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Microbial mercury (Hg) methylation transforms a toxic trace metal into the highly bioaccumulated neurotoxin methylmercury (MeHg). The lack of a genetic marker for microbial MeHg production has prevented a clear understanding of Hg-methylating organism distribution in nature. Recently, a specific gene cluster (hgcAB) was linked to Hg methylation in two bacteria.1 Here we test if the presence of hgcAB orthologues is a reliable predictor of Hg methylation capability in microorganisms, a necessary confirmation for the development of molecular probes for Hg-methylation in nature. Although hgcAB orthologues are rare among all available microbial genomes, organisms are much more phylogenetically and environmentally diverse than previously thought. By directly measuring MeHg production in several bacterial and archaeal strains encoding hgcAB, we confirmed that possessing hgcAB predicts Hg methylation capability. For the first time, we demonstrated Hg methylation in a number of species other than sulfate- (SRB) and iron- (FeRB) reducing bacteria, including methanogens, and syntrophic, acetogenic, and fermentative Firmicutes. Several of these species occupy novel environmental niches for Hg methylation, including methanogenic habitats such as rice paddies, the animal gut, and extremes of pH and salinity. Identification of these organisms as Hg methylators now links methylation to discrete gene markers in microbial communities.

A global metagenome assessment reveals a low risk of methylmercury production in humans and a high potential in Arctic permafrost.

A chemical equilibrium model for Hg complexation in sediments with sulfidic pore waters is presented. The purpose of the model was to explain observed relationships between pore water sulfide, dissolved inorganic Hg (HgD), and bulk methylmercury (MeHg) in surficial sediments of two biogeochemically different ecosystems, the Florida Everglades and Patuxent River, MD. The model was constructed to test the hypothesis that the availability of Hg for methylation in sediments is a function of the concentration of neutral dissolved Hg complexes rather than Hg2+ or total HgD. The model included interaction of mercury with solids containing one or two sulfide groups, and it was able to reproduce observed HgD and bulk MeHg trends in the two ecosystems. The model is consistent with HgS0 as the dominant neutral Hg complex and the form of Hg accumulated by methylating bacteria in sulfidic pore waters. The model-estimated decline in HgS0 with increasing sulfide was consistent with the observed decline in bulk sediments MeHg. Since bacterial Hg uptake rate is one of the factors affecting methylation rate, Hg complexation models such as the one presented are helpful in understanding the factors that control MeHg production and accumulation in aquatic ecosystems.

Methylmercury contamination of fisheries from centuries of industrial atmospheric emissions negatively impacts humans and wildlife worldwide. The response of fish methylmercury concentrations to changes in mercury deposition has been difficult to establish because sediments/soils contain large pools of historical contamination, and many factors in addition to deposition affect fish mercury. To test directly the response of fish contamination to changing mercury deposition, we conducted a whole-ecosystem experiment, increasing the mercury load to a lake and its watershed by the addition of enriched stable mercury isotopes. The isotopes allowed us to distinguish between experimentally applied mercury and mercury already present in the ecosystem and to examine bioaccumulation of mercury deposited to different parts of the watershed. Fish methylmercury concentrations responded rapidly to changes in mercury deposition over the first 3 years of study. Essentially all of the increase in fish methylmercury concentrations came from mercury deposited directly to the lake surface. In contrast, <1% of the mercury isotope deposited to the watershed was exported to the lake. Steady state was not reached within 3 years. Lake mercury isotope concentrations were still rising in lake biota, and watershed mercury isotope exports to the lake were increasing slowly. Therefore, we predict that mercury emissions reductions will yield rapid (years) reductions in fish methylmercury concentrations and will yield concomitant reductions in risk. However, a full response will be delayed by the gradual export of mercury stored in watersheds. The rate of response will vary among lakes depending on the relative surface areas of water and watershed.bioaccumulation ͉ mercury methylation ͉ stable isotopes ͉ whole-ecosystem experimentation ͉ methylmercury

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We propose the use of Desulfovibrio desulfuricans ND132 as a model species for understanding the mechanism of microbial Hg methylation. Strain ND132 is an anaerobic dissimilatory sulfate-reducing bacterium (DSRB), isolated from estuarine mid-Chesapeake Bay sediments. It was chosen for study because of its exceptionally high rates of Hg methylation in culture and its metabolic similarity to the lost strain D. desulfuricans LS, the only organism for which methylation pathways have been partially defined. Strain ND132 is an incomplete oxidizer of short-chain fatty acids. It is capable of respiratory growth using fumarate as an electron acceptor, supporting growth without sulfide production. We used enriched stable Hg isotopes to show that ND132 simultaneously produces and degrades methylmercury (MeHg) during growth but does not produce elemental Hg. MeHg produced by cells is mainly excreted, and no MeHg is produced in spent medium. Mass balances for Hg and MeHg during the growth of cultures, including the distribution between filterable and particulate phases, illustrate how medium chemistry and growth phase dramatically affect Hg solubility and availability for methylation. The available information on Hg methylation among strains in the genus Desulfovibrio is summarized, and we present methylation rates for several previously untested species. About 50% of Desulfovibrio strains tested to date have the ability to produce MeHg. Importantly, the ability to produce MeHg is constitutive and does not confer Hg resistance. A 16S rRNA-based alignment of the genus Desulfovibrio allows the very preliminary assessment that there may be some evolutionary basis for the ability to produce MeHg within this genus.

The Hg-methylating ability of dissimilatory iron-reducing bacteria in the genera Geobacter, Desulfuromonas, and Shewanella was examined. All of the Geobacter and Desulfuromonas strains tested methylated mercury while reducing Fe(III), nitrate, or fumarate. In contrast, none of the Shewanella strains produced methylmercury at higher levels than abiotic controls under similar culture conditions. Geobacter and Desulfuromonas are closely related to known Hg-methylating sulfate-reducing bacteria within the Deltaproteobacteria.