Mechanisms of Microbially Mediated Mercury Methylation and Demethylation in the Environment

The environmental behavior and toxicity of mercury are largely determined by its chemical speciation, and microbially mediated mercury transformations play a crucial role in controlling its mobility, bioavailability, and bioaccumulation. This review systematically examines two key microbial transformation processes: the methylation of inorganic mercury (Hg²⁺) and the demethylation of methylmercury (MeHg), with an emphasis on the major microorganisms involved and the underlying molecular mechanisms. Specifically, this review focuses on the hgcAB-mediated mechanisms of mercury methylation and the methylation potential of different functional microbial groups, as well as the mer system-mediated demethylation of MeHg and the associated microorganisms. It further examines the effects of environmental factors, including the coupling of organic matter, iron, and sulfur cycling, salinity and ionic composition, and climate warming, on microbially mediated mercury methylation and demethylation. This review aims to advance our understanding of microbial regulation of mercury biogeochemical cycling and provide a theoretical basis for mercury pollution risk assessment and the development of bioremediation strategies.