Researchers have long worked under the premise that sulfate-reducing bacteria are responsible for the production of methylmercury. Now, the first large-scale field experiment on the interactions of mercury and sulfate in wetlands, described in the June issue of Environmental Science & Technology, provides the most persuasive evidence yet that acid rain increases production of MeHg.
BackgroundAtmospheric mercury is the dominant Hg source to fish in northern Minnesota and elsewhere. Originating both from local sources such as power plants but also from long range transport (global pollution), atmospherically derived Hg must be methylated prior to accumulating in fish. Sulfate-reducing bacteria are thought to be the primary methylators of Hg in the environment. Previous laboratory and field mesocosm studies have demonstrated an increase in methylmercury (MeHg) levels in sediment and peatland porewaters after "feeding" the microbial community with sulfate.
The new studyWhile the “cause-and-effect relationship” between sulfur and mercury deposition from the atmosphere has been demonstrated before, "this is the first such large-scale ecosystem experiment" says Charles Driscoll of Syracuse University.
The group of researchers around Jeff Jeremiason of Gustavus Adolphus looked at an acre-sized patch of a wetland in Minnesota and prepared half of the site with sprinklers that could simulate rainfall with sulfate loads four times higher than current annual background levels but in the range of historic levels of sulfate deposition in the northeastern U.S..
The team measured sulfate and MeHg content of the wetland the days after having applied 6-h lasting sulfate “rainstorm” to the wetland first in May 2002, followed by 2 more “rains” in July and September of that year.
In comparison to a control section in an upstream and untouched part of the same wetland a jump in sulfate levels followed by a surge of MeHg after the first application was observed, just as theory predicts. The sulfur apparently stimulates certain microbes that transform other forms of mercury to MeHg as they respire. They could also track the concentrations of MeHg exported out of the swampy wetland, which increased threefold.
Then things got more complicated. Following the MeHg flux out of the marsh after the two applications later in the summer, the researchers could no longer observe additional increases in methylmercury production nor could they find evidence of the added sulfate in the marsh.
The group is somehow split on possible explanations for this unexpected observations including a possible change in marsh chemistry and biology with rising temperatures in the summer season. Other speculate that the experiment might have missed all the action by either delaying the measurements to long after application of the rain or by concentrating on the marsh water only.
Anyhow, this newest study complements observations elsewhere, such as the Everglades, where experiments also substantiate the link between sulfate and MeHg. The new study makes the connection though much clearer.
The original study
Jeff D. Jeremiason, Daniel R. Engstrom, Edward B. Swain, Edward A. Nater, Brian M. Johnson, James E. Almendinger, Bruce A. Monson, Randy K. Kolka,
Sulfate Addition Increases Methylmercury Production in an Experimental Wetland, Environ. Sci. Technol., 40/12 (2006) 3800-3806.
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DOI: 10.1128/AEM.66.6.2430-2437.2000 Jeffrey K. King, Joel E. Kostka, Marc E. Frischer, B.F. Michael Saunders, Richard A. Jahnke,
A Quantitative Relationship that Demonstrates Mercury Methylation Rates in Marine Sediments are Based on the Community Composition and Activity of Siulfate-reducing Bacteria, Environ. Sci. Technol., 35/12 (2001) 2491-2496.
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The Influence of Sulfide on Solid-Phase Mercury Bioavailability for Methylation by Pure Cultures of Desulfobulbus propionicus (1pr3), Environ. Sci. Technol., 35/1 (2001) 127-132.
DOI: 10.1021/es001415n J.A. Jay, K.J. Murray, C.C. Gilmour, R.P. Mason, F.M. Morel, A.L. Roberts, H.F. Hemond,
Mercury Methylation by Desulfovibrio desulfuricans ND132 in the Presence of Polysulfides, Appl. Environ. Microbiol., 68 (2002) 5741-5745.
DOI: 10.1128/AEM.68.11.5741-5745.2002 A.L. Bates, W.H. Orem, J.W. Harvew, E.C. Spiker,
Tracing Sources of Sulfur in the Florida Everglades, J. Environ. Qual., 31 (2002) 287-299.
DOI:
10.2134/jeq2002.2870
E.B. Ekstrom, F.M.M. Morel, J.M. Benoit,
Mercury methylation independent of the acetyl-coenzyme. A pathway in sulfate-reducing bacteria, Appl. Environ. Microbiol., 69 (2003) 5414-5422.
DOI: 10.1128/AEM.69.9.5414-5422.2003 D. Acha, V. Iniguez, M. Roulet, J.R.D. Guimaraes, R. Luna, I. Alanoca, S. Sanchez, Sulfate-Reducing Bacteria in Floating Macrophyte Rhizospheres from an Amazonian Floodplain Lake in Bolivia and Their Association with Hg Methylation, Appl. Environ. Microbiol., 71 (2005) 7531-7535 Andreas Drott, Lars Lambertsson,
Erik Björn, Ulf Skyllberg,
Importance of Dissolved Neutral Mercury Sulfides for Methyl Mercury Production in Contaminated Sediments, Environ. Sci. Technol., 41/7 (2007) 2270-2276.
DOI: 10.1021/es061724z
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