Microbially-induced calcium carbonate precipitation by a halophilic ureolytic bacterium and its potential for remediation of heavy metal-contaminated saline environments

Heavy metal contamination is widespread in the environment and has resulted in numerous adverse consequences for the ecosystem. Ureolytic microorganisms are known to immobilize heavy metals by co-precipitation with calcium carbonates, which are formed during microbial hydrolysis of urea. In this work, we described a novel halophilic ureolytic bacterium (Exiguobacterium sp. JBHLT-3) and examined the relationship between ureolytic-driven mineralization of CaCO3 and heavy metal mitigation. The growth of strain JBHLT-3 occurred at salinities between 3 and 12% with an optimum at 6%. This strain was capable of precipitating calcite and vaterite under non-growth conditions. Lead (Pb) was selected as a model heavy metal for this study and amended into the bio-precipitation systems at a starting concentration of 1 mM. The strain JBHLT-3 efficiently removed Pb from saline solutions via incorporation within microbially-induced calcium carbonates. Mineralogical analysis showed that calcite had higher uptake of Pb2+ than vaterite. The increasing solution salinity progressively increased the vaterite fraction in the bio-precipitates but reduced the removal efficiency of Pb2+. Nevertheless, the removal efficiency could be still as high as 89% in the cultures incubated in a 12% salinity medium. Our results highlight the positive role of halophilic ureolytic microbes in remediating heavy metal-contaminated saline environments.