Viral suppression constrains environmentally stimulated microbial methylmercury production in wastewater treatment plants.

Wastewater treatment plants (WWTPs) are hotspots for neurotoxic methylmercury (MeHg) production. While the bottom-up environmental drivers of mercury (Hg) methylation are well-documented, the role of top-down viral predation remains poorly understood. Here, we investigated the interaction between environmental stimulation and viral predation in regulating MeHg production across four geographically distinct WWTPs. Field surveys revealed that while total and dissolved Hg concentrations decreased consistently throughout the treatment process, both total and dissolved MeHg peaked in the biological treatment (BT) units, where reducing conditions favor Hg-methylating microorganisms. Viromic analyses showed a dominance of lytic bacteriophages targeting Hg-methylating microorganisms in the BT units, suggesting that viral predation may suppress environmental stimulation. Mechanistic experiments confirmed the dual regulation of MeHg production: redox potential and organic carbon explained 43.5% and 24.5% of the variance, while viral predation independently accounted for 15.3%. Heat-inactivated virus-like particles (VLPs) had negligible effects, while active VLPs reduced MeHg production by 43%. Targeted infection assays showed that viral lysis disproportionately reduced the relative transcriptional activity of Hg methylators, decreasing the hgcA/16S rRNA transcript ratio by >52%. Finally, dual-stable-isotope tracing confirmed that viral predation constrained MeHg accumulation by 36%, primarily by inhibiting Hg methylation rate. These findings suggest that viral predation has the potential to suppress the increase in MeHg production driven by environmental stimulation, highlighting the need to explore viral dynamics into models of MeHg production hotspots.