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 Ice1) 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 HgII under
phototrophic or fermentative conditions. Fermentative HgII reduction relied on the ability of cells to oxidize pyruvate whereas
phototrophic HgII reduction could be supported even in
the absence of a carbon source. Inhibiting pyruvate fermentation eliminated
HgII reduction in all chemotrophic strains tested, whereas
phototrophic cells remained unaffected. Here we propose a non mer-operon dependent mechanism for Hg0 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.