Distribution of Mercury in Shrimp Ponds and Volatilization of Hg by Isolated Resistant Purple Nonsulfur Bacteria

The aims of this study were to investigate the presence of mercury (Hg) contamination in shrimp ponds in the south of Thailand and to isolate Hg-resistant purple nonsulfur bacteria (PNSB). Contamination by total mercury (HgT) in water and sediment samples ranged from <0.0002 to 0.037 mu g/L and from 30.73 to 398.84 mu g/kg dry weight. In all water and sediment samples, the concentration of Hg-T was less than the Thai, Hong Kong, and Canadian standard guidelines. Of the Hg-resistant PNSB, six strains detoxified Hg2+ by volatilization to Hg-0 using their mercuric reductase enzyme. The ability of PNSB to resist Hg2+ in aerobic dark conditions was better than in microaerobic light, and this corresponded with their Hg reductase activities (dark condition 15.75, 12.62, and 12.16 U/mg protein for strains SSW15-1, SRW1-5, and SSS2-1, respectively). The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values were the same under both incubating conditions at 2.40 mg/L for SRW1-5 and 1.60 mg/L for SSW15-1. However, both values under light condition of SSS2-1 were 3.20 mg/L while under dark-condition MIC and MBC values were 3.20 and 4.00 mg/L. The half maximal inhibitory concentration (IC50) values of Hg2+ on strains SSS2-1, SRW1-5, and SSW15-1 under dark and light conditions were 2.16, 1.23, and 0.90; and 1.66, 1.11, and 0.80 mg/L, respectively. They were identified using 16S ribosomal RNA (rRNA) genes establishing that SSS2-1 and SSW15-1 were Afifella marina, while SRW1-5 was Rhodovulum sulfidophilum. These strains can potentially be used to treat Hg-contaminated shrimp ponds.