It has been established that electrons are derived from water upon photochemical oxidation by PSII or so-called water plastoquinone oxidoreductase (PQOR) and are trans­ferred to the [Fe]-hydrogenase, leading to the photosynthetic hydrogen production in the direct photolysis process. Apart from the previously described PSII-dependent hydrogen production, catabolism of endogenous substrate and the associated oxidative carbon metab­olism in green algae may generate electrons for the photosynthetic systems (Gfeller and Gibbs, 1984; Melis, 2002).

Electrons generated from such an endogenous substrate catabolism flow into the PQ pool between photosystems PSI and PSII (Stuart and GaDron, 1972; Godde and Trebst, 1980). An NADPH-PQOR that has been ascertained in vascular plant chloroplasts supplies elec­trons to the PQ pool (Shinozaki et al., 1986; Kubicki et al., 1996; Neyland and Urbatsch, 1996; Endo et al., 1998; Field et al., 1998; Sazanov et al., 1998). Daylight assimilation by PSI and the associated electron transfer raise the redox potential of these electrons to the equivalent level of ferredoxin and the [Fe]-hydrogenase. Functioning as the terminal electron acceptor, the hydrogen ions (protons) would lead to the production of molecular hydrogen (Gfeller and Gibbs, 1984; Bennoun, 2001; Gibbs et al., 1986). It has been found that in the presence of the PSII inhibitors 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), which impede photosynthetic electron flow from PSII to the PQ pool, the process generates molecular hydrogen and carbon dioxide in a stoichiometric ratio of 2 to 1 (Bamberger et al., 1982). Thus, following a dark incubation of the culture under anaerobic conditions and the ensu­ing induction of the [Fe]-hydrogenase, considerable rates of hydrogen generation can be captured upon illumination of the algae in the presence of DCMU (Happe et al., 1994; Florin et al., 2001).