Carbon-Mediated Rechargeable Operation for Light-Driven Ammonia Production Using Quantum Dot- Azotobacter vinelandii Hybrids

Integrating diazotrophic microorganisms with semiconductor nanomaterials enables nitrogen (N2)-to-ammonia (NH3) conversion under ambient conditions, yet most studies are evaluated using washed cells in carbon-free buffers, obscuring metabolic controls for scalable operation. Here we report that medium carbon status governs light-driven extracellular NH4+ accumulation and long-term production in a quantum dot (QD)-Azotobacter vinelandii hybrid. The hybrid exhibits increased membrane polarization under illumination, accompanied by elevated intracellular NADH/NAD+ and ATP, and NH4+ production is strongly inhibited by a protonophore that dissipates the membrane electrochemical gradient and blocks ATP synthesis, indicating ATP-dependent nitrogenase catalysis. In sucrose-rich medium, extracellular NH4+ accumulation remains low as fixed nitrogen is preferentially assimilated into biomass, while excess carbon is stored as polyhydroxybutyrate (PHB). Upon sucrose depletion, PHB is mobilized and extracellular NH4+ accumulation becomes apparent. Using this carbon switch, intermittent sucrose feeding (1 g L-1) during 12 h dark intervals enabled rechargeable cycling and increased cumulative NH4+ production by ∼2.4-fold over 108 h. These results link QD photoredox input to mediator-assisted electron transfer, cellular bioenergetics, and carbon reserve metabolism and fixed-nitrogen allocation, providing design principles for semiconductor-diazotroph platforms.