Cr(VI)-bioremediation mechanism of a novel strain Bacillus sp. Cr02 with simultaneous aerobic/anaerobic reduction for Cr(VI) from soil to groundwater.

Microbial remediation strategies have been developed to alleviate chromium (Cr) contamination in soil and groundwater. However, the complex redox conditions resulting from variations in oxygen content at different depths of the soil-groundwater system limit the ability of most microorganisms to achieve aerobic/anaerobic sustained bioreduction of Cr(VI) from topsoil to groundwater in contaminated industrial sites. This work screened a novel strain, Bacillus megaterium Cr02, investigated its unique Cr(VI) reduction mechanisms under aerobic and anaerobic conditions, and evaluated its performance in practical soil-groundwater remediation. The results showed that strain Cr02 could rapidly reduce Cr(VI) at an initial concentration of 100 mg/L in groundwater under both conditions. The ChrR enzyme was identified as a key player in aerobic Cr(VI) reduction, with molecular docking showing its hydrophobic interaction with Cr(VI). Conversely, the extracellular reducing metabolite cysteine became the major contributor to anaerobic Cr(VI) reduction. Moreover, their reduction products were further demonstrated to comprise soluble Cr(III) and Cr(OH) precipitates. In soil redox alternations, strain Cr02 stably converted > 90 % of Cr(VI) to residual chromium. These findings highlight the strain's ability to synergistically remediate soil and groundwater, positioning it as a promising candidate for the bioremediation of Cr pollution in dynamic oxygen-containing environments.