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, we found that mercury bio-methylation also occurred in aerobic soil of the Three Gorges Reservoir (TGR). Applied the γ-proteobacterial TGRB and δ-proteobacterial Desulfomicrobium escambiense strains, we evaluated the efficiency of mercury methylation and demethylation using isotope tracer technique. The kinetics simulation showed that the bacterial Hg methylation rate (km ) of TGRB3 was 4.36×10-9pg·cell-1·h-1, which was significantly lower than that of D. escambiense (170.74×10-9pg·cell-1·h-1 ) under the anaerobic condition. Under the facultative and/or aerobic conditions, the D. escambiense could not survive. But, the k m of TGRB3 still were 0.35×10-9 and 0.29×10-9pg·cell-1·h-1, respectively. Furthermore, the bacterial MeHg tolerance threshold of TGRB3 was 3.47×10-9pg·cell-1 , which was 98.6 times lower than of D. escambiense in the anaerobic condition. But the MeHg tolerance threshold of TGRB3 remained at 0.50-0.52×10-9 pg·cell-1 in the facultative and/or aerobic conditions. Notably, the bacterial Hg methylation rates (km ) were higher than the respectively bacterial MeHg demethylation rates (kd ). 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 hazardous material biogeochemical cycle.

Keywords: mercury, methylation/demethylation, RaouLtella terrigena TGRB3, Desulfomicrobium escambiense CGMCC 1.3481, isotopic tracers