Basic Study on Mercury Methylation Under Oxic Conditions: Incubation Experiments Using Seawater from Minamata Bay, Japan

Background: This study investigated the generation of dissolved methylmercury (diss-MeHg) under oxic conditions in the surface waters of Minamata Bay. Although mercury methylation has been extensively investigated under reducing conditions, its occurrence under oxic conditions remains poorly understood. Methods: Incubation conditions capable of artificially inducing mercury methylation were established for the first time. Incubation parameters were optimized using surface seawater samples. Results: Diss-MeHg production was positively correlated with dissolved organic carbon (DOC) concentrations (1–5 mg/L). The highest methylation was observed at 22 °C and 32.5‰ salinity, though no statistically significant differences were observed within the ranges of 18–22 °C and 30.5–34.5‰. Under the optimized conditions (22 °C, 32.5‰, 5 mg/L DOC), seawater samples were sequentially filtered through membranes of various pore sizes. Diss-MeHg was detected in fractions filtered through 0.8- and 3-µm membrane filters but was absent from the 0.45- and 0.2-µm fractions. Additionally, autoclaving the 0.8 µm filtrate prevented diss-MeHg production. Conclusions: These findings suggest the presence of a distinct biological mercury methylation process operating under oxic conditions in surface seawater.