The quantum requirement for H2 production by anoxygenic phototrophic bacteria

Abstract

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 H2 production (QH2). Rates of H2 formation were determined by means of a Clark-type H2 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 H2 with Chlorobium vibrioforme, 7.5 with Rhodospirillum rubrum, and 23.2 with Ectothiorhodospira shaposhnikovii. The physiological efficiency, defined as the measured QH2 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 H2 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 H2 production but a similar quantum requirement. The energetic efficiency of H2 production by various biological and artificial systems is discussed.