REVIEW OF POSSIBLE PATHS FOR ABIOTIC METHYLATION OF MERCURY(II) IN THE AQUATIC ENVIRONMENT

High concentrations of methylmercury in freshwater and marine fish lends an urgency to the problem of how inorganic mercury, the predominant pollutant, is converted to methylmercury in the aquatic environment. Numerous experiments by microbiologists suggest that biotic sediments are major sites for methylation of inorganic mercury and that sulfate-reducing bacteria contribute considerably to methylmercury production. However, the sufficient but low sulfate concentration needed for sulfate-reducing bacteria to produce methylmercury makes it difficult to explain its ubiquitous occurrence in coastal and deep sea marine organisms and sediments. In addition, the high concentration of mercury(II) added to samples allows only mercury-resistant species, which do not predominate in the aquatic environment, to survive. Finally, sterilization methods change chemistry as well as biology, and conclusions based on experiments comparing active and sterile sediments are not definitive. For these reasons there is a strong possibility that abiotic methylation of mercury(II) contributes to the formation of methylmercury. This review emphasizes potential abiotic methylation of mercury(II) by methylcobalamin, methyltin compounds, and humic matter. Although methylcobalamin readily methylates mercury(II) in non-environmental matrices, it is least likely of the three methyl donors because it is not abundant in the aquatic environment. In contrast methyltin compounds (mono-, di-, and trimethyltin) occur in water, biota, and sediments of the marine environment. All three methylate mercury(II) in seawater and may be involved in abiotic methylation. Humic matter is the most promising environmental methylating agent for several reasons. Its concentration in waters and sediments is high. It is associated with the solubility and thus mobility of mercury in freshwaters and marine waters, probably by complexation. In addition, several model studies have shown that humic matter effectively methylates mercury(II) forming methylmercury.