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.

水生食物网的汞（Mercury, Hg）污染，部分受控于具有毒性且可生物富集的甲基汞（methylmercury, MeHg）的形成与积累。甲基汞的生成由携带hgcAB基因对（hgcAB gene pair）的代谢多样性微生物介导，而去甲基化反应则由多种生物及非生物过程介导。然而，这两个过程对甲基汞积累的相对重要性，以及影响它们的环境因素，其特征仍有待明确，在富营养化环境中尤为如此。本研究将某富营养淡水湖泊中的汞甲基化与甲基汞去甲基化过程，与环境甲基汞浓度以及hgcA的丰度和表达水平进行了关联分析。高甲基化速率潜力表明，原位甲基汞生成是水柱中甲基汞的关键来源，这一过程由高hgcA丰度及其转录所驱动。钼酸盐（molybdate）处理降低了甲基化速率潜力，凸显了硫酸盐还原在驱动甲基汞生成中的重要性。尽管硫酸盐还原菌（sulfate-reducing bacteria）在携带hgcA的微生物群落中占比不足10%，但其hgcA基因转录量却占总转录量的50%以上。众多hgcA序列的上游区域存在类arsR转录调控因子，这类调控因子具有转录活性，并与较低的hgcA表达水平相关。综上，本研究阐明了在尚未被充分研究的富营养淡水环境中，影响原位甲基汞生成的微生物与生物地球化学过程。