Grantee Research Project Results
Final 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 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 research project was to determine the parameters that control the flux of elemental mercury from natural waters to the atmosphere. The experimental plan consisted of a series of iterative laboratory and field experiments focused on the principal chemical and biological mechanisms that transform mercury between its dissolved ionic form, Hg(II), and its volatile elemental form, Hg(0): (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 affected by radicals such as superoxide or semiquinones. In addition, at midpoint 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) and the sources of methylmercury to marine, open-ocean fish.
Summary/Accomplishments (Outputs/Outcomes):
The ultimate question in mercury research is how biogeochemical processes and transformations help influence methylmercury exposure to humans and wildlife. We have responded to this challenge by studying three poorly understood processes critical in determining mercury levels in aquatic organisms. Primarily, we studied water column photooxidation, a mechanism that can cause an increased retention time for mercury in the waterbody, leading to an increased likelihood for it to be methylated. Our laboratory also focused on better understanding the physiology and biochemistry of mercury methylation in sulfate-reducing bacteria, the key organisms in freshwater and coastal systems that create the bioaccumulating neurotoxin, methylmercury. Finally, we have sought to better understand the possible sources of methylmercury to open ocean fish, the prime exposure route of mercury to humans.
The oxidation of volatile aqueous Hg(0) in aquatic systems may be important in reducing fluxes of mercury out of aquatic systems. Through laboratory and field experiments on St. Lawrence River water samples, we identified parameters (i.e., chloride concentration, semiquinone inclusion) that regulate the photooxidation of Hg(0). Elemental mercury oxidation was found to be mediated chiefly by ultraviolet (UV) radiation as: (1) “dark” oxidation was not found to be statistically significant; (2) visible light induced a significant but slow photooxidation (k=0.09h-1); and (3) visible plus UV radiation led to a faster photooxidation (k=0.6-0.7 h-1), mainly because of UVA induced reactions. Doubling UV radiation did not increase the reaction rate of Hg(0) photooxidation in natural water samples, indicating that some factor other than photon flux was rate limiting and suggesting that the reaction involves intermediate photo produced oxidant(s). The addition of methanol, a OH scavenger, decreased mercury photooxidation rates by 25 percent in brackish waters and by 19 percent in artificial saline water containing semiquinones, indicating that OH may be partially responsible for Hg(0) oxidation. Photooxidation rates were not affected by oxygen concentrations and did not decrease when samples were heat-sterilized, treated with chloroform, or filtered prior to exposure to light. In the St. Lawrence River, a typical photooxidation flux rate would be 300 pmole m-2 h-1, compared to volatilization flux of 7 pmole m-2 h-1. In coastal waters, the dominant Hg(0) sink is likely to be photooxidation rather than volatilization from the water column during summer days, even in periods of high winds.
Sulfate-reducing bacteria (SRB) in anoxic waters and sediments are the major organisms that transform inorganic mercury, which otherwise would be buried and removed from the watershed into the bioaccumulating neurotoxin, methylmercury. Although a considerable amount of work has addressed the environmental factors that control methylmercury formation and the conditions that control inorganic mercury bioavailability to SRB, little work has been undertaken analyzing the biochemical mechanism of methylmercury production. The acetyl-CoA pathway has been implicated as key to mercury methylation in one SRB strain, Desulfovibrio desulfuricans LS, but this result has not been extended to other SRB species. To probe whether the acetyl-CoA pathway is the controlling biochemical process for methylmercury production in SRB, five incomplete-oxidizing SRB strains and two Desulfobacter strains that do not use the acetyl-CoA pathway for major carbon metabolism were assayed for methylmercury formation and acetyl-CoA pathway enzyme activities. Three of the SRB strains also were incubated with chloroform to inhibit the acetyl-CoA pathway. All species that have been found to have acetyl-CoA activity, including complete oxidizers that require the acetyl-CoA pathway for basic metabolism, methylate mercury. We have identified, however, four incomplete-oxidizing strains that clearly do not utilize the acetyl-CoA pathway for mercury methylation. Mercury methylation is independent of the acetyl-CoA pathway and may not require vitamin B12 in some and perhaps many incomplete-oxidizing SRB strains.
Although the bulk of human exposure to mercury is through the consumption of marine fish, most of what we know about mercury methylation is from studies of freshwaters. We know little of where and how mercury is methylated in the open oceans, and there is currently a debate whether methylmercury concentrations in marine fish have increased along with global anthropogenic mercury emissions. Measurements of mercury concentrations in Yellowfin tuna caught off Hawaii in 1988 show no increase compared to measurements of the same species caught in the same area in 1971. On the basis of the known increase in the global emissions of mercury over the past century and of a simple model of mercury biogeochemistry in the equatorial and subtropical Pacific Ocean, we calculate that the methylmercury concentration in these surface waters should have increased between 9 and 26 percent over this 27 year span if methylation occurred in the mixed layer or in the thermocline. Such an increase is statistically inconsistent with the constant mercury concentrations measured in tuna. We conclude tentatively that mercury methylation in oceans occurs in deep waters or in sediments.
Journal Articles on this Report : 4 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, 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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Ekstrom EB, Morel FMM, Benoit JM. Mercury methylation independent of the acetyl-coenzyme A Pathway in sulfate-reducing bacteria. Applied and Environmental Microbiology 2003;69(9):5414-5422. |
R827915 (Final) |
not available |
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Kraepiel AML, Keller K, Chin HB, Malcolm EG, Morel FMM. Sources and variations of mercury in tuna. Environmental Science & Technology 2003;37(24):5551-5558. |
R827915 (Final) |
Exit Exit |
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Lalonde JD, Amyot M, Orvoine J, Morel FM, Auclair J-C, Ariya PA. Photoinduced oxidation of Hg0(aq) in the waters from the St. Lawrence estuary. Environmental Science & Technology 2004;38(2):508-514. |
R827915 (Final) |
Exit Exit |
Supplemental Keywords:
water, marine, estuary, watershed, fish, tuna, heavy metals, sulfate, microorganisms, environmental chemistry, biology, modeling,, 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:
http://geoweb.princeton.edu/research/biogeochem/biogeochem.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.