In this study the relationships between photosystem. (Ps) II dependent and independent pathways of H-2 photoproduction, cyclic electron transport around PS I, chloro- and mitorespiration, and transmembrane Delta pH were examined by inhibitor analysis in S deprived Chlamydomonas reinhardtii. The rate of non-photochemical reduction of plastoquinones in photosynthetic membranes was significantly diminished under starvation which may explain the minor contribution of the PS II independent pathway of H-2 photoproduction in starved cells. The suppressive effect of the herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea on the long-term H-2 photoproduction was shown to be entirely attributed to the inhibition of electron transport in PS II, whereas non-specific interactions did not take place. Ferredoxin-quinone reductase - dependent cyclic electron transport around PS I slowed down H-2 photoproduction more than two fold. This result was related to the competition between ferredoxin-quinone reductase and hydrogenase for the reduced ferredoxin and to the decrease in transmembrane Delta pH induced by the cyclic electron flow. The Delta pH gradient was shown to down regulate the PS II independent pathway of H-2 photoproduction in starved cells. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.