Possible pathways for mercury methylation in oxic marine waters

Mercury (Hg) is a global contaminant that presents public health risks through seafood consumption primarily in the form of monomethylmercury (MMHg). Methylation of inorganic Hg in water column has been considered a major source of seawater MMHg, but the known Hg methylation by anaerobes possessing the hgcAB gene cluster in anoxic environments could not directly explain the formation and widespread presence of MMHg in seawater where oxic conditions are usually present. In this review, we synthesized the information on previously reported possible pathways to explain the Hg methylation in oxic marine waters, including Hg methylation by (1) methyl donors like organic compounds and organometallic complexes in seawater via abiotic pathways, (2) anaerobic microbes in anoxic microenvironments within oxic seawater, and (3) aerobic microbes in oxic seawater. We evaluated the potential contributions of respective Hg methylation pathways to MMHg in seawaters and discussed the perspectives on future research needs for an improved understanding of seawater Hg methylation. We inferred that while all proposed Hg methylation pathways remain to be further verified, at least one and maybe all of them are plausible depending on ocean conditions. Development and application of new techniques, e.g., quantifying Hg isotope fractionation, would help differentiate (e.g., abiotic versus biotic) Hg methylation pathways. Comprehensive studies toward bridging the gaps between microbial gene screening and Hg methylating capability, between Hg methylation incubation and field MMHg measurement, and between mechanistic Hg methylation studies and environmental relevance will benefit the elucidation of Hg methylation pathways and MMHg distribution in seawater.