Fe₃O₄@PVA-regulated electron transfer and microbial communication intensify denitrification in algae-bacteria...

Low carbon-to-nitrogen (C/N) wastewater constraints biological nitrogen removal due to limited electron transfer and unstable redox microenvironments, particularly in algae-bacteria symbiotic systems (ABSS) where photosynthetically generated oxygen suppresses anaerobic pathways. Here, a Fe₃O₄@PVA composite was employed to regulate electron transfer and microbial communication, thereby intensifying denitrification under carbon-limited conditions. Five sequencing batch reactors were operated for 40 days with varying Fe₃O₄@PVA dosages to systematically evaluate nitrogen transformation performance, microbial community assembly, and functional gene expression. At an optimal dosage of 20 g L^-1 Fe₃O₄@PVA, the system achieved a total nitrogen (TN) removal efficiency of 98.4 ± 0.6% during stable operation, alongside a simultaneous nitrification-denitrification (SND) efficiency of 99.32%. However, excessive loading led to microbial inhibition and subsequent performance deterioration. Mechanistic analyses revealed that Fe₃O₄@PVA acted as a sustained electron mediator and redox regulator, promoting Fe²⁺ release, reshaping extracellular polymeric substance composition, and has the potential to enhance quorum sensing-mediated microbial interconnectivity. These effects collectively facilitated the formation of localized hypoxic niches, strengthened denitrification, thereby establishing an appropriate microenvironment for anammox bacteria. This study provides mechanistic insights into material-assisted regulation of electron transfer and microbial interactions, offering an effective process intensification pathway for nitrogen removal in electron-transfer-limited biological systems.