Oxygen stress drives overcompensation of carbon sources for enhanced polymer denitrification

Dissolved oxygen (DO) is traditionally considered as a strong inhibitor of denitrification. However, its impact on polymer-based denitrification, where carbon bioavailability is rate-limiting, remains poorly understood. Here, we investigated the response of a polycaprolactone (PCL)-supported biofilm system to long-term DO stress (2-8 mg/L). Contrary to conventional expectations, elevating DO from 2 to 8 mg/L significantly accelerated nitrate removal (from 8.06 to 10.50 mg N/L) rather than suppressing it. Stoichiometric modeling and metabolomic analysis revealed an oxygen-induced carbon release mechanism, where high DO stimulated the secretion of extracellular esterases and intensified the β-oxidation pathway, increasing polymer carbon release by 17.71 mg/L. This excess carbon overcompensated for aerobic consumption, effectively alleviating electron donor limitations. Metagenomics further confirmed a structural shift towards an oxygen-tolerant consortium, with significant enrichment of dual-function genera (e.g., Pseudoxanthomonas) and enhanced coupling of respiratory chain complexes (I-III). The biofilm achieved spatial decoupling, utilizing the outer aerobic layer for rapid hydrolysis and oxygen consumption to protect the inner anoxic denitrification zone. These findings overturn the strict anoxic requirement for denitrification, providing a robust strategy for advanced nitrate removal in oxygen-fluctuating tailwaters.