The contribution of PS II to H
2 photoproduction by several unicellular green algae was measured both when O
2 evolution and photophosphorylation were unimpaired and also when these processes had been eliminated by Cl-CCP. As judged by the effects of DCMU, a PS II contribution was found under both sets of experimental conditions for several strains of
Chlorella, Ankistrodesmus and
Scenedesmus. However, H
2 photoproduction by
Chlamydomonas moewusii was insensitive to DCMU and thus was entirely due to PS I. With cells treated with Cl-CCP, the relative amount of PS II contribution varied from zero in autotrophically grown
Chlamydomonas reinhardii, to
20% in photoheterotrophically grown and
50% in autotrophically grown cells of
Ankistrodesmus braunii, Chlorella fusca, Chlorella vulgaris and
Scenedesmus obliquus. The dehydrogenation of reduced H-donors by PS II of
Scenedesmus treated with Cl-CCP showed the same biphasic kinetics previously described for H
2 photoproduction by PS I of this alga.
These studies were supported by contract No. AT-(40-1)-2687 from the U.S. Atomic Energy Commission.