THE QUANTUM REQUIREMENT FOR H-2 PRODUCTION BY ANOXYGENIC PHOTOTROPHIC BACTERIA

Cultures from three groups of phototrophic bacteria (green sulphur bacteria, purple non-sulphur bacteria and purple sulphur bacteria) were investigated in respect of the quantum requirement for H-2 production (Q(H2)). Rates of H-2 formation were determined by means of a Clark-type H-2 electrode with dense suspensions of whole cells and malate, acetate or sulphide as electron donor. At low light intensities (0-3 W.m-2 of monochromatic light) the minimum quantum requirement was 8.6 quanta per H-2 with Chlorobium vibrio-forme, 7.5 with Rhodospirillum rubrum, and 23.2 with Ectothiorhodospira shaposhnikovii. The physiological efficiency, defined as the measured Q(H2) compared to the theoretical value calculated from the energy requirement of the physiological processes involved, was 94%, 88%, or 28%, respectively. With increasing light intensities the quantum requirement also increased. Various hydrogenase inhibitors either inhibited both H-2 uptake and production (Cu2+, NO), or affected neither of these activities (CO, C2H2, N2O, ethylenedinitrilotetraacetate). An uptake hydrogenase-deficient Hup--mutant of R. rubrum had higher rates of net H-2 production but a similar quantum requirement. The energetic efficiency of H-2 production by various biological and artificial systems is discussed.