Microbial transformations of mercury species in rice paddy soils: methylation, demethylation and reduction

Abstract:

Rice paddy soils are “hotspots” for converting inorganic mercury to neurotoxic methylmercury (MeHg) which accumulates in rice grains, resulting in high risk of MeHg exposure. Although Hg methylation and reduction have been in focus for decades, we still lack fundamental understanding of the organisms involved in those processes. Here we combined laboratory incubations with stable isotope tracers to investigate the roles of anaerobic microbes to Hg transformations of different species across a contamination gradient. Results showed that sulfate-reduction was the main driver of MeHg formation at control site and methanogenesis had an important and complex role in MeHg cycling as Hg concentrations increase. The inhibition of methanogenesis at the mining sites led to an increase in MeHg production up to 16.6 folds and a decrease in MeHg degradation by up to 77%, suggesting that methanogenesis is associated with MeHg degradation as Hg concentrations increased. Sulfate-reduction was suggested to be the principle pathway for Hg(II) reduction at abandoned Hg mining sites, while methanogenesis was more important for MeHg reduction at the control site. This study broadens our understanding of the roles of microbes on Hg cycling in rice paddies and provides insights into Hg mitigation in contaminated areas.