Engineering interfacial electron flux in transition metal-microbe systems for anaerobic nitrogen transformation.

Anaerobic nitrogen transformation under electron-limited conditions, including low C/N and low-temperature environments, suffers from insufficient electron flux, compromising kinetics, pathway completion, and N2 selectivity. Conventional carbon supplementation is costly and unsustainable, driving demand for alternative electron supply. Photo-driven transition metal-microbe hybrids enable regulated interfacial electron delivery. This review establishes interfacial electron flux as a governing concept for the engineering of electron-limited nitrogen transformation. Bidirectional electron exchange couples microbial hole scavenging with photoelectron injection: microbial electrons consume photogenerated holes and thereby help sustain photocatalytic activity, while photoexcited electrons provide reducing equivalents for nitrate, nitrite, and downstream nitrogen intermediates. This coupling also remodels denitrifying metabolism by modulating EPS conductivity, functional gene expression, energy conservation, and community assembly, thereby controlling pathway selectivity. Metal oxides, sulfides, and composites are compared for electron generation, interfacial coupling, biocompatibility, and long-term stability. Life cycle assessment is integrated to define practical boundaries, including material sustainability, electron utilization efficiency, N2O mitigation, and reactor feasibility. By linking material photochemistry to microbial nitrogen metabolism through interfacial electron flux, this review delivers a design framework for scalable, sustainable biohybrid technologies for carbon and energy limited anaerobic nitrogen transformation.