Bio-promoter mediated denitrification recovery from Cd(II) stress: microbial activity resilience, electron behavior enhancement and community collaboration evolution

Biological denitrification was fragile to toxic substances, while currently, there are few regulation strategies for toxic substance-stressed denitrification. This study proposed three bio-promoters (basic bio-promoter composed of cytokinin, biotin, L-cysteine, and flavin adenine dinucleotide (FAD); phosphomolybdic acid, PMo12; and combined bio-promoter composed of basic bio-promoter and PMo12) to recover denitrification performance after Cd(II) stress. Through inhibiting 58.02% and 48.84% of nitrate reductase (NR) and nitrite reductase (NIR) activities, Cd(II) caused all the influent nitrogen to accumulate in the system as NO3--N and NO2--N. Combined bio-promoter performed the best effect compared with self-recovery, shortening 21 recovery cycles and improving 10.36% nitrogen removal efficiency due to the synergistic effect of the basic bio-promoters and PMo12. Basic bio-promoter enhanced the activities of peroxidase (POD) and catalase (CAT) for reactive oxygen species (ROS) clearance, accelerating cell membrane remediation and promoting microbial detoxification to complete self-remediation. Basic bio-promoter significantly recovered 23.30% of nicotinamide adenine dinucleotide (NADH), ensuring the electron supply for NO3--N reduction. Meanwhile, PMo12 recovered the content of FAD, flavin mononucleotide (FMN), and cytochrome c (cyt.c), enhancing 106.48% of electron transfer activity compared with self-recovery. Bio-promoters significantly enhanced the relative abundance of Seminibacterium and Dechloromonas, positively correlated denitrifiers, thus facilitating rapid recovery of denitrification performance.