Application of Microbial Transformation to Remediate Hg-Contaminated Water: Strain Isolation and Laboratory Microcosm Study

The groundwater at an abandoned industrial site located in Southern Taiwan has been polluted by mercury (Hg), which endangers human health and ecosystems. The objectives of this study were to (1) isolate Hg- and salt-tolerant microbial strains from the Hg-polluted site, (2) perform microcosm tests to evaluate the efficiency of Hg2+ reduction using the isolated strains, (3) determine the potential for enhancing the microbial reduction of Hg2+ using different media, and (4) apply a quantitative real-time polymerase chain reaction (PCR) technique to quantify and correlate the variations in merA (mercury reductase) concentrations during the Hg reduction process. Three bacterial strains were isolated and designated B7, A5, and A6, and these strains were able to grow well in media with 3.5% salinity. Of these strains, B7 had a salinity resistance that was greater than those of the other two strains. Strains B7 and A6 displayed acceptable Hg resistance, and they could sustain a Hg2+ shock as high as 100 mg/L without significant inhibition in nutrient broth (NB) medium. The 16s rRNA sequencing of the strains revealed that Strain B7 was closely related to Enterobacter cloacae, and Strains A5 and A6 were associated with Pseudomonas sp. The results of merA gene quantification analyses revealed that the merA gene of the three strains could be induced by increased Hg2+ concentrations. Up to 92% Hg2+ removal was obtained in microcosms inoculated with Strain B7 and incubated in Luria-Bertani medium, given an initial Hg2+ concentration of 120 mg/L. The results indicate that there is potential to use the isolated Hg-reducing strains in the development of on-site or in situ bioremediation technologies for practical remediation of Hg-contaminated groundwater.