Activated Carbon Reshapes the Relationship between Mercury Methylation and Sulfate Reduction in a Sediment Bioelectrochemical Treatment System

Mercury (Hg) methylation is tightly linked to the activity of sulfate-reducing bacteria (SRB). Bioelectrochemical systems (BESs) have been proposed to remediate excess sulfate in freshwater systems, but their effects on coupled Hg and sulfur cycling remain poorly understood. Here, we developed a novel BES to investigate how applied voltage and activated carbon (AC) interact to regulate sulfate reduction and Hg(II) methylation kinetics in sediments from a sulfate-impacted stream. Methylation potentials did not track bulk sulfate reduction across treatments: AC suppressed methylation by up to 80%, applied voltage stimulated sulfate reduction but slightly decreased methylation in AC– mesocosms, and the combined AC and voltage treatment maintained methylation at levels comparable to voltage alone. This observation was reflected in shifts among methylating taxa. Desulfobulbus, a genus capable of direct electron transfer to graphite electrodes, was preferentially enriched in RNA relative to DNA across all mesocosms, and the hgcAB genes associated with Geobacteraceae were elevated under applied voltage. These results indicate that applied voltage enriches electroactive Hg methylators rather than broadly stimulating SRB activity, a process that may be facilitated by electron conduction through AC. This feature of BES operation should be considered when coapplying sorbent amendments and electrochemical potential in Hg-contaminated freshwater systems.