Anaerobic mercury methylators inhabit sinking particles of oxic water columns

Abstract

Increased concentration of mercury, particularly methylmercury, in the environment is a worldwide concern because of its toxicity in severely exposed humans. Although the formation of methylmercury in oxic water columns has been previously suggested, there is no evidence of the presence of microorganisms able to perform this process, using the hgcAB gene pair, in such environments. Here we show the prevalence of hgc+ microorganisms in sinking particles of the oxic water column of Lake Geneva (Switzerland and France) and its anoxic bottom sediments. Sinking particles found in oxic waters exhibited relatively high proportion of hgc genes taxonomically assigned to Firmicutes. In contrast hgc+ of Nitrospirae, Chloroflexota and PVC superphylum were prevalent in anoxic sediment while hgc+ Desulfobacterota were found in both environments. Altogether, the description of the diversity of putative mercury methylators in the oxic water column and anoxic sediments of Lake Geneva highlight that both environments have capability for Hg methylation expanding our understanding of environmental factors triggering MeHg formation in aquatic environments and at a global scale.