Resolving the electron bottleneck in denitrification for N₂O mitigation: The roles of algal-mediated metabolic...

Excessive nitrogen (N) discharge from agriculture and industry causes severe ecological harm. Moreover, conventional nitrogen removal processes often generate N₂O, adding to the environmental burden. While algal-bacterial consortium has emerged as a low-carbon alternative to address these issues, the bioenergetic mechanisms governing their N₂O-mitigation performance remain poorly understood. In this study, a Chlorella sorokiniana-Paracoccus denitrificans consortium was established to clarify the mechanism underlying enhanced N₂O mitigation. The consortium achieved superior total nitrogen removal (86.6%) while mitigating N₂O emissions by 67.7% compared with the bacterial monoculture system, effectively overcoming the metabolic limitations of denitrification. The consortium established a coordinated nitrogen-metabolic network, which not only upregulated assimilatory pathways but, crucially, enhanced the expression of downstream reductase genes to prevent intermediate accumulation. Comprehensive analysis revealed that the enhanced nitrogen transformation and N₂O mitigation in the MIC&BAC system were driven by a dual mechanism involving assimilation shunt and downstream denitrification reinforcement. In this mechanism, algal-bacterial coupling increased ATP and NAD(P)H availability and promoted nitrogen assimilation into biomass, thereby reducing the nitrogen flux entering the denitrification pathway. Meanwhile, improved reducing-equivalent supply and electron-transfer regulation strengthened terminal N₂O reduction to N₂. Crucially, we identified a key extracellular "redox capacitor" mechanism mediated by extracellular polymeric substances (EPS). The specific enrichment of humic/fulvic acid components and c-type cytochromes formed a redox-active interfacial network with enhanced charge-transfer capacity. The enhanced extracellular electron transfer (EET) process supported more electron delivery to downstream denitrifying reductases while minimizing N₂O accumulation. These findings provide new insights into the bioenergetic and electron-transfer mechanisms governing algal-bacterial symbiosis.