Recent advances in microbial mercury methylation: A review on methylation habitat, methylator, mechanism, and influencing factor

In natural environments, inorganic mercury (Hg2+) can be converted to the neurotoxin methylmercury (MeHg) mainly via the microbial Hg methylation process with anaerobic microorganisms. This process is a cellular reaction, and is influenced by cellular activity of Hg methylating microorganisms and by cellular Hg2+ uptake. In the present article, we review research progress of Hg bio-methylation, which includes: sites for Hg bio-methylation, Hg methylating microorganisms, mechanisms of Hg bio-methylation, and factors affecting Hg bio-methylation. Microbial Hg methylation occurs in a variety of sites such as the freshwater and marine environments as well as some unrecognized habitats. Sulfate-reducing bacteria using sulfate as their terminal electron acceptors are a group of the most extensively studied Hg methylators, followed by iron-reducing bacteria. The cellular uptake of Hg2+ before methylation may proceed via passive and/or active transport. Most Hg methylators have a pivotal gene pair encoding proteins required for methylation process. The cellular activity and bioavailable Hg2+ concentration for methylation can be determined primarily by many environmental factors i.e., temperature, redox potentials, pH value, existence of dissolved organic matter (DOM) and sulfur-containing organic compounds. A comprehensive understanding of methylation habitat, methylator, mechanism, and influencing factor is urgently required and will provide guidance for regulating net MeHg production within natural environments.