Biohybrid photoheterotrophic metabolism for significant enhancement of biological nitrogen fixation in pure microbial cultures

Abstract

We induce the coating of biocompatible cadmium sulfide (CdS) nanoparticles (NPs) on the living cell surface of a versatile photoheterotrophic bacterium, Rhodopseudomonas palustris. The photo-induced electrons from the CdS NPs significantly improve the biological nitrogen fixation in pure cultures of R. palustris, as shown by increased nitrogenase activity, additional H₂ evolution, elevated reducing equivalents, and increased intracellular ammonia and L-amino acids. As a result, an additional 153% of solid biomass is accumulated by the biohybrid cells, with outstanding photosynthetic efficiency of 6.73% and a nearly unaffected malate usage efficiency of 0.06 g h⁻¹. The number of NPs and the cross-membrane interface both play important roles in the efficient generation and transduction of electrons. The biohybrid cells continuously fix N₂ when sufficient N is available, thus revealing excessive reducing power. The Calvin cycle also contributes 28.1% to the additional solid biomass in the presence of available CO₂. The CdS-coated photoheterotrophic cells exhibit excellent practical feasibility with an industrial waste carbon source under a solar/dark cycle. This study provides a facile and expandable strategy for other studies of visible-light-driven ambient N₂ fixation and advanced solar-to-chemical conversion.