Microbial selective response to photogenerated electrons stabilizes nitrogen removal in oxygenic photogranules under...

Efficient nitrogen removal from low carbon-to-nitrogen (C/N) ratio wastewater remains challenging due to insufficient reducing power and unstable microbial metabolism. Oxygenic photogranules (OPGs) enable aeration-free nitrogen removal through phototroph-heterotroph coupling, yet their performance deteriorates under low C/N conditions. Here, we integrated OPGs with a visible-light-responsive g-C₃N₄/TiO₂ photocatalyst to supply photogenerated electrons and examined electron-associated metabolic responses within the microbial consortium. The biohybrid system achieved enhanced nitrogen removal efficiencies of 75.2% and 69.4% at C/N ratios of 5 and 3, respectively, corresponding to volumetric nitrogen removal rates of 95.40 and 143.56 mg N/(L·d), following a single catalyst addition. Photoelectrochemical and biochemical analyses indicated that photogenerated electrons were efficiently transferred across a stable extracellular polymeric substances-mediated biotic-abiotic interface and redistributed via extracellular electron shuttles. Community-resolved metatranscriptomic analysis revealed that photogenerated electron-associated metabolic responses were non-uniform and preferentially linked to functionally prioritized populations and nitrogen transformation pathways. At C/N = 5, the input of photogenerated electrons was associated with enhanced assimilatory-denitrification coupling across cyanobacterial and heterotrophic populations, whereas at C/N = 3, photogenerated electron-associated responses were preferentially linked to energy-conserving nitrogen transformation pathways. Overall, these findings suggest that photogenerated electrons function as context-dependent redox regulators of microbial metabolism in OPGs, establishing a selective electron routing paradigm that enables adaptive allocation of reducing power at low C/N ratios and stabilizes solar-driven nitrogen removal.