The Genetic Basis for Bacterial Mercury Methylation

TLDR

Mercury is most commonly released into the food web as methylmercury, a highly toxic organic form that accumulates up the food chain, but it is only produced from inorganic Hg²⁺ by anaerobic microorganisms such as sulfur‑reducing bacteria. Parks et al. used comparative genomics to identify a two‑gene cluster encoding a corrinoid and an iron‑sulfur protein that is present in all known Hg‑methylating bacteria and absent in non‑methylators.

Abstract

Mercury Methylating Microbes Mercury (Hg) most commonly becomes bioavailable and enters the food web as the organic form methylmercury, where it induces acute toxicity effects that can be magnified up the food chain. But most natural and anthropogenic Hg exists as inorganic Hg 2+ and is only transformed into methylmercury by anaerobic microorganisms—typically sulfur-reducing bacteria. Using comparative genomics, Parks et al. (p. 1332 , published online 7 February; see the Perspective by Poulain and Barkay ) identified two genes that encode a corrinoid and iron-sulfur proteins in six known Hg-methylating bacteria but were absent in nonmethylating bacteria. In two distantly related model Hg-methylating bacteria, deletion of either gene—or both genes simultaneously—reduced the ability for the bacteria to produce methylmercury but did not impair cellular growth. The presence of this two-gene cluster in several other bacterial and lineages for which genome sequences are available suggests the ability to produce methylmercury may be more broadly distributed in the microbial world than previously recognized.