Global Prevalence and Distribution of Genes and Microbes involved in Mercury Methylation
It is well known that the methylation of mercury (Hg) is mediated by bacteria and produces neurotoxic methylmercury (MeHg), which is also highly bioaccumulative in living organisms. However, the specific environments or locations in which MeHg is created are not well understood or identified. The recent finding of the specific genes (hgcAB) involved in Hg methylation provides a potential tool for scientists to identify the specific environments or locations where MeHg is created. Because the hgcAB genes are highly conserved, a team of scientists from Oak Ridge National Laboratory, Smithsonian Environmental Research Center, and Texas A&M University realized that they had a foundation for broadly evaluating spatial and niche-specific patterns of microbial Hg-methylation potential in natural environments. The team primarily used assembled and annotated data publicly available from the Department of Energy’s Joint Genome Institute to query hgcAB diversity and distribution in >3,500 publically available microbial metagenomes, encompassing a broad range of global environments. 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 and mammalian microbiomes, suggesting a low risk of endogenous MeHg production. New potential methylation habitats were identified, including invertebrate digestive tracts, thawing permafrost, coastal “dead zones,” soils, sediments, and extreme environments, suggesting multiple routes for MeHg entry into food webs. Several new taxonomic groups capable of Hg methylation emerged, including lineages having no cultured representatives. Phylogenetic analysis points to an evolutionary relationship between hgcA and genes encoding the corrinoid iron-sulfur proteins functioning in the ancient Wood-Ljungdahl carbon fixation pathway, suggesting that methanogenic archaea may have been the first to perform these biotransformations.
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. 2015. “Global Prevalence and Distribution of Genes and Microorganisms involved in Mercury-Methylation,” Science Advances 1(9), e1500675. DOI: 10.1126/sciadv.1500675.