Enhancement of bio-promoters on Cr(VI) inhibited sulfur-driven denitrification: repairing damage, accelerating electron transfer, and reshaping microbial collaboration

Proposing recovery strategies to recover heavy-metal-inhibited sulfur-driven denitrification and revealing recovery mechanisms can provide technical support for the stable operation of bio-systems. Compared with self-recovery, the basic bio-promoter (L-cysteine, biotin, and cytokinin) and mediator-promoter (basic bio-promoter and AQDS) proposed in this study effectively shortened the recovery time of NO3--N reduction (18 and 21 cycles) and NO2--N reduction (42 and 27 cycles) in 20 and 40 mg/L Cr(VI) inhibited sulfur-driven denitrification. Mediator-promoter repaired microbial damage by promoting intracellular chromium efflux. Two promoters quenched the accumulated ROS and O2- by stimulating the secretion of CAT, POD, and SOD, thereby reaching equilibrium in oxidative-antioxidant system. Basic bio-promoter and mediator-promoter quickly restored S2O32- oxidation to ensure sufficient electron donors and improved electron transfer rate by increasing cyt.c contents. There is a positive correlation between Thiobacillus (sulfur-oxidizing bacterium) and Simplicispira (denitrifying bacterium), facilitating rapid denitrification recovery and long-term stable operation of recovered systems.