Selenium Metabolizing Capabilities of the Twelve Bacterial Strains Isolated from Urban Environmental Samples Not Contaminated with Selenium

The role of bacterial selenium metabolism in non-polluted environments remains underexplored within the selenium biogeochemical cycle. In this study, selenium-metabolizing bacteria were isolated from urban environmental samples not affected by elevated levels of toxic elements. Among twelve isolates, ten were identified as Citrobacter spp., while the remaining two were Scandinavium hiltneri and Klebsiella aerogenes. The Citrobacter isolates demonstrated high selenium-removal efficiency, removing over 95% of 5 mM selenium from the aqueous phase within one week. In contrast, S. hiltneri K24-1 and K. aerogenes K24-4 removed only 19% and 69%, respectively. A detailed investigation of five representative isolates, C. freundii K21-1, S. hiltneri K24-1, C. braakii K24-2, K. aerogenes K24-4, and C. freundii K24-5, revealed that Citrobacter spp. efficiently reduced selenate directly to elemental selenium, with minimal accumulation of selenite intermediates. These results highlight Citrobacter spp. as key selenium reducers and suggest their potential as bioindicators of selenium metabolic capacity in the environment.