C02 + 4H20 + P > (CH20) + 02 + 3H20 (1) A mass spectrometer inlet and an oxygen electrode in the same vessel allowed the continuous recording of the gases exchanged (H2, C02, 02) by hydrogenase-containing anaerobically adapted Scenedesmius obliquus strain D, (Gaffron) and Chlorella fusca Shihira et Krauss (= pyrenoidosa) 211-15. A light intensity which produces more photosynthetic oxygen than the cells can re-reduce to water leads to de-adaptation and the substitution of normal photosynthesis for photoreduction. The sequence of these metabolic events was recorded in a matter of a few minutes. Upon exposure of these adapted algae to light, an evolution of hydrogen lasting up to 60 seconds preceded any other light-dependent gas exchange. In the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, this initial hydrogen production was inhibited approximately 50%, pointing to a contribution of electrons by photosystem II. At very low hydrogen tensions (0