Respiration-coupled redox metabolism supports Cr(VI) reduction by an indigenous Bacillus in soil-groundwater systems

Reliable in situ remediation of Cr(VI)-contaminated soil-groundwater systems remains challenging because introduced microorganisms often show poor persistence, competitiveness, and functional stability under site-specific subsurface conditions. Indigenous microorganisms may provide a more adaptable alternative, yet how their respiratory metabolism, ecological fitness, and community interactions support sustained Cr(VI) reduction and immobilization under field-relevant conditions remains insufficiently understood. Here, an indigenous Cr(VI)-reducing strain GS was isolated and evaluated in aqueous batch cultures, soil microcosms, and a pilot-scale in situ trial, supported by multi-omics and microbiome analyses. Strain GS maintained high Cr(VI) reduction across wide temperature and pH ranges in aqueous cultures. In soil microcosms (100-1000 mg kg^-1 Cr(VI)), strain GS bioaugmentation achieved 66-77% Cr(VI) reduction and shifted chromium from labile pools to more stable solid-phase fractions, indicating effective immobilization. Transcriptomic and metabolomic profiling indicated that Cr(VI) stress activated a respiration-coupled redox response, characterized by enhanced heme/porphyrin metabolism, increased respiration-associated redox activity, and strengthened carbon/redox metabolism. Community analyses identified Bacillus as a key taxon associated with Cr(VI) reduction. A 180-day field trial achieved rapid Cr(VI) depletion in soil and groundwater within 30-120 days and showed no rebound over 180 days. These findings support indigenous Bacillus-based in situ bioremediation as a sustainable strategy for Cr(VI)-contaminated soil-groundwater systems.