Actual and potential rates of hydrogen photoproduction by continuous culture of the purple non-sulphur bacterium Rhodobacter capsulatus

The influence of (NH^sub 4^)^sub 2^SO^sub 4^ concentration and dilution rate (D) on actual and potential H^sub 2^ photoproduction has been studied in ammonium-limited chemostat cultures of Rhodobacter capsulatus B10. The actual H^sub 2^ production in a photobioreactor was maximal (approx. 80mlh^sup -1^l^sup -1^) at D=0.06h^sup -1^ and 4mM (NH^sub 4^)^sub 2^SO^sub 4^. However, it was lower than the potential H^sub 2^ evolution (calculated from hydrogen evolution rates in incubation vials), which amounted to 100-120 ml h^sup -1^l^sup -1^ at D=0.03-0.08h^sup -1^. Taking into account the fact that H^sub 2^ production in the photobioreactor under these conditions was not limited by light or lactate, another limiting (inhibiting) factor should be sought. One possibility is an inhibition of H^sub 2^ production by the H^sub 2^ accumulated in the gas phase. This is apparent from the non-linear kinetics of H^sub 2^ evolution in the vials or from its inhibition by the addition of H^sub 2^; initial rates were restored in both cases after the vials had been refilled with argon. The actual H^sub 2^ production in the photobioreactor at D= 0.06h^sup -1^ was shown to increase from approximately 80mlh^sup -1^l^sup -1^ to approximately 100ml h^sup -1^l^sup -1^ under an argon flow at 100mlmin^sup -1^. Under maximal H^sub 2^ production rates in the photobioreactor, up to 30% of the lactate feedstock was utilised for H^sub 2^ production and 50% for biomass synthesis.[PUBLICATION ABSTRACT]