Photolability of Photosynthesis in Two Separate Mutants of Scenedesmus obliquus

Two separate mutants of the green alga, Scenedesmus obliquus, are described in which photosynthesis is sensitive to moderate intensities of white light (100 mw cm(-2)). Heterotrophic cultures of both mutants lose photosynthetic activity when exposed to white light; the site of at least the initial phase of this inactivation is within photosystem I. Although all whole cell and cell-free reactions typical of photosystem I examined are inhibited by irradiation, the principal component of photosystem I affected is P-700. In light-sensitive-4 the inactivation of P-700 activity is restored during the subsequent dark period. This recovery is prevented by various antibiotics and by anaerobic conditions. In light-sensitive-41 P-700 activity is recovered only after a complete cell division and new growth. Irradiation periods which inhibit photosynthesis in both mutants are without effects upon the activity or presence of ferredoxin, ferredoxin-NADP(+) oxidoreductase, plastocyanin, cytochrome f(552), cytochrome b-562 or cytochrome b-559.Prolonged irradiation of cells of light-sensitive-41 causes the disappearance of photosystem II activity, alpha-tocopherol, and plastoquinone. Some decrease of both the chlorophylls and carotenoids occurs but there is no preferential deletion of any particular carotenoid.