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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
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 29
Environmental Science & Technology
1
Contrasting effects of marine and terrestrially
2
derived dissolved organic matter on mercury
3
speciation and bioavailability in seawater
4 5 6
Amina T. Schartup1*, Udonna C. Ndu2, Prentiss H. Balcom3,4, Robert P. Mason3 and Elsie M.
7
Sunderland1,4
8
1
9
United States
Department of Environmental Health, Harvard School of Public Health, Boston MA 02215,
10
2
11
Brunswick NJ 08901, United States
12
Department of Environmental Science, Rutgers University, 14 College Farm Road, New
3
Department of Marine Sciences, 1080 Shennecossett Rd Groton CT 06340, United States
13 14
4School
15
*Corresponding author
16
[email protected]
of Engineering and Applied Sciences, Harvard University, Cambridge MA, 02138
17
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Environmental Science & Technology
Abstract
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Methylmercury (MeHg) is the only species of mercury (Hg) to biomagnify in aquatic
26
food-webs to levels that are a widespread concern for human and ecological health. Here we
27
investigate the association between dissolved organic matter (DOM) in seawater and Hg
28
speciation and uptake using experimental data and field measurements from Long Island Sound
29
(LIS) and the Northwestern Atlantic continental margin (NWA). We measured differences in
30
DOM composition across sampling stations using excitation emission matrix (EEM)
31
fluorescence spectroscopy and further separated DOM into terrestrial and marine components
32
using Parallel Factor Analysis (PARAFAC). Highest MeHg concentrations were found in the
33
estuarine stations (LIS) with highest DOM concentrations due to enhanced external inputs from
34
the watershed and rivers. For stations on the shelf and slope, MeHg in plankton increased
35
linearly with a decreasing fraction of fluorescence attributable to DOM components with a
36
terrestrial rather than marine origin. These results are corroborated by experimental data showing
37
higher MeHg uptake by cells in the presence of predominantly marine DOM compared to
38
terrestrial DOM. Highest fractions of dissolved gaseous mercury were also found at stations with
39
the highest marine DOM content, suggesting a greater reducible fraction of divalent inorganic
40
Hg. These data suggest DOM composition is a critical driver of Hg reactivity and bioavailability
41
in offshore marine waters.
42
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Introduction Mercury (Hg) is a naturally occurring heavy metal with a biogeochemical cycle that has
46
been substantially perturbed by human activity.1,2 Methylmercury (MeHg) is the only Hg species
47
to biomagnify in aquatic food-webs and accumulate at levels that are a widespread concern for
48
human and ecological health.3,4 Dissolved organic matter (DOM) binds strongly with both Hg
49
and MeHg in natural ecosystems and affects bioavailability.5 Freshwater systems exhibit much
50
wider gradients in DOM concentrations (1700 µM)6 than marine systems (34-80 µM).7
51
Molecular structure and primary sources marine DOM are much more diverse than in terrestrial
52
systems.8–10 Here we investigate how differences in marine DOM composition affect Hg
53
reactivity and biological uptake of MeHg in estuarine and continental shelf waters.
54
In aquatic ecosystems, divalent inorganic mercury (HgII) can be converted to MeHg by a
55
variety of microbial claves or reduced to elemental mercury (Hg0) through a combination of
56
photolytic and biological reactions.11–14 Strong binding of HgII to DOM has been hypothesized to
57
affect the reactive pool available for both net methylation and net reduction.8,9,15,16 DOM may
58
also enhance the activity of methylating microbes in oligotrophic ecosystems by providing a
59
substrate for bacterial activity, while under eutrophic conditions impacts on binding to HgII may
60
be more important.17,18–20 Previous experimental and field studies report mixed effects of DOM
61
on biological uptake of MeHg.21–24 We hypothesize here that variability in DOM composition
62
helps to explain these results.
63
Excitation emission matrix (EEM) fluorescence spectroscopy is widely used to
64
characterize differences in the composition of DOM. These data can be used to identify DOM
65
that originates from soils, rivers and marine productivity according to their characteristic
66
fluorescence intensities and differences in the wavelength maxima.25 Parallel Factor Analysis
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Environmental Science & Technology
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(PARAFAC) has been widely used to further separate DOM into chemically distinct
68
components, based on spectra of known compounds or organic matter structure, that are difficult
69
to distinguish in the EEMs.26 Here we use such an analysis to distinguish broadly between
70
sources with a terrestrial and marine origin in order to minimize over interpretation of the
71
individual PARAFAC components.27–29
72
The main objective of this work is to better understand how DOM composition affects
73
Hg reactivity and MeHg bioavailability in marine ecosystems. To do this, we collected data on
74
Hg and DOM concentrations and composition in filtered seawater from two estuarine stations in
75
Long Island Sound (LIS), and along a gradient of terrestrial and marine influenced locations of
76
the Northwest Atlantic continental margin (abbreviated as NWA from hereon). We compare
77
identified components of DOM using EEM and PARAFAC analysis to Hg concentrations
78
measured at each site and uptake of MeHg by plankton. We complement these data with
79
laboratory experiments that further investigate how DOM composition affects Hg binding using
80
a titration experiment and biological uptake using a bioreporter.
81 82
Methods
83
Field sampling
84
Figure 1 shows seawater sampling locations for DOM and Hg species in LIS and the
85
NWA, which includes the Gulf of Maine, shelf and slope. We collected Long Island Sound
86
seawater using acid-washed, Teflon-lined General Oceanics GO-FLO sampling bottles and
87
Teflon coated messengers deployed on a Kevlar line following trace-metal clean protocols.30
88 89
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
91
17-26 August 2008 and 29 September-7 October 2009. Total Hg, MeHg and dissolved gaseous
92
mercury (DGM) concentrations in seawater and MeHg in plankton were measured at stations
93
shown in Figure 1. We use particulate MeHg measured in LIS as a proxy for concentrations in
94
microseston (> 0.2 µm).31
95
We measured dissolved organic carbon (DOC) concentrations and DOM composition for
96
all LIS cruises and at stations shown in Figure 1 on the NWA cruise in 2008 (Table S2). We
97
measured Hg species concentrations for LIS across seasons. For the NWA, we used 2008-2009
98
data from Soerensen et al.8 on total Hg and DGM and 2008-2009 data from Hammerschmidt et
99
al.31 on MeHg in seawater and MeHg in plankton.
100
Page 6 of 29
Total mercury and methylmercury analyses
101
All LIS seawater for Hg species analysis was transferred into acid washed FEP bottles in
102
the field prior to filtration, and stored double-bagged inside iced coolers. We filtered samples in
103
a laminar flow hood
Article
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
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 29
Environmental Science & Technology
1
Contrasting effects of marine and terrestrially
2
derived dissolved organic matter on mercury
3
speciation and bioavailability in seawater
4 5 6
Amina T. Schartup1*, Udonna C. Ndu2, Prentiss H. Balcom3,4, Robert P. Mason3 and Elsie M.
7
Sunderland1,4
8
1
9
United States
Department of Environmental Health, Harvard School of Public Health, Boston MA 02215,
10
2
11
Brunswick NJ 08901, United States
12
Department of Environmental Science, Rutgers University, 14 College Farm Road, New
3
Department of Marine Sciences, 1080 Shennecossett Rd Groton CT 06340, United States
13 14
4School
15
*Corresponding author
16
[email protected]
of Engineering and Applied Sciences, Harvard University, Cambridge MA, 02138
17
ACS Paragon Plus Environment
1
Environmental Science & Technology
Page 2 of 29
18 19
20 21
TOC art
22
ACS Paragon Plus Environment
2
Page 3 of 29
23 24
Environmental Science & Technology
Abstract
25
Methylmercury (MeHg) is the only species of mercury (Hg) to biomagnify in aquatic
26
food-webs to levels that are a widespread concern for human and ecological health. Here we
27
investigate the association between dissolved organic matter (DOM) in seawater and Hg
28
speciation and uptake using experimental data and field measurements from Long Island Sound
29
(LIS) and the Northwestern Atlantic continental margin (NWA). We measured differences in
30
DOM composition across sampling stations using excitation emission matrix (EEM)
31
fluorescence spectroscopy and further separated DOM into terrestrial and marine components
32
using Parallel Factor Analysis (PARAFAC). Highest MeHg concentrations were found in the
33
estuarine stations (LIS) with highest DOM concentrations due to enhanced external inputs from
34
the watershed and rivers. For stations on the shelf and slope, MeHg in plankton increased
35
linearly with a decreasing fraction of fluorescence attributable to DOM components with a
36
terrestrial rather than marine origin. These results are corroborated by experimental data showing
37
higher MeHg uptake by cells in the presence of predominantly marine DOM compared to
38
terrestrial DOM. Highest fractions of dissolved gaseous mercury were also found at stations with
39
the highest marine DOM content, suggesting a greater reducible fraction of divalent inorganic
40
Hg. These data suggest DOM composition is a critical driver of Hg reactivity and bioavailability
41
in offshore marine waters.
42
ACS Paragon Plus Environment
3
Environmental Science & Technology
43 44 45
Page 4 of 29
Introduction Mercury (Hg) is a naturally occurring heavy metal with a biogeochemical cycle that has
46
been substantially perturbed by human activity.1,2 Methylmercury (MeHg) is the only Hg species
47
to biomagnify in aquatic food-webs and accumulate at levels that are a widespread concern for
48
human and ecological health.3,4 Dissolved organic matter (DOM) binds strongly with both Hg
49
and MeHg in natural ecosystems and affects bioavailability.5 Freshwater systems exhibit much
50
wider gradients in DOM concentrations (1700 µM)6 than marine systems (34-80 µM).7
51
Molecular structure and primary sources marine DOM are much more diverse than in terrestrial
52
systems.8–10 Here we investigate how differences in marine DOM composition affect Hg
53
reactivity and biological uptake of MeHg in estuarine and continental shelf waters.
54
In aquatic ecosystems, divalent inorganic mercury (HgII) can be converted to MeHg by a
55
variety of microbial claves or reduced to elemental mercury (Hg0) through a combination of
56
photolytic and biological reactions.11–14 Strong binding of HgII to DOM has been hypothesized to
57
affect the reactive pool available for both net methylation and net reduction.8,9,15,16 DOM may
58
also enhance the activity of methylating microbes in oligotrophic ecosystems by providing a
59
substrate for bacterial activity, while under eutrophic conditions impacts on binding to HgII may
60
be more important.17,18–20 Previous experimental and field studies report mixed effects of DOM
61
on biological uptake of MeHg.21–24 We hypothesize here that variability in DOM composition
62
helps to explain these results.
63
Excitation emission matrix (EEM) fluorescence spectroscopy is widely used to
64
characterize differences in the composition of DOM. These data can be used to identify DOM
65
that originates from soils, rivers and marine productivity according to their characteristic
66
fluorescence intensities and differences in the wavelength maxima.25 Parallel Factor Analysis
ACS Paragon Plus Environment
4
Page 5 of 29
Environmental Science & Technology
67
(PARAFAC) has been widely used to further separate DOM into chemically distinct
68
components, based on spectra of known compounds or organic matter structure, that are difficult
69
to distinguish in the EEMs.26 Here we use such an analysis to distinguish broadly between
70
sources with a terrestrial and marine origin in order to minimize over interpretation of the
71
individual PARAFAC components.27–29
72
The main objective of this work is to better understand how DOM composition affects
73
Hg reactivity and MeHg bioavailability in marine ecosystems. To do this, we collected data on
74
Hg and DOM concentrations and composition in filtered seawater from two estuarine stations in
75
Long Island Sound (LIS), and along a gradient of terrestrial and marine influenced locations of
76
the Northwest Atlantic continental margin (abbreviated as NWA from hereon). We compare
77
identified components of DOM using EEM and PARAFAC analysis to Hg concentrations
78
measured at each site and uptake of MeHg by plankton. We complement these data with
79
laboratory experiments that further investigate how DOM composition affects Hg binding using
80
a titration experiment and biological uptake using a bioreporter.
81 82
Methods
83
Field sampling
84
Figure 1 shows seawater sampling locations for DOM and Hg species in LIS and the
85
NWA, which includes the Gulf of Maine, shelf and slope. We collected Long Island Sound
86
seawater using acid-washed, Teflon-lined General Oceanics GO-FLO sampling bottles and
87
Teflon coated messengers deployed on a Kevlar line following trace-metal clean protocols.30
88 89
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
ACS Paragon Plus Environment
5
Environmental Science & Technology
90
S1 for further details of sampling locations). Samples from the NWA were collected between
91
17-26 August 2008 and 29 September-7 October 2009. Total Hg, MeHg and dissolved gaseous
92
mercury (DGM) concentrations in seawater and MeHg in plankton were measured at stations
93
shown in Figure 1. We use particulate MeHg measured in LIS as a proxy for concentrations in
94
microseston (> 0.2 µm).31
95
We measured dissolved organic carbon (DOC) concentrations and DOM composition for
96
all LIS cruises and at stations shown in Figure 1 on the NWA cruise in 2008 (Table S2). We
97
measured Hg species concentrations for LIS across seasons. For the NWA, we used 2008-2009
98
data from Soerensen et al.8 on total Hg and DGM and 2008-2009 data from Hammerschmidt et
99
al.31 on MeHg in seawater and MeHg in plankton.
100
Page 6 of 29
Total mercury and methylmercury analyses
101
All LIS seawater for Hg species analysis was transferred into acid washed FEP bottles in
102
the field prior to filtration, and stored double-bagged inside iced coolers. We filtered samples in
103
a laminar flow hood
