Global relative frequency of HgcA based on metagenomic projects. Overlay is the estimated continental emission of mercury (in tons), based on the United Nations Environment Programme Global Mercury Assessment 2013 report. Diamonds represent pelagic ocean water samples, and circles represent all other samples.
Most natural and anthropogenic mercury (Hg) exists as inorganic Hg2+ that can be transformed into toxic methylmercury by anaerobic microbes. Recent discovery of the two genes responsible for this transformation—hgcA and hgcB—provides a genetic tool to identify the capacity and distribution of mercury-methylating bacteria across the globe. This knowledge would in turn help improve insights into the potential cycling and health impacts of environmental mercury. To gain an understanding of the global distribution and abundance of hgcAB, we queried the diversity and distribution of these genes in >3,500 publicly available microbial metagenomes encompassing a broad range of environments (see figure).
Results suggest that in situ or in-body mercury methylation is extremely rare (if not absent) in all mammals including humans, indicating that human methylmercury poisoning probably occurs only through ingestion of the toxin. New potential methylation habitats also were identified, including invertebrate guts, thawing permafrost, coastal "dead zones," soils, sediments, and extreme environments, suggesting multiple routes for methylmercury entry into food webs. In addition, this study begins to address long-standing evolutionary questions about mercury methylation while generating a new global view of its potential.
The hgcAB genes were found in nearly all anaerobic, but not aerobic, environments including oxygenated layers of the open ocean. Critically, hgcAB was effectively absent in ~1500 human microbiomes, suggesting a low risk of endogenous methylmercury production. Several new taxonomic groups capable of mercury methylation emerged, including lineages having no cultured representatives.
Podar, M., C. C. Gilmour, C. C. Brandt, A. Soren, S. D. Brown, B. R. Crable, A. V. Palumbo, A. C. Somenahally, and D. A. Elias. "Global prevalence and distribution of genes and microorganisms involved in mercurymethylation." Science Advances 1(9):e1500675. [DOI:10.1126/sciadv.1500675]