COMPARISON OF HG(II) UPTAKE AND METHYLATION ACROSS DIVERSE ANAEROBIC MICROORGANISMS

The intracellular transport of Hg(II) is a critical step in the production and accumulation of methylmercury, and thus it is important to understand how Hg speciation influences the bioavailability to different microbes. Methylmercury is produced by a variety of obligate anaerobes including members of the δ-Proteobacteria, Methanomicrobia, and most recently, Firmicutes. Washed cell experiments with model organisms within the Proteobacteria support the accidental active uptake of Hg(II) through heavy metal transporters such as those involved in the uptake of Zn(II). Experiments are currently under way to determine whether this mechanism is widely distributed across diverse phyla, including the Firmicutes and Methanomicrobia. Interestingly, differences in Hg bioavailability were observed across taxa and respiratory guild. For instance, a fermentative, firmicute Ethanoligenes harbinense had low affinity for Hg(II) thiol or chloride complexes, exhibiting the lowest uptake and methylation rates of organisms examined. Low methylation rates were also observed for an iron-reducing firmicute, Desulfitobacterium metallireducens; preliminary data suggests similar trends in uptake are observed in both firmicute strains. In contrast, iron-reducing δ-proteobacteria, Geobacter sulfurreducens, had high rates of Hg uptake and methylation when Hg(II) was complexed to some thiols, such as cysteine, but not to other thiols such as penicillamine or glutathione. The sulfate-reducing bacterium Desulfovibrio sp. ND132, isolated from highly sulfidic estuarine waters, had the highest affinity for Hg(II) thiol complexes and was able to take up and methylate Hg bound to a wide variety of thiols, including both penicillamine and glutathione, neither of which support Hg uptake in iron-reducing bacteria. These experiments may explain the observed results from field studies suggesting the relative importance of the sulfate-reducing bacteria in the methylation of Hg. Similar experiments are being conducted with the methanogen, Methanospirillum hungatei to determine conditions promoting uptake and methylation in archaea. These results are critical for understanding how different microbial populations respond to changes in Hg speciation in order to better predict methylmercury accumulation in the environment.