Occurrence of methylmercury in aerobic environments: Evidence of mercury bacterial methylation based on simulation experiments

Abstract

Methylmercury (MeHg) is mainly produced by anaerobic δ-proteobacteria such as sulfate-reducing bacteria (SRB). However, mercury bio-methylation has also been found to occur in the aerobic soil of the Three Gorges Reservoir (TGR). Using γ-proteobacterial TGR bacteria (TGRB) and δ-proteobacterial Desulfomicrobium escambiense strains, the efficiency of mercury methylation and demethylation was evaluated using an isotope tracer technique. Kinetics simulation showed that the bacterial Hg methylation rate (km) of TGRB3 was 4.36 × 10−9 pg·cell−1·h−1, which was significantly lower than that of D. escambiense (170.74 ×10−9 pg·cell−1·h−1) under anaerobic conditions. Under facultative and/or aerobic conditions, D. escambiense could not survive, while the km of TGRB3 were 0.35 × 10−9 and 0.29 × 10−9 pg·cell−1·h−1, respectively. Furthermore, the bacterial MeHg tolerance threshold of TGRB3 was 3.47 × 10−9 pg·cell−1, which was 98.6-fold lower than that of D. escambiense under anaerobic conditions. However, the MeHg tolerance threshold of TGRB3 remained at 0.50–0.52 × 10−9 pg·cell−1 under facultative and/or aerobic conditions. Notably, bacterial Hg methylation rates (km) were higher than the corresponding bacterial MeHg demethylation rates (kd1). These results establish the contribution of some aerobic and/or facultative anaerobic bacteria to net environmental MeHg production in terrestrial ecosystems and provide a novel understanding of the biogeochemical cycle of MeHg. SynopsisHg methylation of facultative and/or aerobic bacteria may contribute to the net production of environmental methylmercury in terrestrial ecosystems.