Microbial reduction of [Co(III)-EDTA]⁻ by Bacillus licheniformis SPB-2 strain isolated from a solar salt pan.

Naturally stressed habitats are known to be repositories for novel microorganisms with potential bioremediation applications. In this study, we isolated a Co(III)-EDTA reducing bacterium Bacillus licheniformis SPB-2 from a solar salt pan that is exposed to constant cycles of hydration and desiccation in nature. Co(III)-EDTA generated during nuclear waste management process is difficult to remove from the waste due to its high stability and solubility. It is reduced form i.e. Co(II)-EDTA is less stable though it is toxic. This study showed that B. licheniformis SPB-2 reduced 1mM Co(III)-EDTA in 14 days when grown in a batch mode. However, subsequent cycles showed an increase in the reduction activity, which was observed up to four cycles. Interestingly, the present study also showed that Co(III)-EDTA acted as an inducer for B. licheniformis SPB-2 spore germination. Vegetative cells germinated from the spores were found to be involved in Co(III)-EDTA reduction. More detailed investigations showed that after Co(III)-EDTA reduction, i.e. Co(II)-EDTA complex was removed by B. licheniformis SPB-2 from the bulk liquid by adsorption phenomenon. The bacterium showed a D10 value (radiation dose required to kill 90% cells) of ∼250 Gray (Gy), which signifies the potential use of B. licheniformis SPB-2 for bioremediation of moderately active nuclear waste.