Biophotolysis of water: the light saturation curves

Photosynthetic light saturation curves for the steady-state simultaneous photoproduction of molecular hydrogen and oxygen have been measured for the first time. Both in vitro and in vivo systems have been studied. The in vitro system was composed of isolated spinach chloroplasts, ferredoxin and hydrogenase. The in vivo systems were the anaerobically adapted intact eukaryotic green algae Scenedesmus D 3 and Chlamydomonas reinhardtii. The following results have been obtained: (1) the stoichiometric ratio of hydrogen to oxygen is light-dependent and, in general, not equal to two; (2) the ratios of the light-saturated rates were greater than 2 for all of the systems studied; (3) far-red 700 nm) light, as expected, greatly increased the ratio of hydrogen to oxygen; (4) the light-saturation curves for the simultaneous photoproduction of hydrogen and oxygen do not have the same analytical shapes; a higher light intensity is required to saturate the hydrogen curve than is required for the oxygen curve. Although the measured light-saturated rate of electron transport in the biophotolytic mode (hydrogen and oxygen evolution) is much less than that of normal photosynthesis, these experiments indicate that the biophotolysis of water is a strict analog of normal photosynthesis.