Microbial Interactions with the Mercury Cycle

All forms of mercury (Hg) are toxic to the environment and to all organisms, particularly the organometallic neurotoxin methylmercury. Microbes play key roles in controlling the form and fate of mercury in the environment and therefore in mediating mercury toxicity. They are the sole natural source of methylmercury, forming this toxin from mostly anthropogenic mercury deposited in the atmosphere globally. Microbes can also detoxify and recycle mercury from sediments or waters into the atmosphere, thereby acting both to immobilize and mobilize this element in its biogeochemical cycle. Phylogenetic analyses support the interpretation that microorganisms have been interacting in such ways with the element mercury since very early in the geological evolution of the earth, where most of the mercury in the environment would have been emitted from volcanoes or seafloor hydrothermal vents. Recent advances in metagenomics, bioinformatics, and computational modeling, coupled with extensive cultivation-driven experimental research, have yielded important insights into which microorganisms are capable of transforming mercury species by changing their valence state and/or speciation, including the formation or destruction of organometallic mercury species. These studies have also generated new conceptual models of the proteins that mediate these transformations and the biochemical mechanisms involved. Still, many long-standing questions remain to be answered; this concise review attempts to illustrate recent progress and identify current questions in the study of the geomicrobiology of mercury.