CO2 FIXATION AND PHOTOEVOLUTION OF H-2 AND O-2 IN A MUTANT OF CHLAMYDOMONAS LACKING PHOTOSYSTEM-I

Although mutant B4 of Chlamydomonas reinhardtii lacks photosystem I (P S I), it is capable of photoautotrophic assimilation of atmospheric ca rbon dioxide and sustained simultaneous photo-evolution of molecular o xygen and hydrogen. Here we report that at saturating light intensitie s, carbon dioxide reduction is stable under anaerobiosis but unstable in air. At lower light intensities, carbon dioxide reduction is stable in both atmospheres. The data indicate that OS I is not necessary for autotrophic photosynthesis. One interpretation of these results is th at oxygenic photosynthesis developed as a single-light-reaction proces s, presumably from a bacterium with a phaeophytin-quinone reaction cen tre, but became unstable as oxygen in the Earth's atmosphere accumulat ed. PS I was the seconde light reaction, added to confer stability in oxygen-containing atmospheres. Viewed from this perspective, the well- known Z scheme of modern photosynthesis is seen as a specialized adapt ation for performing low-potential reductive photochemistry in oxygen- containing atmospheres, but is not an irreducible necessity for satisf ying the thermodynamic and mechanistic requirements of carbon dioxide photoreduction using water as the source of reductant.