Sulfamethoxazole disrupts denitrification pathway and enhances nitrous oxide (N₂O) accumulation through enzymes and...

The pervasive occurrence of sulfamethoxazole (SMX) in wastewater poses serious challenges for biological nitrogen removal and contributes to greenhouse gas emissions. This study systematically elucidates the inhibitory mechanisms of SMX on denitrification performance and nitrous oxide (N₂O) dynamics through both short-term and long-term exposure experiments in sequencing batch reactors (SBRs). Short-term SMX exposure markedly inhibited NO₂^--N and N₂O reduction, with N₂O reduction rates decreasing by 37.63% at an SMX concentration of 20 mg/L. Under long-term SMX exposure, a broader deterioration of denitrification performance was observed with NO₃^--N, NO₂^--N, and N₂O-N removal efficiencies declining by 53.4%, 48.8%, and 15.9%, respectively. This result revealed that SMX inhibited extracellular polymeric substance (EPS) formation, disrupted sludge granule integrity. Meanwhile, SMX inhibited the activity of the electron transport system (ETSA), weakening the electron transfer to the terminal enzymes, which further suppressed the function of nitric oxide reductase (NOR), leading to impaired electron flow. Microbial community analysis indicated that a selective enrichment of antibiotic-resistant genera (e.g., Thauera) and depletion of key denitrifiers (e.g., Pseudomonas), which collectively drove incomplete denitrification and N₂O accumulation. Overall, these findings highlight the dual risk of compromised nitrogen removal and enhanced N₂O emissions under antibiotic stress, providing mechanistic insight for mitigating the environmental impacts of antibiotic pollution in wastewater treatment systems.