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.