Mercury methylation and demethylation in aerobic surface waters

Potential rates of methylmercury formation and demethylation were studied in runoff waters, sediment trap material, and aerobic water layers of five small forest lakes by radioisotope methods. In addition, the roles of microbes and particulate matter in methylation and demethylation processes were examined. Methylation rates were low (≤0.12%∙d−1) in all sites studied. Water pH (4.9–6.9) had no clear effect on methylmercury formation. Methylation took place in filtered water (nominal pore size 0.2 μm) and in formaldehyde-treated or autoclaved samples, but was strongly suppressed by both dissolved organic matter (color) and particulate matter. Demethylation rates in surface waters (≤13.2%∙d−1) were decreased by low temperatures and the process was blocked by sterile filtration, formaldehyde treatment, and autoclaving. The results indicate that mercury methylation in aerobic humic waters is an accidental process caused by bacterial exoenzymes or other dissolved compounds. Methylmercury demethylation was clearly microbial in nature.