Advantages of partial denitrification-anaerobic ammonium oxidation system under sulfamethoxazole stress: Adaptive mechanisms and synergistic metabolism.

The widespread use of antibiotics has led to their persistence in aquatic environments, posing serious challenges to biological treatment systems. This study systematically compared the performance and adaptive mechanisms of partial denitrification (PD)/anaerobic ammonium oxidation (anammox) and single anammox systems under long-term sulfamethoxazole (SMX) stress over 193 days. At an influent SMX concentration of 3 mg/L, the PD/anammox system retained 75% of its initial total inorganic nitrogen (TIN) removal efficiency, significantly higher than that of the single anammox system (49%). The PD/anammox achieved an SMX degradation efficiency of 80%, substantially exceeding that of the single system (39%). Metagenomic analyses revealed higher abundances of key nitrogen metabolism genes (hzs, hdh, narG/H/I, napA/B, nirK/S) and SMX degradation genes (sadA, sadC, tmoABCDEF, dmpB/D) in the PD/anammox system. The enhanced performance was closely associated with the enrichment of the denitrifying microbiome (e.g., Thauera, Zoogloea, unclassified_f_Rhodocyclaceae), which provided a stable nitrite supply and carried SMX degradation genes. Both systems relied on extracellular polymeric substances (EPS) as a protective barrier under low SMX stress (1 mg/L). Under higher SMX concentrations (>1 mg/L), the PD/anammox system exhibited dynamic enrichment of sulfonamide resistance genes (sul1). These results demonstrated the superiority of the PD/anammox system over the single anammox system. The combined effects of a diverse microbiome, multi-level stress-response mechanisms involving EPS and antibiotic resistance genes, and efficient functional gene expression make PD/anammox a robust and promising technology for the treatment of antibiotic-containing wastewater.