Final Report: Microbiology of Mercury Methylation in Sediments

EPA Grant Number: R825433C015
Subproject: this is subproject number 015 , established and managed by the Center Director under grant R825433
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: EERC - Center for Ecological Health Research (Cal Davis)
Center Director: Rolston, Dennis E.
Title: Microbiology of Mercury Methylation in Sediments
Investigators: Nelson, Douglas
Institution: University of California - Davis
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1996 through September 30, 2000
RFA: Exploratory Environmental Research Centers (1992) RFA Text |  Recipients Lists
Research Category: Center for Ecological Health Research , Targeted Research


The objectives of this research project were to: (1) explore the relationships between the microbial methylation of mercury and the reduction of sulfate in Clear Lake sediments, thereby elucidating whether sulfate-reducing bacteria are the dominant producers of this toxic compound; (2) establish linkages between acid-rock drainage, fluxes of inorganic mercury to the lake, and production of methylmercury in lake sediments near the Sulphur Bank Mercury Mine; and (3) determine whether bacteria other than sulfate-reducing bacteria are capable of mercury methylation in Clear Lake sediments.

At the start of this project, the microbial literature almost dogmatically held that sulfate-reducing bacteria are the dominant agents catalyzing the conversion of divalent inorganic mercury into mono-methylmercury in both estuarine and freshwater sediments, where the great majority of environmental methylation occurs. Methylmercury becomes biomagnified at higher trophic levels in mercury-contaminated ecosystems, and thereby may reach levels at which its neurotoxic properties impact top carnivores, including humans. Sulfate-reducing bacteria are expected to dominate anaerobic microbial food chains in sulfate-impacted sediments. For this research project, conditions favoring sulfate-reducing bacteria are expected to occur in the immediate vicinity of the Sulphur Bank Mine (Clear Lake, CA) as a result of the highly localized concentrations of sulfate ions derived from acid-rock drainage. The cooccurrence of high concentrations of inorganic mercury at these same sites gave rise to an initial hypothesis that, if sulfate-reducing bacteria are the major agents of mercury methylation in general, the region of the lake nearest the mine should exhibit intense production of this toxic mercury compound.

Summary/Accomplishments (Outputs/Outcomes):

Assays of Sulfate Reduction and Mercury Methylation in Clear Lake Sediments

Clear Lake is a U.S. Environmental Protection Agency Superfund site because of mercury biomagnification in its food chain, and it has been estimated from recent core analyses that as much as a metric ton of fresh mercury enters the lake sediments each year. We used radiotracer techniques (35S-sulfate) in laboratory core-tube incubations to identify the most active sediment strata and net rates of bacterial sulfate reduction therein. These assays were performed at four stations in the lake throughout 1.5 annual cycles. The resultant data provide the most complete set available to date that allow correlation analyses of these two microbial processes in anoxic lake sediments. Throughout an annual cycle, it appears that sulfate reduction and mercury methylation are very strongly correlated (r2 > 0.9) at our Lower Arm Station; more weakly, but still significantly, positively correlated at our Oaks Arm and Upper Arm Stations; and uncorrelated at the Narrows Station. Although intriguing, these correlations do not prove causal linkage because many microbial processes wax and wane in synchrony seasonally because of variation in temperature and organic input. Our data also showed that detected methylation potential rates were not well correlated with ambient sediment levels of total mercury, but were extremely sensitive to, and positively correlated with, concentrations of added inorganic mercury. This last observation may explain the role of mine site-derived floc in stimulating the evolution of methylmercury from Clear Lake sediments.

Comparing integrated rates of sulfate reduction with the total sulfate content of Clear Lake yields additional insights. Given an average water column sulfate concentration, the volume of the lake, the mean residence time of water in the lake, and annually integrated sulfate reduction rates, we estimated an apparent annual deficit of 6.4 x 108 moles of sulfate lost to reduction and not regenerated by influent streams. However, there is a poorly constrained flow of acid mine drainage to Clear Lake from the Sulphur Bank Mine that can be estimated to be 1.2 X 104 L/minute (4,000 gpm) if one assumes that it is the source of this additional sulfate. Caveats for this estimate are provided in the full report.

Bacteria Other Than Sulfate Reducers are Dominant Agents of Methylation at Alkaline Sediment Sites in Clear Lake

To obtain a more precise picture of the microbial agents of methylation, we used molybdate anions, which are a structural analog of sulfate, as a specific inhibitor of sulfate reduction. The surprising conclusion from those studies is that anaerobic microbial processes other than sulfate reduction appear to be responsible for 50 to 75 percent of the methylation in Clear Lake sediments. We are in the early stages of identifying and characterizing the specific microbes responsible for this portion of mercury methylation.

Early in the project, studies with structured sediment microcosms revealed that sediment from the Oaks Arm "floc" site (OA-F) site emitted methylmercury to the overlying water at rates roughly 20-fold higher than sediments from the other test sites in the lake. More recently, we established that this "hot spot" for methylation is directly impacted by acid-rock drainage originating near the Sulphur Bank Mine. Extensive surveys of sediment cores indicated that the characteristics of the OA-F sediment porewaters are pH < 4.0 and [SO4=] = 20-80 mM, but the bulk of Clear Lake sediments are characterized by near-surface porewaters of slightly alkaline pH and sulfate concentrations of approximately 0.1 mM. At site OA-F, the concentrations of sulfate and protons increase with increasing sediment depth, which gives additional support for diffusive or advective flow of acid mine drainage into the lake at these sites. Additional mapping to determine the full extent of these "hot spot footprints" would be extremely useful in estimating the net annual flows of sulfate and mercury from the Sulphur Bank Mine into Clear Lake.

Recently, sulfate-reducing bacteria and iron-reducing bacteria have been isolated from site OA-F and another hot spot in mine-impacted sediments of Clear Lake. We are in the process of characterizing them for mercury methylation potential and acid tolerance, but already have preliminary evidence that some of the iron reducers are capable of significant methylation of divalent mercury. This would be a novel finding for this metabolic group, and would be a good start toward identifying the "missing" methylators in Clear Lake sediments.

The following activities were accomplished:

• We have compiled the most complete data set we know of comparison of rates of mercury methylation and sulfate reduction in anoxic lake sediments over a full annual cycle at multiple sites in a lake. Throughout an annual cycle, the processes are strongly correlated at three of the four stations sampled. However, these sorts of correlation analyses do not prove that sulfate reducers are performing the bulk of mercury methylation in Clear Lake sediments; other types of studies were used to discern the relationships between sulfate reducers and the bulk of mercury methylation.

• We found that during sulfate reduction assays, the addition of excess divalent cations caused apparent rates of sulfate reduction to be considerably higher than in parallel incubations free of this addition. We believe that this highlights the previously unappreciated extent of reoxidation of sulfide in incubations of sediments lacking these "sulfide trapping" cations, and provides a potentially useful new method for measuring the gross rate of sulfate reduction as well as the net rate in sediment-water slurries. To date, the traditional use of radiotracer sulfate yields a net rate of reduction because any regeneration of sulfate during the course of the incubation (e.g., via the oxidation of the resultant sulfide) does not contribute to the final calculation of sulfate reduction.

• For Clear Lake sediments, we have shown that mercury methylation potential is extremely sensitive to modest levels of added inorganic divalent mercury, but the rates of this process are not closely correlated with ambient sediment levels of total mercury, even if they are high. These findings have strong implications for remedial strategies near the mine site because continued addition of fresh mercury seemingly constitutes a greater threat than the presence of "aged" mercury that his been in lake sediments for longer periods of time.

• From our annually integrated sulfate reduction rates extrapolated throughout Clear Lake sediments, we estimated that this process is consuming sulfate equal to 4.6 times the entire content of the lake. Mean residence time of water in the lake suggests a large apparent annual deficit of sulfate. If the “missing” sulfate actually entered via acid-rock drainage from the Sulphur Bank Mine and vicinity, mass balance yields a rough estimate of water flow from these sources as 1.2 X 104 L/minute (4,000 gpm). Knowledge of the annual rate of water flow from the Herman Pit to Clear Lake is essential for a rational design of site remediation; however, the range of estimates produced by others using several different approaches is large, with the higher flow rates being extremely controversial. Our mass balance approach provides an independent estimate that matches well with some of the highest estimates made by other Center researchers (Project R825433C017).

• Studies using radiotracers, determinations of changes in methylmercury concentration, and measurements of net rates of sulfate reduction indicate that anaerobic microbial processes other than sulfate reduction appear to be responsible for 50 to 75 percent of the mercury methylation in Clear Lake sediments. This is in strong contrast to the literature dogma that sulfate reducers are dominant methylators in freshwater and estuarine sediments, and potentially may have widespread implications for mercury research and abatement in Clear Lake. We are in the process of identifying the other methylating bacteria in our system.

• Structured sediment microcosm (core tube) studies revealed that sediment from the OA-F site emitted methylmercury to the overlying water at rates roughly 20-fold higher than the slightly alkaline sediments from the other stations in the lake. This finding demonstrates a linkage between the Sulphur Bank Mine and this important microbial process. The OA-F site is about 30 m offshore from a portion of the waste rock pile of the Sulphur Bank Mine, and we now have established that this hot spot for methylation is directly impacted by acid-rock drainage that originates near the mine. Whether the driver of this enhanced methylation is the increased solubility of mercury at acidic pH, the presence of novel microbes or the abundance of sulfate at this site has not yet been determined. This finding will help target mitigation efforts more specifically to the most prominent mercury methylation sources.

Supplemental Keywords:

ecosystem, ecosystem protection, environmental exposure and risk, geographic area, international cooperation, water, terrestrial ecosystems, aquatic ecosystem, aquatic ecosystem restoration, aquatic ecosystems and estuarine research, biochemistry, ecological effects, ecological indicators, ecological monitoring, ecology and ecosystems, environmental chemistry, restoration, state, water and watershed, watershed, watershed development, watershed land use, watershed management, watershed modeling, watershed restoration, watershed sustainability, agricultural watershed, exploratory research environmental biology, California, CA, Clear Lake, Lake Tahoe, anthropogenic effects, aquatic habitat, biogeochemical cycling, ecological assessment, ecology assessment models, ecosystem monitoring, ecosystem response, ecosystem stress, environmental stress, environmental stress indicators, fish habitat, hydrologic modeling, hydrology, integrated watershed model, lake ecosystems, lakes, land use, nutrient dynamics, nutrient flux, water management options, water quality, wetlands., RFA, Scientific Discipline, INTERNATIONAL COOPERATION, Waste, Water, TREATMENT/CONTROL, ECOSYSTEMS, Ecosystem Protection/Environmental Exposure & Risk, POLLUTANTS/TOXICS, Aquatic Ecosystems & Estuarine Research, Water & Watershed, Contaminated Sediments, mercury transport, Treatment Technologies, Restoration, Aquatic Ecosystem, Environmental Microbiology, Biochemistry, Terrestrial Ecosystems, Bioremediation, Ecology and Ecosystems, Soil Contaminants, Aquatic Ecosystem Restoration, Mercury, Watersheds, anthropogenic stress, mercury uptake, contaminant exposure, biodiversity, microbial degradation, watershed management, contaminated marine sediment, aqueous mercury, Clear Lake watershed, acid mine drainage, agricultural watershed, contaminated sediment, restoration strategies, Clear Lake, fish-eating birds, marine biogeochemistry, contaminants in soil, methylmercury, bioremediation of soils, integrated watershed model, aquatic ecosystems, contaminated groundwater, environmental stress, mercury methylation, ecological impact, ecosystem stress, mercury chemistry, ecological research, lake ecosystems, watershed restoration

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Progress and Final Reports:

Original Abstract
  • 1997
  • 1998
  • 1999

  • Main Center Abstract and Reports:

    R825433    EERC - Center for Ecological Health Research (Cal Davis)

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
    R825433C001 Potential for Long-Term Degradation of Wetland Water Quality Due to Natural Discharge of Polluted Groundwater
    R825433C002 Sacramento River Watershed
    R825433C003 Endocrine Disruption in Fish and Birds
    R825433C004 Biomarkers of Exposure and Deleterious Effect: A Laboratory and Field Investigation
    R825433C005 Fish Developmental Toxicity/Recruitment
    R825433C006 Resolving Multiple Stressors by Biochemical Indicator Patterns and their Linkages to Adverse Effects on Benthic Invertebrate Patterns
    R825433C007 Environmental Chemistry of Bioavailability in Sediments and Water Column
    R825433C008 Reproduction of Birds and mammals in a terrestrial-aquatic interface
    R825433C009 Modeling Ecosystems Under Combined Stress
    R825433C010 Mercury Uptake by Fish
    R825433C011 Clear Lake Watershed
    R825433C012 The Role of Fishes as Transporters of Mercury
    R825433C013 Wetlands Restoration
    R825433C014 Wildlife Bioaccumulation and Effects
    R825433C015 Microbiology of Mercury Methylation in Sediments
    R825433C016 Hg and Fe Biogeochemistry
    R825433C017 Water Motions and Material Transport
    R825433C018 Economic Impacts of Multiple Stresses
    R825433C019 The History of Anthropogenic Effects
    R825433C020 Wetland Restoration
    R825433C021 Sierra Nevada Watershed Project
    R825433C022 Regional Transport of Air Pollutants and Exposure of Sierra Nevada Forests to Ozone
    R825433C023 Biomarkers of Ozone Damage to Sierra Nevada Vegetation
    R825433C024 Effects of Air Pollution on Water Quality: Emission of MTBE and Other Pollutants From Motorized Watercraft
    R825433C025 Regional Movement of Toxics
    R825433C026 Effect of Photochemical Reactions in Fog Drops and Aerosol Particles on the Fate of Atmospheric Chemicals in the Central Valley
    R825433C027 Source Load Modeling for Sediment in Mountainous Watersheds
    R825433C028 Stress of Increased Sediment Loading on Lake and Stream Function
    R825433C029 Watershed Response to Natural and Anthropogenic Stress: Lake Tahoe Nutrient Budget
    R825433C030 Mercury Distribution and Cycling in Sierra Nevada Waterbodies
    R825433C031 Pre-contact Forest Structure
    R825433C032 Identification and distribution of pest complexes in relation to late seral/old growth forest structure in the Lake Tahoe watershed
    R825433C033 Subalpine Marsh Plant Communities as Early Indicators of Ecosystem Stress
    R825433C034 Regional Hydrogeology and Contaminant Transport in a Sierra Nevada Ecosystem
    R825433C035 Border Rivers Watershed
    R825433C036 Toxicity Studies
    R825433C037 Watershed Assessment
    R825433C038 Microbiological Processes in Sediments
    R825433C039 Analytical and Biomarkers Core
    R825433C040 Organic Analysis
    R825433C041 Inorganic Analysis
    R825433C042 Immunoassay and Serum Markers
    R825433C043 Sensitive Biomarkers to Detect Biochemical Changes Indicating Multiple Stresses Including Chemically Induced Stresses
    R825433C044 Molecular, Cellular and Animal Biomarkers of Exposure and Effect
    R825433C045 Microbial Community Assays
    R825433C046 Cumulative and Integrative Biochemical Indicators
    R825433C047 Mercury and Iron Biogeochemistry
    R825433C048 Transport and Fate Core
    R825433C049 Role of Hydrogeologic Processes in Alpine Ecosystem Health
    R825433C050 Regional Hydrologic Modeling With Emphasis on Watershed-Scale Environmental Stresses
    R825433C051 Development of Pollutant Fate and Transport Models for Use in Terrestrial Ecosystem Exposure Assessment
    R825433C052 Pesticide Transport in Subsurface and Surface Water Systems
    R825433C053 Currents in Clear Lake
    R825433C054 Data Integration and Decision Support Core
    R825433C055 Spatial Patterns and Biodiversity
    R825433C056 Modeling Transport in Aquatic Systems
    R825433C057 Spatial and Temporal Trends in Water Quality
    R825433C058 Time Series Analysis and Modeling Ecological Risk
    R825433C059 WWW/Outreach
    R825433C060 Economic Effects of Multiple Stresses
    R825433C061 Effects of Nutrients on Algal Growth
    R825433C062 Nutrient Loading
    R825433C063 Subalpine Wetlands as Early Indicators of Ecosystem Stress
    R825433C064 Chlorinated Hydrocarbons
    R825433C065 Sierra Ozone Studies
    R825433C066 Assessment of Multiple Stresses on Soil Microbial Communities
    R825433C067 Terrestrial - Agriculture
    R825433C069 Molecular Epidemiology Core
    R825433C070 Serum Markers of Environmental Stress
    R825433C071 Development of Sensitive Biomarkers Based on Chemically Induced Changes in Expressions of Oncogenes
    R825433C072 Molecular Monitoring of Microbial Populations
    R825433C073 Aquatic - Rivers and Estuaries
    R825433C074 Border Rivers - Toxicity Studies