[Identification of a Facultative Bacterium Strain with the Ability to Methylate Mercury Under Both Aerobic and...

A strain with the ability to methylate mercury under both the aerobic and anaerobic conditions was isolated from soil of the water-level-fluctuation-zone in the Three Gorge Reservoir in Shibaozhai Village, Zhongxian Country, Chongqing, China (E108°12'3″ and N30°24'36″). The soil was classified as Purple soil with a pH of 7.97 (0-20 cm depth). The isolation was performed under 1.0 mg·L^-1 HgCl₂ conditions. After its morphological and physiological characterization, and its phylogenetic analysis using 16S rDNA gene sequence, the strain was identified as Pseudomonas fluorescens sp., and named as Pseudomonas fluorescens XD-MeHg-B₂ (GenBank accession number: KU954349). On one hand, at 30℃ under aerobic condition, the concentration of methylmercury (MeHg) in the PBS (phosphate buffer saline) solution, which was inoculated with 1×10¹¹ cfu·mL^-1 suspension of P. fluorescens XD-MeHg-B₂ and an initial Hg²⁺ of 200 ng·L^-1, was exponentially increased to 1.22 ng·L^-1±0.15 ng·L^-1 after 60 min incubation and then approached to the maximum of 3.85 ng·L^-1±0.33 ng·L^-1 160 min after incubation. The largest mercury methylation rate was 1.93%. On the other hand, at 30℃ under anaerobic condition, the concentration of MeHg in the PBS solution, which was also inoculated with 1×10¹¹ cfu·mL^-1 suspension of P. fluorescens XD-MeHg-B₂ and an initial Hg²⁺ of 200 ng·L^-1, was 2.86 ng·L^-1±0.73 ng·L^-1 and the largest mercury methylation rate was 1.43% 180 min after incubation. As a result, P. fluorescens XD-MeHg-B₂ showed its ability to methylate mercury under both aerobic and anaerobic conditions while with a comparatively hysteretic and lower ability of mercury methylation. These results demonstrated that P. fluorescens XD-MeHg-B₂ could be a promising candidate for further studies on mercury biogeochemical cycle, particularly under dry-wet alternative conditions.