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 hgcAB 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 hgcA abundance and expression. High methylation rate potentials
indicated in situ MeHg formation was a key source
of MeHg to the water column, driven by high hgcA 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 hgcA gene transcription, despite representing
less than 10% of the hgcA-carrying microbial community.
An arsR-like transcriptional regulator preceded many hgcA sequences; these were transcriptionally active and
linked to lower hgcA expression. Overall, this study
elucidates the microbial and biogeochemical processes that influence
the in situ formation of MeHg in understudied eutrophic
freshwater environments.