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

Denitrification is fragile to toxic substances, while currently there are few regulation strategies for toxic substance-stressed denitrification. This study proposed a combined bio-promoter composed of basic bio-promoter (cytokinin, biotin, L-cysteine, and flavin adenine dinucleotide) and phosphomolybdic acid (PMo12 ) to recover cadmium(II) (Cd(II)) stressed denitrification. By inhibiting 58.02% and 48.84% of nitrate reductase and nitrite reductase activities, Cd(II) caused all the influent nitrogen to accumulate as NO3 - -N and NO2 - -N. Combined bio-promoter shortened the recovery time by 21 cycles and improved nitrogen removal efficiency by 10% as the synergistic effect of basic bio-promoter and PMo12 . Basic bio-promoter enhanced antioxidant enzyme activities for reactive oxygen species clearance and recovered 23.30% of nicotinamide adenine dinucleotide for sufficient electron donors. Meanwhile, PMo12 recovered electron carriers' contents, increasing the electron transfer activity by 60.81% compared with self-recovery. Bio-promoters enhanced the abundance of Seminibacterium and Dechloromonas, denitrifiers positively correlated with rapid recovery of denitrification performance.