Photocatalytic Fe3O4@CDs Drives Nitrite-Independent extracellular respiration of anammox via efficient bidirectional electron transfer.

Anaerobic ammonium oxidation (Anammox) process relying on extracellular electron transfer (EET) overcomes nitrite scarcity in practical wastewater, but inefficient EET rate constrains nitrogen removal. Herein, Fe3O4 with carbon dot shell (Fe3O4@CDs) was self-assembled with anammox bacterial as a photocatalytic hybrid. Building on bacteria inward uptake of photogenerated electrons from CDs and outward transfer of respiratory electrons to Fe3O4, a bidirectional electron pathway was established firstly. This novel route achieved efficient nitrite-independent Anammox, enabling direct and rapid removal of 80 mg/L NH4+-N. To elucidate efficient nitrogen removal essence, metagenomics was employed to reveal photogenerated electrons fate and their mediated nitrogen metabolism mechanism. Results showed that, following assembly via -PO bonds, bacteria internalized CDs photogenerated electrons (0.82 µA/cm) into menaquinone (MQ) pool. This influx activated energy-generation route constructed by bc1 and Rnf enzymes, increasing intracellular ATP level by 3.36---6.51-fold. Consequently, energy drove electrons pumping from MQ pool to cytochrome c, followed by transport outward via CDs, pili and flavin, amplifying electrons eflux by 1.77-fold. Such efflux generated MQ pool electron vacancies, which were efficiently replenished by electrons from hydrazine synthase- and hydrazine dehydrogenase-catalyzed NH4+-N oxidation to N2 without nitrite. Underpinned by the photogenerated electron-driven EET process, Anammox bacteria was enriched from 25.26% to 48.02%, thus sustaining a total nitrogen removal efficiency of > 97% for over 80 days-far exceeding the performance of existing system. This technology provides an efficient and sustainable theoretical framework for the application of anammox in practical wastewater treatment.