Grantee Research Project Results
2000 Progress Report: Understanding the Role of Sulfur in the Production and Fate of Methylmercury in Watersheds
EPA Grant Number: R827653Title: Understanding the Role of Sulfur in the Production and Fate of Methylmercury in Watersheds
Investigators: Mason, Robert P.
Current Investigators: Mason, Robert P. , Gilmour, Cynthia C.
Institution: University of Maryland Center for Environmental Science , Chesapeake Biological Laboratory
Current Institution: Chesapeake Biological Laboratory , Academy of Natural Sciences
EPA Project Officer: Chung, Serena
Project Period: October 1, 1999 through September 30, 2002 (Extended to October 1, 2003)
Project Period Covered by this Report: October 1, 1999 through September 30, 2000
Project Amount: $779,786
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 principal objective of this research is to understand the role of sulfur, especially in the form of sulfide, in mercury (Hg) methylation and methylmercury (MMHg) fate and transport in watersheds, including the factors controlling the efflux of MMHg from sediments and soils. In order to do so, the more specific objectives are to investigate and contrast Hg complexation with sulfide, as a control over bioavailability of Hg to methylating microorganisms, with the role of microbial community structure and microbial activity in net MMHg production using bacterial cultures, and natural sediments and soils. In addition, the project will quantify how MMHg production and accumulation will change for a given change in sulfur loading to ecosystems, using mesocosms. Furthermore, the controls over biotic demethylation using bacterial cultures and natural sediments, and over the abiotic decomposition of MMHg in natural waters will be investigated. A model of Hg and MMHg cycling in aquatic systems will be constructed incorporating the factors controlling bioavailability, methylation and MMHg fate and decomposition.
Progress Summary:
A large fraction of the work within the first year has been related to the question of how speciation in water and bioavailability from sediments controls Hg methylation. The results of studies of the binding of inorganic Hg with dissolved organic carbon isolates from the Florida Everglades have been submitted for publication and the work on the role of sulfide as a methylation control are detailed a number of manuscripts (see publications list). Our understanding of the role of sulfide in influencing Hg methylation has increased enormously. Further studies are looking at the potential interactions of Hg and MMHg with solid oxide phases and with solid sulfide phases. This work has begun but has not yet been completed. The interaction with the Canadian group on the "bioreporter" studies has begun. A student visited Cindy Gilmour's laboratory in the summer of 2000 and some preliminary experiments were performed that tended to confirm the hypothesis that Hg is taken up in the presence of chloride by passive diffusion of neutral complexes. More work is being done to transfer the bioreporter genes to a sulfide-tolerant organism so that the hypothesis of passive uptake of Hg-sulfide by bacteria can be tested using this technique. The developments are such that this should be possible in the near future.
We have modified existing procedures for the determination of the speciation (partitioning) of Hg to sediment phases and have done sequential extraction experiments with both Florida Everglades sediment and with lake sediment. In both cases, the organic matter appears, for the solid phase, to be an important locale for Hg sequestration, especially in the high organic content Everglades sediment. Sediments from the Experimental Lakes Area in Canada, which have less than 5% organic matter, have the Hg more evenly distributed between the organic (37%) reactive (25%) and pyrite (38%) fractions. Clearly, other phases increase in importance in high sulfide or low organic content sediments.
Abiotic demethylation studies in estuarine water suggest that MMHg is relatively stable to degradation. These results contrast those from oligotrophic lakes and from the Florida Everglades. It is hypothesized that the enhanced stability in low-DOC saline waters is due to the stability of the chloride complex relative to that of the hydroxide. Also, the presence of DOC will be important if there is enough present so that MMHg complexation to DOC dominates. Under these conditions, the DOC may enhance degradation by acting as a mediator for electron transfer. This notion is being perused.
Our sediment core incubation flux experiments tend to confirm the results of others that MMHg is mobile under conditions where the overlying waters become anoxic, or of extremely low oxygen. This supports our hypothesis that it is the oxic surface layer that acts as a barrier to MMHg transport and that complexation to oxide phases, or complexation to organic matter bound to oxide phases, is controlling the movement of MMHg at the sediment-water interface. Our complexation studies discussed above will provide the information necessary to study and model MMHg movement across a redox gradient and determine the conditions under which MMHg becomes mobile. These studies are on-going.
We are and will continue to use stable isotopes of Hg in these investigations and development of the techniques necessary to measure the low levels of Hg and the associated isotope ratios by ICP-MS has been an important part of the required analytical developments needed for this project to succeed. The method development required was not trivial and constant attention is required to avoid cross-contamination and other problems inherent with the use of stable isotopes. We now have two ICP-MS instruments that we can use in these studies, one at each laboratory of the PI's. Methods are now in place for the analysis of both Hg and MMHg.
Future Activities:
Activities in the next year will focus on completing the experiments looking at the role of DOC and solid oxide phases in Hg and MMHg complexation. Additionally, further studies of the binding of Hg to the solid phase in sediments and soils is planned for the summer of 2001. These experiments, along with further bioreporter studies will complete the first section of the proposal plan which focused on the role of complexation in modifying mercury methylation, and will to some degree complete the objectives under the section of the proposal looking at factors controlling the exchange of Hg and MMHg at the sediment-water interface.
One of the major activities for the next year is a focus on the role of microorganisms and microbial activity in modifying methylation rates in sediments. Currently it is not clearly known whether limitation of microbial processes by such factors as carbon supply limitation is more important in determining Hg methylation in sediments in contrast to changes in bioavailability as a result of changes in Hg speciation. We hope to screen a number of organisms for both their methylating and demethylating activity and will use stable isotope spike experiments to do this. These biotic experiments will be complimented with experiments looking further at the importance of abiotic demethylation processes.
The mesocosm dosing experiments and the field work related to looking at controls on methylation under natural conditions will begin in the summer of 2001 and continue to the end of the project.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
Other project views: | All 23 publications | 8 publications in selected types | All 7 journal articles |
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Benoit JM, Mason RP, Gilmour CC, Aiken GR. Constants for mercury binding by dissolved organic matter isolates from the Florida Everglades. Geochimica et Cosmochimica Acta 2001;65(24):4445-4451. |
R827653 (2000) R827653 (2001) R827653 (Final) R827631 (2000) R827631 (2001) R827631 (Final) |
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Benoit JM, Gilmour CC, Mason RP. The influence of sulfide on solid-phase mercury bioavailability for methylation by pure cultures of Desulfobulbus propionicus (1pr3). Environmental Science & Technology 2001;35(1):127-132. |
R827653 (2000) R827653 (2001) R827653 (Final) R827631 (2000) R827631 (2001) R827631 (Final) |
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Benoit JM, Gilmour CC, Mason RP. Aspects of bioavailability of mercury for methylation in pure cultures of Desulfobulbus propionicus (1pr3). Applied and Environmental Microbiology 2001;67(1):51-58. |
R827653 (2000) R827653 (2001) R827653 (Final) R827631 (2001) |
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Supplemental Keywords:
water, sediment, estuary, watersheds, coastal, bioavailability, mercury, methylmercury, sulfate, sulfide, Scientific Discipline, Water, Waste, Ecosystem Protection/Environmental Exposure & Risk, Hydrology, Bioavailability, Environmental Chemistry, Remediation, Contaminated Sediments, Fate & Transport, Air Deposition, Ecology and Ecosystems, Mercury, fate and transport, contaminated sediment, soils, sulfur, biogeochemical cycling, methylmercury, watershed influences, terrestrial and aquatic fate, sulfide, atmospheric deposition, microbiological aspectsRelevant Websites:
http://cbl.umces.edu/~mason/welcome.html Exit
http://www.anserc.org/index.html Exit
Synthesis Report of Research from EPA’s Science to Achieve Results (STAR) Grant Program: Mercury Transport and Fate Through a Watershed (PDF) (42 pp, 760 K)
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.