A Genome Catalogue of Mercury-Methylating Bacteria and Archaea from
\nSediments of a Boreal River Faced by Human Disturbances

Methylmercury (MeHg), the most bioavailable form of mercury (Hg), is a neurotoxin produced by anaerobic microbes. MeHg generated in aquatic sediments can be transferred to aquatic organisms and biomagnified along food webs, ultimately reaching fish consumers. This is a particular concern for rivers, as they are connective bodies for aquatic ecosystems and play crucial roles in the transport of nutrients. Moreover, rivers exhibit heightened susceptibility to environmental disturbances within their watershed, which have been linked to increased Hg-methylation. Rivers impacted by run-of-river dams hold specific significance, given the growing preference for these dam types over reservoir dams. Early studies have identified sulfate reducers, methanogens, and iron reducers as the main contributors to Hg methylation. More recently, proteins encoded by the hgcAB genes have been found to confer the ability to methylate Hg. Recent metagenomic studies have expanded our knowledge of hgcAB-carrying lineages in the environment. Nevertheless, genome-based exploration of Hg-methylators remains limited, particularly in the context of river systems. To fill this knowledge gap, we created a genome catalogue of Hg-methylating microorganisms from the sediments of a river impacted by two run-of-river dams, logging, and a forest fire. We assessed the taxonomic and metabolic diversity of these putative Hg-methylators. Additionally, we assessed their abundance and diversity across sites along the river that were subject to different disturbances to gain insight into the ecological impact on Hg-methylators. For a deeper understanding of the environmental factors shaping Hg-methylator
\ndiversity, we juxtaposed the genome catalogue with the wider microbial community to which these methylators belong. We uncovered a unique and diverse assemblage of Hg-methylators dominated by members of metabolically versatile and fermentative Bacteroidota. This assemblage was particularly enriched in butyrate fermentative, carbon fixing and nitrite reducing microbes. We found that sites affected by press-like disturbances such as logging were particularly favorable to the establishment of a Hg-methylating niche. Lastly, we argue that the effects of watershed disturbances are likely not specific to Hg-methylators, but rather shared across the greater microbial community.

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