Sulfate Reduction Drives Elevated Methylmercury Formation in the Water Column of a Eutrophic Freshwater Lake

Mercury (Hg) contamination of aquatic food webs is controlled in part by the formation and accumulation of toxic and bioaccumulative methylmercury (MeHg). MeHg production is mediated by metabolically diverse microorganisms carrying the <i>hgcAB</i> gene pair, while the demethylation reaction is mediated by several biotic and abiotic processes. However, the relative importance of these two processes on MeHg accumulation and the environmental factors that influence them are poorly characterized, especially in eutrophic environments. In this study, both Hg methylation and MeHg demethylation in a eutrophic freshwater lake were linked to ambient MeHg concentrations and <i>hgcA</i> abundance and expression. High methylation rate potentials indicated <i>in situ</i> MeHg formation was a key source of MeHg to the water column, driven by high <i>hgcA</i> abundance and transcription. Molybdate treatment decreased methylation rate potentials, highlighting the importance of sulfate reduction in driving MeHg formation. Sulfate-reducing bacteria accounted for over 50% of the <i>hgcA</i> gene transcription, despite representing less than 10% of the <i>hgcA</i>-carrying microbial community. An <i>arsR</i>-like transcriptional regulator preceded many <i>hgcA</i> sequences; these were transcriptionally active and linked to lower <i>hgcA</i> expression. Overall, this study elucidates the microbial and biogeochemical processes that influence the <i>in situ</i> formation of MeHg in understudied eutrophic freshwater environments.