Photolability of Photosynthesis in Two Separate Mutants of Scenedesmus obliquus: Preferential Inactivation of Photosystem I.

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.