Preserving Denitrification in High-Dissolved-Oxygen Environments: A Novel Sulfur–Polycaprolactone–Polyurethane Sponge Composite Carrier with Multi-Omics Insights

Abstract

To address the denitrification challenges in the high-dissolved-oxygen (DO) aquatic environment system caused by the secondary effluent of wastewater treatment plants, this study innovatively developed a sulfur–polycaprolactone (S0-PCL) dual-electron donor composite carrier embedded in polyurethane sponge to mitigate high DO stress. The system demonstrated remarkable resilience, with a denitrification efficiency decline of only 11.3% under a DO gradient (0–6.5 mg/L), significantly outperforming previously reported efficiency losses exceeding 90%. Metatranscriptomic analysis revealed that the millimeter-scale pores in the sponge created potential anoxic microzones, providing a microenvironment conducive to maintaining key denitrification gene expression. The anoxic stage was dominated by the sulfur-autotrophic denitrification (SAD) microorganism Sulfurovum (abundance 68.1%, expression 55.1%). In high-DO conditions, Alicycliphilus (55.3, 41.1%) and Paracoccus (31.6%, 31.8%) synergistically drove denitrification, while Hyphomonas (8.7%, 24.1%) assisted in DO and organics consumption. Gene expression data further revealed that Alicycliphilus hydrolyzed PCL to enter the tricarboxylic acid cycle, while Paracoccus simultaneously utilized S0 for SAD, establishing a dual-electron donor transfer pathway to sustain denitrification activity. The S0-PCL–polyurethane sponge provided resilience via oxygen isolation, sustained carbon release, and metabolic diversification, enabling microbial and functional gene reconfiguration. This study provides a novel technological pathway for deep nitrogen removal in oxygen-rich environments.