dissolved organic matter (DOM) in seawater and Hg
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speciation and uptake using experimental data and field measurements from Long Island Sound
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(LIS) and the Northwestern Atlantic continental margin (NWA). We measured differences in
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DOM composition across sampling stations using excitation emission matrix (EEM)
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fluorescence spectroscopy and further separated DOM into terrestrial and marine components
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using Parallel Factor Analysis (PARAFAC). Highest MeHg concentrations were found in the
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estuarine stations (LIS) with highest DOM concentrations due to enhanced external inputs from
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the watershed and rivers. For stations on the shelf and slope, MeHg in plankton increased
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linearly with a decreasing fraction of fluorescence attributable to DOM components with a
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terrestrial rather than marine origin. These results are corroborated by experimental data showing
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higher MeHg uptake by cells in the presence of predominantly marine DOM compared to
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terrestrial DOM. Highest fractions of dissolved gaseous mercury were also found at stations with
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the highest marine DOM content, suggesting a greater reducible fraction of divalent inorganic
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Hg. These data suggest DOM composition is a critical driver of Hg reactivity and bioavailability
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in offshore marine waters.
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Introduction Mercury (Hg) is a naturally occurring heavy metal with a biogeochemical cycle that has
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been substantially perturbed by human activity.1,2 Methylmercury (MeHg) is the only Hg species
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to biomagnify in aquatic food-webs and accumulate at levels that are a widespread concern for
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human and ecological health.3,4 Dissolved organic matter (DOM) binds strongly with both Hg
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and MeHg in natural ecosystems and affects bioavailability.5 Freshwater systems exhibit much
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wider gradients in DOM concentrations (1700 µM)6 than marine systems (34-80 µM).7
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Molecular structure and primary sources marine DOM are much more diverse than in terrestrial
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systems.8–10 Here we investigate how differences in marine DOM composition affect Hg
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reactivity and biological uptake of MeHg in estuarine and continental shelf waters.
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In aquatic ecosystems, divalent inorganic mercury (HgII) can be converted to MeHg by a
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variety of microbial claves or reduced to elemental mercury (Hg0) through a combination of
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photolytic and biological reactions.11–14 Strong binding of HgII to DOM has been hypothesized to
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affect the reactive pool available for both net methylation and net reduction.8,9,15,16 DOM may
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also enhance the activity of methylating microbes in oligotrophic ecosystems by providing a
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substrate for bacterial activity, while under eutrophic conditions impacts on binding to HgII may
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be more important.17,18–20 Previous experimental and field studies report mixed effects of DOM
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on biological uptake of MeHg.21–24 We hypothesize here that variability in DOM composition
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helps to explain these results.
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Excitation emission matrix (EEM) fluorescence spectroscopy is widely used to
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characterize differences in the composition of DOM. These data can be used to identify DOM
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that originates from soils, rivers and marine productivity according to their characteristic
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fluorescence intensities and differences in the wavelength maxima.25 Parallel Factor Analysis
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(PARAFAC) has been widely used to further separate DOM into chemically distinct
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components, based on spectra of known compounds or organic matter structure, that are difficult
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to distinguish in the EEMs.26 Here we use such an analysis to distinguish broadly between
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sources with a terrestrial and marine origin in order to minimize over interpretation of the
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individual PARAFAC components.27–29
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The main objective of this work is to better understand how DOM composition affects
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Hg reactivity and MeHg bioavailability in marine ecosystems. To do this, we collected data on
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Hg and DOM concentrations and composition in filtered seawater from two estuarine stations in
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Long Island Sound (LIS), and along a gradient of terrestrial and marine influenced locations of
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the Northwest Atlantic continental margin (abbreviated as NWA from hereon). We compare
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identified components of DOM using EEM and PARAFAC analysis to Hg concentrations
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measured at each site and uptake of MeHg by plankton. We complement these data with
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laboratory experiments that further investigate how DOM composition affects Hg binding using
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a titration experiment and biological uptake using a bioreporter.
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Methods
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Field sampling
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Figure 1 shows seawater sampling locations for DOM and Hg species in LIS and the
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NWA, which includes the Gulf of Maine, shelf and slope. We collected Long Island Sound
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seawater using acid-washed, Teflon-lined General Oceanics GO-FLO sampling bottles and
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Teflon coated messengers deployed on a Kevlar line following trace-metal clean protocols.30
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LIS samples were obtained from two sites on three cruises: (1) 6-7 August 2009, (2) 09 November-14 December 2009, and (3) 24-27 May 2010 (see Supporting Information (SI) Table
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S1 for further details of sampling locations). Samples from the NWA were collected between
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17-26 August 2008 and 29 September-7 October 2009. Total Hg, MeHg and dissolved gaseous
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mercury (DGM) concentrations in seawater and MeHg in plankton were measured at stations
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shown in Figure 1. We use particulate MeHg measured in LIS as a proxy for concentrations in
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microseston (> 0.2 µm).31
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We measured dissolved organic carbon (DOC) concentrations and DOM composition for
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all LIS cruises and at stations shown in Figure 1 on the NWA cruise in 2008 (Table S2). We
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measured Hg species concentrations for LIS across seasons. For the NWA, we used 2008-2009
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data from Soerensen et al.8 on total Hg and DGM and 2008-2009 data from Hammerschmidt et
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al.31 on MeHg in seawater and MeHg in plankton.
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Total mercury and methylmercury analyses
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All LIS seawater for Hg species analysis was transferred into acid washed FEP bottles in
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the field prior to filtration, and stored double-bagged inside iced coolers. We filtered samples in
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a laminar flow hood
