Hydrogen production by photosystem I of Scenedesmus: Effect of heat and salicylaldoxime on electron transport and photophosphorylation.

Photosynthesis, photoreduction, the p-benzoquinone Hill reaction, and glucose uptake by whole cells, as well as cyclic photophosphorylation (with PMS) by chloroplast particles were strongly inhibited by 10(-2) M salicylaldoxime or by heating whole cells for 1-2 min at 55°. In contrast, H2 photoproduction by whole cells of mutant No. 11 and wild type Scenedesmus and PS I-mediated MR reduction by chloroplast particles were either stimulated or not significantly inhibited by these agents. H2 production by mutant No. 8 was slightly depressed by salicylaldoxime. DCMU inhibited H2 photoproduction with 10(-2) M salicylaldoxime approximately 20%, indicating some contribution of electrons by endogenous organic compounds to photosystem II between the O2-evolving mechanism and the DCMU-sensitive site. We conclude that photohydrogen production by PS I of Scenedesmus does not require cyclic photophosphorylation but is due to non-cyclic electron flow from organic substrate(s) through PS I to hydrogenase where molecular H2 is released.