Salinity-gradient stress driven microbial succession and cooperation toward enhanced nitrogen metabolism for treating electroplating tail wastewater employing combined anoxic-oxic process.

High salinity severely constrains biological nitrogen removal in electroplating tail wastewater, yet the ecological mechanisms driving microbial community assembly and functional succession during long-term acclimation remain elusive. Here, a hydrolytic acidification coupled two-stage anoxic/oxic bioreactor was operated under stepwise salinity acclimation (1‰-10‰) to treat actual electroplating tail wastewater. The system maintained robust performance, consistently keeping effluent chemical oxygen demand (COD) below 50 mg/L and satisfying industrial discharge standards of China for nitrogen (ammonium nitrogen (NH4+-N) < 8 mg/L and total nitrogen (TN) < 15 mg/L) (GB 21900-2008). Intriguingly, long-term salinity pressure significantly decreased total extracellular polymeric substances (EPS) and the protein-to-polysaccharide ratio (PN/PS), while simultaneously promoting the accumulation of compatible solutes (ectoine and glutamate) within the cells. This indicated that microorganisms reduced their reliance on EPS-mediated extracellular defenses, while intracellular osmotic adaptation was enhanced. Quantitative ecological modeling revealed that increasing salinity suppressed dispersal limitation while accelerating homogenizing dispersal and selection, but stochasticity remains an important factor shaping community succession during prolonged salt stress (Normalized Stochasticity Ratio (NST) decreased from 78% ± 8% at stage SS1 to 33% ± 7% at stage SS10). This environmental filtering selectively enriched salt-tolerant functional groups with potential complementary roles (principally Nitrosomonas, Nitrospira, and Thauera) and increased the proportion of positive co-occurrence associations (61.7%). Overall, this study demonstrates that progressive salinity acclimation fosters a resilient, functional complementarity potential microbiome capable of sustaining efficient wastewater purification, providing both mechanistic insights and a practical strategy for treating complex industrial wastewater.