Photoproduction of hydrogen by photosystem I of Scenedesmus

Anaerobically adapted and illuminated Scenedesmus evolves molecular hydrogen from endogenous organic compounds. This photoproduction of H2 does not require photosystem II, since 5x10(-6) M DCMU, which inhibited normal photosynthesis almost completely, did not significantly inhibit the photoevolution of H2. The relative efficiencies in far-red light of photosynthesis, photoreduction and H2 production were determined. Photohydrogen evolution was comparatively the most efficient of these three processes. Three mutants of Scenedesmus (isolated and characterized by Dr. N. I. Bishop) were also tested. Mutant PS-50, which lacks cytochrome 552, did not photoproduce H2. Mutant No. 11, blocked in photosystem II, showed rates of H2 production comparable to those of the wild type. Cl-CCP, an uncoupler of photophosphorylation, caused an apparent stimulation of H2 production by mutant No. 11 and wild-type cells. Mutant No. 8, which is partially blocked in photosystem I, showed a diminished photohydrogen production which was inhibited by Cl-CCP. These results suggest that photoproduction of hydrogen by photosystem I is due either to cyclic photophosphorylation, which supplies energy needed for a dark, H2-yielding reaction, or to a more direct photooxidation of organic compounds by the photosynthetic electron transfer chain.