Microbial potential to mitigate neurotoxic methylmercury accumulation in farmlands and rice.

Toxic methylmercury (CHHg) is produced by microbial conversion of inorganic mercury in hypoxic environments such as rice paddy soils, and can accumulate in rice grains. Although microbial demethylation has been recognized as a crucial pathway for CHHg degradation, the identities of microbes and pathways accountable for CHHg degradation in soil remain elusive. Here, we combine CHHg-DNA stable-isotope probing experiments with shotgun metagenomics to explore microbial taxa and associated biochemical processes involved in CHHg degradation in paddy and upland soils. We identify Pseudarthrobacter, Methylophilaceae (MM2), and Dechloromonas as the most significant taxa potentially engaged in the degradation of CHHg in paddy soil with high mercury contamination. We confirm that strains affiliated with two of those taxa (species Dechloromonas denitrificans and Methylovorus menthalis) can degrade CHHg in pure culture assays. Metagenomic analysis further reveals that most of these candidate CHHg degraders carry genes associated with the Wood-Ljungdahl pathway, dicarboxylate-hydroxybutyrate cycle, methanogenesis, and denitrification, but apparently lack the merB and merA genes involved in CHHg reductive demethylation. Finally, we estimate that microbial degradation of soil CHHg contributes to 0.08-0.64 fold decreases in CHHg accumulation in rice grains across China (hazard quotient (HQ) decrements of 0.62-13.75%). Thus, our results provide insights into microorganisms and pathways responsible for CHHg degradation in soil, with potential implications for development of bioremediation strategies.