ICMGP - Conference Presentation

Abstract Information :

Mercury (Hg) is a global pollutant and a potent neurotoxin in its methylated form, methylmercury (MeHg). Some anaerobic microorganisms are responsible for the formation of MeHg that biomagnifies in food webs. Other microorganisms reduce Hg(II) to volatile Hg(0) which serves as a cellular detoxification mechanism and may also limit Hg(II) substrate availability to Hg methylators. We discovered a novel, non mer-mediated, Hg(II) reduction pathway by anoxygenic photoheterotrophic and fermentative Heliobacteria. They are found in terrestrial ecosystems ranging from rice paddies and gardens to volcanic soils. To uncover aspects of their metabolism that drive Hg(II) reduction, we used a combination of microbial physiology and genome editing experiments. We found that the amount of Hg(II) reduced is influenced by nutrient and metabolic conditions. Carbon source oxidation appears to be necessary for Hg(II) reduction during fermentation but not during photoheterotrophy. This suggests that in the absence of an organic carbon source, light energy transduction may supply electrons for the reduction of Hg(II). Experiments with mutants unable to photosynthesize exposed to Hg(II) in the absence of a fermentable carbon source were also unable to reduce Hg(II). Together, results suggest that photosynthesis and fermentation provide electrons through common redox active enzyme(s) that are the site of Hg(II) reduction. We are now in the process of testing the role of ferredoxin:NAD(P)H oxidoreductase in Hg(II) reduction, which receives electrons from fermentation and phototrophy. Anaerobic microbial processes driving Hg(II) reduction deserve further study as they stand to influence the environmental fate of Hg pollution, yield novel bioremediation technologies, and reveal how Heliobacteria, which perform one of the oldest known forms of photosynthesis, have interacted with Hg over billions of years of evolution.