Periphyton Biofilms Influence Net Methylmercury Production in an Industrially Contaminated System

Mercury (Hg) methylation and methylmercury (MMHg) demethylation activity of periphyton biofilms from the industrially contaminated East Fork Poplar Creek, Tennessee (EFPC) were measured during 2014-2016 using stable Hg isotopic rate assays. ²⁰¹Hg^II and MM²⁰²Hg were added to intact periphyton samples in ambient streamwater and the formation of MM²⁰¹Hg and loss of MM²⁰²Hg were monitored over time and used to calculate first-order rate potentials for methylation and demethylation. The influences of location, temperature/season, light exposure and biofilm structure on methylation and demethylation potentials were examined. Between-site differences in net methylation for samples collected from an upstream versus downstream location were driven by differences in the demethylation rate potential (k_d). In contrast, the within-site temperature-dependent difference in net methylation was driven by changes in the methylation rate potential (k_m). Samples incubated in the dark had lower net methylation due to lower k_m values than those incubated in the light. Disrupting the biofilm structure decreased k_m and resulted in lower net methylation. Overall, the measured rates resulted in a net excess of MMHg generated which could account for 3.71-7.88 mg d^-1 MMHg flux in EFPC and suggests intact, actively photosynthesizing periphyton biofilms harbor zones of MMHg production.