Metabolic Pathways for Photobiological Hydrogen Production by Nitrogenase- and Hydrogenase-containing Unicellular Cyanobacteria Cyanothece

Current biotechnological interest in nitrogen-fixing cyanobacteria stems from their robust respiration and capacity to produce hydrogen. Here we quantify both dark-and light-induced H-2 effluxes by Cyanothece sp. Miami BG 043511 and establish their respective origins. Dark, anoxic H-2 production occurs via hydrogenase utilizing reductant from glycolytic catabolism of carbohydrates (autofermentation). Photo-H-2 is shown to occur via nitrogenase and requires illumination of PSI, whereas production of O-2 by co-illumination of PSII is inhibitory to nitrogenase above a threshold pO(2). Carbohydrate also serves as the major source of reductant for the PSI pathway mediated via nonphotochemical reduction of the plastoquinone pool by NADH dehydrogenases type-1 and type-2 (NDH-1 and NDH-2). Redirection of this reductant flux exclusively through the proton-coupled NDH-1 by inhibition of NDH-2 with flavone increases the photo-H-2 production rate by 2-fold (at the expense of the dark-H-2 rate), due to production of additional ATP (via the proton gradient). Comparison of photobiological hydrogen rates, yields, and energy conversion efficiencies reveals opportunities for improvement.