Bell-shaped response of mercury methylation to sulfate loading in urban sewer systems: Implications for source-level control.

Urban sewer systems act as incubators for mercury-methylating (hgcA) microorganisms, yet how sulfate-an abundant and variable sewage constituent-drives this process remains unclear. Here, we combined controlled bioreactor experiments, batch incubations, and genome-resolved metagenomics to demonstrate that Hg methylation potential follows a nonlinear, bell-shaped response to sulfate loading. The MeHg production rate peaked at moderate sulfate concentrations (75-150 mg/L), reaching levels 1.2-5.4 times higher than those observed under low sulfate conditions (6-30 mg/L). This enhancement arose from distinct community responses: at 75 mg/L, a phylogenetically diverse hgcA consortium emerged, with methanogens and fermenters complementing SRB, whereas at 150 mg/L, SRB-methylators such as Desulfobulbus dominated, indicating functional specialization. Outside this range, low sulfate (<30 mg/L) suppressed most hgcA populations due to electron acceptor scarcity, while high sulfate (>300 mg/L) favored non-methylating SRB like Desulfobacter postgatei, thereby reducing overall methylation potential. Importantly, by integrating our findings with reported sewage data, we show that sulfate concentrations in most domestic sewage fall within the optimal range for hgcA proliferation, explaining their consistently high abundances worldwide. Our results also highlight the potential basis for source-level interventions, such as substituting sulfate-free coagulants or restricting sulfate-rich industrial discharges, to reduce hgcA proliferation and mitigate downstream MeHg risks in urban water systems.