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

The influence of (NH4)(2)SO4 concentration and dilution rate (D) on actual and potential H-2 photoproduction has been studied in ammonium-limited chemostat cultures of Rhodobacter capsulatus B10. The actual H-2 production in a photobioreactor was maximal (approx. 80 ml h(-1) l(-1)) at D = 0.06 h(-1) and 4 mM (NH4)(2)SO4 However, it was lower than the potential H-2 evolution (calculated from hydrogen evolution rates in incubation vials), which amounted to 100-120 ml h(-1) l(-1) at D = 0.03-0.08 h(-1). Taking into account the fact that H-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-2 production by the H-2 accumulated in the gas phase. This is apparent from the non-linear kinetics of H-2 evolution in the vials or from its inhibition by the addition of H-2; initial rates were restored in both cases after the vials had been refilled with argon. The actual H-2 production in the photobioreactor at D = 0.06 h(-1) was shown to increase from approximately 80 ml h(-1) l(-1) to approximately 100 ml h(-1) l(-1) under an argon flow at 100 ml min(-1). Under maximal H-2 production rates in the photobioreactor, up to 30% of the lactate feedstock was utilised for H-2 production and 50% for biomass synthesis.