REDOX TRANSFORMATIONS OF MERCURY BY COMETABOLIC MICROBIAL PROCESSES

The redox state of inorganic mercury (Hg), transitioning between Hg^II and Hg⁰, greatly influences its mobility and bioavailability in the environment. Research over the last half century has focused on specific microbial and photochemical redox transformations, e.g., enzymatic detoxification by mercuric reductase and reduction by photochemically produced halogen radicals, and their role in Hg geochemistry.

Evidence collected in recent years, however, clearly suggests that microbes partake in Hg redox cycling by performing electron producing or consuming processes central to their metabolism that indirectly affect Hg by so called co-metabolic pathways. Examples include the reduction of Hg^II by anaerobic bacteria seeking electron acceptors in respiration, phototrophs that obtain carbon from reduced organics and thus accrue excess NAD(P)H to create an intracellular growth-inhibiting redox imbalance, and chemotrophic ferrous iron oxidizing thiobacilli with a cytochrome C oxidase that uses Hg^II in addition to oxygen as a terminal electron acceptor. On the oxidative side of redox cycling, Hg⁰ may be oxidized by enzymes of the oxidative stress response in aerobic bacteria and among strict anaerobes that lack known oxidative response systems. Initial evidence suggests that oxidation by such anaerobes and in anoxic environments may be related to an initial bonding of Hg⁰ to thiol moieties in cell walls or the cytoplasm followed by its reduction.

This diversity of microbial pathways leading to Hg redox transformations raises the question of their importance in geochemical cycling in the environment. Such pathways may contribute to hitherto unexplained observations of Hg⁰ accumulation in low to no-light environments such as bottom waters of boreal lakes, under sea ice in polar regions, and in groundwater aquifers. Contrary to assumptions that Hg⁰ production equates with removal from active cycling by evasion to the atmosphere, Hg redox transformations may lead to its retention and subsequent methylation in the biosphere. In light of the broad occurrence of these non-specific processes, more knowledge is needed for the integration of microbial co-metabolic processes into Hg biogeochemical and environmental management paradigms.