Grantee Research Project Results
2001 Progress Report: The Redox Cycle of Mercury in Natural Waters
EPA Grant Number: R827915Title: The Redox Cycle of Mercury in Natural Waters
Investigators: Morel, Francois M.
Institution: Princeton University
EPA Project Officer: Packard, Benjamin H
Project Period: October 11, 1999 through October 10, 2002 (Extended to October 10, 2003)
Project Period Covered by this Report: October 11, 2001 through October 10, 2002
Project Amount: $726,318
RFA: Mercury: Transport and Fate through a Watershed (1999) RFA Text | Recipients Lists
Research Category: Watersheds , Heavy Metal Contamination of Soil/Water , Water , Safer Chemicals
Objective:
The main objective of this project is to elucidate the parameters that control the flux of elemental mercury from natural waters to the atmosphere. The experimental plan consists of a series of iterative laboratory and field experiments focusing on the principal, chemical, and biological mechanisms that transform mercury between its dissolved ionic form, Hg(II), and its volatile elemental form, Hg(0) as in: (1) the biological reduction of Hg(II) to Hg(0) by transmembrane metal reductases in photosynthetic microorganisms; (2) the chemical reduction of Hg(II) by high energy reductants (typically formed in the light) such as the superoxide anion or an organic radical; and (3) the oxidation of elemental mercury Hg(0) to Hg(I), likely effected by radicals such as superoxide or semiquinones. In addition, at mid-point during the project, we expanded the scope of our work to investigate the biochemical mechanism of Hg(II) methylation in sulfate-reducing bacteria (which now appears to be the key factor controlling the concentration of methylmercury in natural waters).
Progress Summary:
Photooxidation of Hg(0)
We have conducted a series of laboratory and field experiments to investigate the factors influencing photooxidation of Hg(0), the least understood of the redox processes affecting the release of Hg(0) from water bodies. The experiments were conducted, in part, with natural water samples covering a range of salinity, because we have shown the effect of chloride on photooxidation (Lalonde, et al. Environmental Science and Technology 2001; 35(7):1367-1372). We established that, in brackish waters:
- Hg(0) photooxidation follows a pseudo first-order kinetics and oxidation rates are higher in saline waters than in freshwaters.
- Photooxidation may largely outweigh volatilization as a loss mechanism of Hg(0) in the top meter.
- Photooxidation is mainly induced by UV-A radiation.
- In addition to semiquinones, OH radical may oxidize Hg, but O2 does not seem to do so.
- Photooxidation is mainly abiotic.
Bioreduction and Photoreduction of Hg(II)
In addition, we took part in a major international research initiative called Mercury Experiment To Assess Atmospheric Loading In Canada and the United States (METAALICUS). The METAALICUS project aims to clearly establish the link between atmospheric deposition of mercury and mercury concentrations in fish. Our focus within this initiative is to elucidate the role of redox processes on Hg fluxes at the air/water interface.
As part of this initiative, we conducted an enclosure experiment in Lake 239 at the Experimental Lakes Area. Our specific goal was to follow redox transformations of added 200 HgCl2. Dissolved gaseous mercury (DGM, mainly Hg(0)) concentrations reached very high surface levels (up to 6 ng/L) during the days following the spike (total Hg 20 ng/L), leading to important losses to the atmosphere. These losses may explain the decrease in total Hg observed in the enclosures over time. Photochemical experiments using enclosure waters revealed that photoreduction rates were high after spiking (1 ng/L/h) and decreased during the summer, with low rates observed in August (0.01 ng/L/h). These low rates may be caused by photobleaching of dissolved organic carbon (DOC) in the enclosures. These results indicate that newly deposited Hg may be efficiently recycled back to the atmosphere under certain conditions.
As part of the METAALICUS project, we also participated in the whole ecosystem experiment. Three stable isotopes were added to a lake, its wetland and its upland, and their transport and fate were followed. We monitored the temporal and spatial distributions of DGM, the concentration of which influences air/water Hg exchange. In August and September, DGM concentrations were the highest near the surface, then decreased in the epilimnion and in the higher part of the metalimnion. In both months, an important peak was observed in the bottom of the metalimnion located at 5.5 meters, just below the 1 percent light cut-off. DGM concentrations also increased above the sediments. Mid-day increases in DGM levels at the surface and at 5.5 meters likely are partly photomediated, because these peaks greatly decreased at night. The metalimnetic and hypolimnetic increases are poorly explained solely by photochemical processes. Photosynthetical activity of organisms may be invoked in the metalimnetic peaks of DGM and the microbial activity of heterotrophs bacteria might be involved just above the sediments.
Hg(II) Methylation
Previous research on the biochemistry of Hg methylation by sulfate-reducing bacteria (SRB) using the strain D. desulfovibrio LS has implicated the acetyl-CoA pathway as the route of MeHg production. However, it is not clear whether this is the sole or even primary metabolic process responsible for Hg methylation. The purpose of this part of our work is to ascertain whether the acetyl-CoA pathway is an exclusive route for MeHg production in SRB, and if the occurrence of this pathway controls Hg-methylation in aquatic sediments. To explore these questions, we have set out to: (1) demonstrate a universal presence of the acetyl-CoA pathway in SRB capable of Hg methylation, and (2) establish a link between the acetyl-CoA pathway and MeHg production in aquatic sediments.
Our initial goal has been to determine whether this pathway is of primary importance across Hg-methylating SRB species. Ten strains of SRB from several genera, representing both the complete and incomplete acetate-oxidizing groups, have been screened for the existence of the acetyl-CoA pathway via the presence of carbon monoxide dehydrogenase (CODH) and formate dehydrogenase (FDH), two enzymes that are considered to be diagnostic of the pathway. These strains also were tested for the ability to methylate Hg in vivo. Our results show that there is a link between the ability to methylate Hg and the presence of the acetyl-CoA pathway in Desulfovibrio species and in several complete-oxidizing SRB. However, Desulfobulbus species methylate mercury without the presence of the acetyl-CoA pathway. This study is the first step in understanding the complex biochemistry of mercury methylation in SRB.
Analytical Advances
One of the main obstacles in redox studies of Hg done in the field comes from the limitation of current analytical methods. These methods typically involve long delays (about 0.5-1 hours) between collection and analysis, which may limit our ability to detect fast redox processes. Therefore, we designed and tested a prototype instrumentation for the high temporal resolution (every 5 minutes) in situ analysis of elemental Hg. This system is automated and has a detection limit of 8.5 pg L-1. By using solar radiation as a predictor of DGM concentrations in aquatic systems, we demonstrated the fast response time of the analytical system to fluctuations in ambient DGM at sunrise.
Future Activities:
During the final year of the project, we will continue our promising work on Hg redox transformations, both in the laboratory and in the field. To contrast the results at low latitudes to those already obtained at high latitudes, we will perform experiments in the Rio Negro (high DOC) and the Amazon (low DOC), and at the confluence of these two Brazilian large rivers. The aim of this study will be to establish a connection between physical and chemical properties of DOC and redox processes affecting Hg. Also, as part of COMERN-related projects, we will establish an empirical model predicting the potential for Hg(II) reduction and evasion of boreal lakes, and be involved in determining the role of macrophytes as potential Hg reducers.
For our methylation work, we have identified two promising specific inhibitors for investigating the metabolic pathway(s) responsible for MeHg production in the natural microbial populations of aquatic sediments. Chloroform has been reported to inhibit the acetyl Co-A pathway, and fluoroacetate has been reported to inhibit acetate metabolism via the TCA cycle. However, these inhibitors had not been explicitly tested on SRB. Using our pure strains, we have verified the effectiveness and specificity of these inhibitors. To determine the importance of the acetyl-CoA pathway for Hg methylation in freshwater and estuarine sediments, we will examine the effect of these specific inhibitors in intact sediment cores collected in the Experimental Lakes Area (Canada) and Long Island Sound (USA). These studies will better characterize the population structure of SRB that are responsible for Hg methylation.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
Other project views: | All 20 publications | 7 publications in selected types | All 7 journal articles |
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Type | Citation | ||
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Amyot M, Lean DRS, Poissant L, Doyon MR. Distribution and transformation of elemental mercury in the St. Lawrence River and Lake Ontario. Canadian Journal of Fisheries and Aquatic Sciences 2000;57:(Suppl. 1)155-163. |
R827915 (2001) |
not available |
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Amyot M, Auclair JC, Poissant L. In situ high temporal resolution analysis of elemental mercury in natural water. Analytica Chimica Acta 2001;447(1-2):153-159. |
R827915 (2001) R827915 (Final) |
not available |
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Jay J, Morel FMM, Hemond HF. Mercury speciation in the presence of polysulfides. Environmental Science and Technology 2000;34:2196-2200. |
R827915 (1999) R827915 (2001) R824778 (Final) R827634 (Final) |
not available |
Supplemental Keywords:
photooxidation, photoreduction, volatilization, methylation, mercury, water, Scientific Discipline, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Hydrology, Environmental Chemistry, Fate & Transport, Microbiology, Mercury, fate and transport, photosynthetic microorganisms, emissions, phytoplankton, biogeochemical cycling, methylmercury, enzymatic reduction, atmospheric deposition, mercury vaporRelevant Websites:
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.