A genome catalogue of mercury-methylating bacteria and archaea from sediments of a boreal river facing human disturbances

<p>Methyl mercury is a toxic compound produced by anaerobic microbes that biomagnifies in aquatic food webs, impacting animal and human health. Genome-based explorations of Hg methylators remain limited, particularly in the context of river ecosystems. To fill this knowledge gap, we created a genome catalogue of putative Hg-methylating microorganisms (based on the presence of <em>hgcAB</em>) from the sediments of a river impacted by two run-of-river hydroelectric dams, logging, and a wildfire. By using genome-resolved metagenomics, we uncovered a unique and diverse assemblage of Hg methylators dominated by members of the metabolically versatile Bacteroidota and particularly enriched in butyrate fermentative microbes. By comparing diversity and abundance of Hg methylators between sites that were subjected to different disturbances, we found that ongoing disturbances, such as input of organic matter related to logging activities. were particularly favorable to the establishment of a Hg-methylating niche. Lastly, for a deeper understanding of the environmental factors shaping Hg methylator diversity, we juxtaposed the Hg-methylating genome catalogue with the wider microbial community. The results suggest that Hg methylators respond to environmental conditions similarly to overall microbial community, and therefore it is crucial to interpret the diversity and abundance of Hg methylators within their specific ecological context.</p>