Novel photoelectron-driven nitrate reduction in anammox granules using photosensitive semiconductor iron mineral for wastewater treatment.

The accumulation of nitrate byproducts and limited electron availability fundamentally constrain the efficacy of anaerobic ammonium oxidation (anammox) processes. While iron minerals regulate electron transfer, their potential to drive anammox via semiconductive photoexcitation remains underexplored. Here, we establish a novel "Photo-Chemo-Bio" strategy to overcome these thermodynamic bottlenecks using light-excited hematite (α-Fe2O3). Among tested minerals, hematite exhibited superior band-structure suitability, achieving a 3.65-fold photocurrent enhancement (4.06 μA·cm-2) upon bio-hybridization, facilitated by the active recruitment of photo-electrons via upregulated outer-membrane c-type cytochromes. Crucially, this photo-enhanced electron supply boosted the total nitrogen removal rate by 27.4% while suppressing nitrate yield by 42.8%. Kinetic analysis revealed a precise metabolic decoupling: solar irradiation did not accelerate ammonia oxidation but specifically diverted electron flow toward nitrate reduction pathways. Genome-resolved metagenomics unraveled the molecular basis of this synergy, identifying a "hardwired" cooperative network: flanking Desulfobacillus-like species, characterized by a specific metabolic truncation (absence of nor genes), act as obligate "net NO providers" to fuel the anammox core; concurrently, heterotrophic Casimicrobiaceae unexpectedly encode Photosystem II (psbA), functioning as auxiliary "energy antennas" to harvest photons. These findings demonstrate how mineral-microbe hybrids can orchestrate electron flux to close the nitrogen loop, offering a sustainable, carbon-free strategy for high-efficiency wastewater treatment.