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

Summary 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.The following abbreviations were used CI-CCP carbonyl cyanide m-chlorophenylhydrazone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DCPIP dichlorophenol-indophenol - MR methyl red - PMS phenazine methosulfate - PS photosystem This work was supported by contract AT-(40-1)-2687 from the U.S. Atomic Energy Commission to Professor H. Gaffron.