Microbial mercury methylation and demethylation: biogeochemical mechanisms and metagenomic perspectives in freshwater ecosystems

Elevated concentration of methylmercury (MeHg) in fish is a worldwide concern due to its detrimental effects on human health. Although Hg methylation is a key issue regarding MeHg contamination, neither abiotic nor microbial methylation mechanisms are well understood. The overall objective of this study was to link the potential for microbial methylation and demethylation to the molecular characterization of microbial communities in two typical freshwater ecosystems and to gain in-depth understanding of Hg methylation mechanisms by syntrophy. Sunday Lake is a remote and “pristine” forest lake exposed to Hg mostly through atmospheric deposition in the Adirondack Mountains, New York. This study demonstrated that floating Sphagnum moss mats near the lake water front were hot spots for MeHg accumulation and microbial methylation, and sub-habitats where sulfate reducing bacteria (SRB) community was highly developed. SRB were identified as a major group of Hg methylators, as sulfate addition to the mat samples doubled the potential Hg methylation rates while molybdate significantly inhibited them. The dominant distribution of Syntrophobacter spp. in the Sphagnum mats led to the investigation of syntrophy in Hg methylation. By incubating mono- or co-cultures of Syntrophobacter spp., with Desulfovibrio spp., this study was the first to demonstrate that a Syntrophobacter-Desulfovibrio coculture significantly increased growth of both syntrophic partners and stimulated MeHg synthesis compared to activities of Desulfovibro spp. monocultures. Syntrophy could stimulate MeHg synthesis by two pathways: Desulfovibrio growing with methanogens in sulfate-free environments, and Desulfovibrio growing with Syntrophobacter in sulfate-limited environments where sources of energy and carbon are limited. In the South River, an industrially Hg-contaminated site in Virginia, high Hg methylation rates and low demethylation activities were observed in nine sites downstream from the contaminating source, partially explaining why fish in this river have high MeHg levels. 16S rRNA sequencing from sediment cDNA showed that at least three groups of SRB and one group of Geobacter-like iron reducing bacteria (IRB) that were closely affiliated to known Hg methylators, were active in the sediments. Further metabolic inhibition and stimulation experiments confirmed that both SRB and IRB were involved in the microbial methylation in South River sediments.Ph.D.Includes bibliographical referencesby Riqing Y