Abstract
Mercury methylation produces the neurotoxic, highly bioaccumulative methylmercury (MeHg). Recent identification of the methylation genes (hgcAB) provides the foundation for broadly evaluating microbial Hg-methylation potential in nature without making explicit rate measurements. We queried hgcAB diversity and distribution in all available microbial metagenomes, encompassing most environments. The genes were found in nearly all anaerobic, but not in aerobic, environments including oxygenated layers of the open ocean. Critically, hgcAB was effectively absent in ~1500 human microbiomes, suggesting a low risk of endogenous MeHg production. New potential methylation habitats were identified, including invertebrate guts, thawing permafrost, coastal “dead zones”, soils, sediments, and extreme environments, suggesting multiple routes for MeHg entry into food webs. Several new taxonomic groups potentially capable of Hg-methylation emerged, including lineages having no cultured representatives. We begin to address long-standing evolutionary questions about Hg-methylation and ancient carbon fixation mechanisms while generating a new global view of Hg-methylation potential.
