Bridge-Mediated Electron Transfer: Bioinspired Redox Communication for Sustained Nitrite-Independent Anaerobic Ammonium Oxidation

The lack of nitrite (NO₂^-) in real wastewater severely limits the sustainable nitrogen removal of anaerobic ammonium oxidation (anammox). Although microbial extracellular electron transfer (EET) provides a new approach for NO₂^--independent anammox, the slow electron transfer at the microbe-material interface hinders its engineering application. Herein, this study proposes a strategy to enhance the electrical contact between microorganisms and insoluble acceptors through conductive bridging materials (e.g., Fe₂O₃ nanoparticles encapsulated by flavin-rich extracellular polymeric substances). Results showed that the interface between anammox bacteria (AMX1) and Fe₂O₃ exhibited a high electron flux (6.86 mA·cm^-2), considerably higher than all reports to date, achieving stable ammonium (NH₄⁺) removal of approximately 97.90% and operating continuously for over 150 days. Building on the efficient EET, Fe₂O₃ was further triggered into Fe²⁺/Fe³⁺ redox signaling for microbial metabolic coordination. Specifically, Fe²⁺ signals channeled reducing power into coenzyme A/biotin synthesis in symbiotic bacteria (VER2) and fed back to the carbon fixation enzyme (FC = 1.1-fold) of AMX1 through chemotaxis migration and cross-feeding, while Fe²⁺ was reconverted to Fe³⁺. The Fe³⁺ signals induced gene expression (Log₂FC > 0) of EET-associated proteins and simultaneously facilitated the conversion of electricity to critical chemical energy, accelerating the autotrophic growth of AMX1. In this way, anammox bacteria not only survived but also thrived in NO₂^--limited environments, with relative abundance increasing by 127.22% to sustain NH₄⁺ removal. This study offers a novel solution to the NO₂^- supply challenge in wastewater treatment, advancing industry toward carbon neutrality goals.