Global Survey of Mercury Methylation and Demethylation Microbial Communities in Wastewater and Activated Sludge

Wastewater treatment plants (WWTPs) are an understudied source of mercury methylating and demethylating microbes to downstream aquatic and terrestrial environments, where methylmercury production and subsequent bioaccumulation in the food web occur. To identify methylators and demethylators and evaluate their occurrence in WWTPs, metagenomic and metatranscriptomic analyses of raw sewage, activated sludge, and effluent samples from WWTPs across the globe were conducted. Results indicated that hgcA- and merB-carriers were widespread in WWTPs, with higher abundance in raw sewage and sludge compared to treated effluent. Bdellovibrionota were identified as merB-carriers, linking them to demethylation for the first time. Novel conserved motifs of hgcB and fused hgcAB were also identified. 30% of hgcA genes were colocalized with arsenic-resistance operons on the same contig, while all merB-carriers contained arsenic resistance genes (ars), though merB and arsR were not colocated. Antibiotic resistance genes were also present in the genomes of multiple hgcA- amd merB-carriers, including one sample where hgcA and the antiseptic/antibiotic resistant gene (qacG) were colocated on the same contig, suggesting possible coselection in environments containing antibiotics. Mobile genetic element-mediated horizontal gene transfer was identified as a mechanism facilitating the genetic transfer of hgcA. Overall, these findings highlight WWTPs as reservoirs of genes involved in mercury methylation and demethylation, with potential implications for mercury cycling in downstream environments.