Understanding the Role of Sulfur in the Production and Fate of Methylmercury in Watersheds

EPA Grant Number: R827653
Title: Understanding the Role of Sulfur in the Production and Fate of Methylmercury in Watersheds
Investigators: Mason, Robert P. , Gilmour, Cynthia C.
Institution: Chesapeake Biological Laboratory , Academy of Natural Sciences
EPA Project Officer: Hiscock, Michael
Project Period: October 1, 1999 through September 30, 2002 (Extended to October 1, 2003)
Project Amount: $779,786
RFA: Mercury: Transport and Fate through a Watershed (1999) RFA Text |  Recipients Lists
Research Category: Water and Watersheds , Mercury , Water , Safer Chemicals


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. We hypothesize that the decreased methylation of Hg in high sulfide environments results from changes in Hg bioavailability to the methylating organisms while low production in sulfatelimited systems is driven by limitation of microbial activity. We will therefore investigate the influence of sulfide, and other parameters, and the relative importance of microbial community structure and activity to net MMHg production in natural sediments and soils. As demethylation plays an important role in MMHg biogeochemistry, we will investigate the controls over biotic demethylation using bacterial cultures and natural sediments, and over the abiotic decomposition of MMHg in natural waters. We will examine the hypothesis that biotic demethylation ability is widespread among sulfate-reducing bacteria and is proportional to overall sulfate-reducing activity, while methylation is more strongly related to sulfide concentrations, and bacterial community composition. Finally, as methylation principally occurs in sediments, we will probe the factors controlling the efflux of MMHg from sediments, using sediment flux chambers and sediment core incubations. We propose that the redox status of the sediments is the most important control on the diffusive flux of MMHg from sediments to the water column. Overall, these combined activities will lead to an better understanding of the factors controlling the formation, fate and transport of MMHg in watersheds.


To achieve our objectives we will investigate how Hg complexation with sulfide effects bioavailability of Hg to methylating microorganisms, using bacterial cultures, and natural sediments and soils, combining laboratory, field and mesocosm experiments. We will also make use of engineered microorganisms (so-called Bioreporters) to test our hypotheses about the factors controlling Hg uptake by methylating bacteria. In the culture studies, the medium will be controlled so that the chemical speciation of Hg can be estimated using chemical equilibrium modeling. These studies will be designed to specifically address the hypotheses related to Hg bioavailability controls on methylation and to examine the role of microbial community structure and the responsible methylating organisms, and the factors promoting methylation and/or demethylation. Additional studies will be used to assess the importance of ancillary variables such as organic and iron content, in influencing MMHg formation and fate in sediments, and how these factors effect exchange between the sediment and the water column. We will use appropriate spike additions of both stable and radio isotopes of Hg to media to test our hypotheses about the relative importance of methylation versus demethylation in controlling net MMHg formation in the environment.

Expected Results:

This study will provide the information needed to build a predictive understanding of the factors controlling the formation, degradation, fate and transport of MMHg in watersheds. This information provides the crucial link for our understanding of the relationship between atmospheric deposition of Hg to watersheds and Hg bioaccumulation in piscivorous fish. The results of the experiments proposed here will provide significant new information for computer models needed by managers and regulators so that they may assess the impact of remediation strategies on the Hg content o fish across watersheds. When combined with models of bioaccumulation, this new information will help answer the question posed in the STAR Announcement - For a given amount of mercury transported into a watershed, what is the predicted concentration of MMHg in fish?

Publications and Presentations:

Publications have been submitted on this project: View all 20 publications for this project

Journal Articles:

Journal Articles have been submitted on this project: View all 5 journal articles for this project

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 aspects

Relevant 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:

  • 2000 Progress Report
  • 2001 Progress Report
  • 2002
  • Final Report