Björn, E. | bioRxiv Science

bioRxiv2022

Abundance and expression of hgcAB genes and mercury availability jointly explain methylmercury formation in stratified brackish waters

Neurotoxic methylmercury (MeHg) is formed by microbial methylation of inorganic divalent Hg (HgII) and constitutes severe environmental and human health risks. The methylation is enabled by hgcA and hgcB genes, but it is not known if the associated molecular-level processes are rate-limiting or enable accurate prediction of MeHg formation in nature. In this study, we investigated the relationships between hgcA genes and MeHg across redox stratified water columns in the brackish Baltic Sea. We found that the abundance of hgcA genes and transcripts combined with the concentration of dissolved HgII-sulfide species were strong predictors of both HgII methylation rate and MeHg concentration, implying their roles as principal joint drivers of MeHg formation in these systems. In establishing relationships between hgcA genes and MeHg, we advance the fundamental understanding of mechanistic principles governing MeHg formation in nature and enable refined predictions of MeHg levels in coastal seas in response to the accelerating spread of oxygen deficient zones.

ecology↗

bioRxiv2018

Mercury methylating microbial communities of boreal forest soils

The formation of the potent neurotoxic methylmercury (MeHg) is a microbially mediated process that has raised much concern because MeHg poses threats to wildlife and human health. Since boreal forest soils can be a source of MeHg in aquatic networks, it is crucial to understand the biogeochemical processes involved in the formation of this pollutant. High-throughput sequencing of 16S rRNA and the mercury methyltransferase, hgcA, combined with geochemical characterisation of soils, were used to determine the microbial populations contributing to MeHg formation in forest soils across Sweden. The hgcA sequences obtained were distributed among diverse clades, including Proteobacteria, Firmicutes, and Methanomicrobia, with Deltaproteobacteria, particularly Geobacteraceae, dominating the libraries across all soils examined. Our results also suggest that MeHg formation is linked to the composition of also non-mercury methylating bacterial communities, likely providing growth substrate (e.g. acetate) for the hgcA-carrying microorganisms responsible for the actual methylation process. While previous research focused on mercury methylating microbial communities of wetlands, this study provides some first insights into the diversity of mercury methylating microorganisms in boreal forest soils.\n\nImportanceDespite a global state of awareness that mercury, and methylmercury in particular, is a neurotoxin that millions of people continue to be exposed to, there are sizable gaps in our fundamental understanding of the processes and organisms involved in methylmercury formation. In the present study we shed light on the diversity of the microorganisms responsible for methylmercury formation in boreal forest soils. All the microorganisms identified have a relevant role on the processing of organic matter in soils. Moreover, our results show that the formation of methylation formation is not only linked to mercury methylating microorganisms but also to the presence of non-mercury methylating bacterial communities that contribute to methylmercury formation by the appropriate substrate to the microorganisms responsible for the actual methylation process. This study improves current knowledge on the diversity of organisms involved in methylmercury formation in soils.

microbiology↗