The kinetics of hydrogen photoproduction by adapted Scenedesmus

In our earlier work we have shown that hydrogen photoproduction by photosystem I of Scenedesmus does not require O2 evolution or cyclic photophosphorylation but must be due to non-cyclic electron flow from organic substrate(s) through photosystem I to hydrogenase, where molecular H2 is released. The kinetics of this reaction are rather complex, in that H2 photoproduction by Scenedesmus evidently occurs in two phases: a rapid initial phase which depends upon the dehydrogenation of a "pool" of H donors, and a later and slower second phase which is limited by the flow of electrons from fermentation. When adapted cells were incubated in the dark with an inhibitor (Cl-CCP or salicylaldoxime), the pool utilized by photosystem I gradually disappeared. However, the pool gave a rapid rate of hydrogen photoproduction when the adapted cells were illuminated immediately after adding the inhibitor. The rate at which the pool was utilized depended upon the light intensity and was not light-saturated at the highest intensity tested (3.4×10(3) μW cm(-2)).With light of at least medium intensity (1.67×10(3) μW cm(-2)), the pool was rapidly exhausted and the reaction became dependent upon the "leak" of electrons from fermentation. The size of the "leak" was found to depend upon the level of reduced organic compounds in the cell, since this process was depressed by starving the cells and was much enhanced by adding glucose or by growing the cells heterotrophically. A quantitative relationship was found between the amount of glucose added and the resulting stimulation of H2 photoproduction, in that one μmole of glucose gave about 0.5 μmole of H2 gas.