Enhanced anti-toxicity memory of Cr(VI)-4-CP stressed denitrification by bio-promoter: Microbial cooperation and multi-path electron transfer drive toxics transformation-migration.

Coexisting heavy metals and organic pollutants in industrial wastewaters posed synergistic inhibition to denitrification by activating dissimilatory nitrate reduction and disrupting electron supply-consumption balance. Taking Cr(VI) and 4-chlorophenol (4-CP) as representative pollutants, this study proposed a combined bio-promoter composed of growth factors and phosphomolybdic acid (PMo₁₂) to accelerate recovery and establish anti-toxicity memory under compound stress. The promoter restored over 90% nitrogen removal within 9 T and maintained 63.6% nitrogen removal under Cr(VI)-4-CP re-stress. Compared to first-stress, the recovered system reduced 37.44 mg/L more Cr(VI) and kept 4-CP below 5 mg/L, thus rapidly relieving Cr(VI)-4-CP toxicity and increasing the supply of direct electron donor nicotinamide adenine dinucleotide (NADH, 65.5%) and energy source adenosine triphosphate (ATP, 27.8%). Meanwhile, the enhanced extracellular polymeric substance (EPS) ensured 11.15 mg/g mixed liquid suspended solids (MLSS) more chromium immobilization with 97.9% distributed intercellularly, preventing Cr(VI) from invading cells and minimizing intracellular oxidative damage. The biofilm-fixed Mo (4.28 mg/g MLSS) shortened electron transfer distance to NO₃⁻-N, which, combined with a 17.3% increase in cytochrome (cyt.c), formed a new mode of multi-path electron transfer. Microbacterium with glucose-4-CP co-metabolism and denitrification functions contributed 13.0% of the recovered community, transforming glucose and 4-CP competitive metabolism into collaborative metabolism, further enhancing the anti-toxicity memory, and ensuring efficient denitrification performance.