Unveiling in situ methylmercury production and degradation in aquaculture sediments: Transformation rates, functional genes and microbial methylators.

Methylmercury (MeHg) is recognized as a deleterious neurotoxin with the traits of biomagnification through the food chain and accumulation in edible aquatic products. However, the in situ production of MeHg in aquaculture environments has not been well understood. Herein, the sediments were collected from aquaculture ponds with different rearing operations. Isotope-based tracer analysis showed that Hg methylation and MeHg demethylation rate constants in the aquaculture sediments were 0.001-0.022 d-1 and 0.11-0.40 d-1, respectively. Most of bacterial Hg methylators (> 97.0 %) in aquaculture sediments were assigned to Firmicutes and Actinobacteria phyla. Four functional genes responsible for Hg transformation (hgcAB and merAB) could be detected in the aquaculture sediments. In particular, Hg methylation rate constants were positively and significantly correlated with the levels of hgcAB genes (p < 0.05). Inhibitive reagent addition assays and correlation analysis consistently demonstrated that sulfate-reducing bacteria (SRB) were the main methylators in aquaculture sediments, and antibiotic use could fortify the resistance of Hg methylators to antibiotics. These findings suggest that the in situ production of MeHg in aquaculture sediments may be effectively reduced via inhibiting SRB activities, and both hgcAB genes are useful markers of MeHg production in aquaculture environments.