Heliobacteria Reveal Fermentation As a Key Pathway for Mercury Reduction in Anoxic Environments

The accumulation of mercury (Hg) in rice, a dietary staple for over half of the world's population, is rapidly becoming a global food safety issue. Rice paddies support the anaerobic production of toxic methylmercury that accumulates in plant tissue, however the microbial controls of Hg cycling in anoxic environments remain poorly understood. In this study, we reveal a novel reductive Hg metabolism in a representative of the family Heliobacteria (Heliobacterium modesticaldum Ice 1) that we confirm in model chemotrophic anaerobes. Heliobacteria served as our initial model because they are a family of spore-forming fermentative photoheterotrophs commonly isolated from terrestrial environments. We observed that H. modesticaldum reduced up to 75% of Hg-II under phototrophic or fermentative conditions. Fermentative Hg-II reduction relied on the ability of cells to oxidize pyruvate whereas phototrophic Hg-II reduction could be supported even in the absence of a carbon source. Inhibiting pyruvate fermentation eliminated Hg-II reduction in all chemotrophic strains tested, whereas phototrophic cells remained unaffected. Here we propose a non mer-operon dependent mechanism for Hg-0 production in anoxic environments devoid of light where external electron acceptors are limited. These mechanistic details provide the foundation for novel bioremediation strategies to limit the negative impacts of Hg pollution.