Deciphering bio-Pd0 mediated aerobic-intracellular/anaerobic-extracellular Cr(VI) reduction pathways

Chromate bioreduction is a promising bioremediation strategy, but its application in wastewater treatment is limited by the Cr(VI) biotoxicity, particularly due to the generation of Cr(VI)-induced reactive oxygen species (ROS). To mitigate these adverse effect, this study synthesized bio-Pd⁰ in Citrobacter freundii to modulate distinct Cr(VI) aerobic/anaerobic reduction pathways. With the incorporation of bio-Pd⁰, aerobic and anaerobic Cr(VI) specific removal rate increased by 18.1–21.2 and 51.5–87.9 times, respectively. The genomics, RT-qPCR, respiratory inhibition experiments, ROS levels and antioxidant enzyme activity assays demonstrated that bio-Pd⁰ enhanced intracellular Cr(VI) aerobic bioreduction mainly via upregulating the chromate multi-electron reductase (Cr-MER) (related with YieF, NfsA and NemA), together with the energy-dependent resistance system (antioxidant enzyme superoxide dismutase and catalase, efflux pump Emr, DNA repair protein RecG). Cr-MER and resistance system effectively reduced the generation and accumulation of ROS, respectively. For anaerobic conditions, bio-Pd⁰ facilitated hydrogenase-mediated short respiratory chain (S-chain) to reduce Cr(VI) extracellularly (related with HypE, RutF and OmcG), avoiding ROS generation and the energy consumption of the intracellular detoxification system, and maximized the energy utilization for Cr(VI) extracellular reduction. This study provides a new strategy for Cr(VI) bioreduction and elucidates the underlying mechanism.