A series of mutant strains of Scenedesmus obliquus with abnormal carotenoid compositions.

Several mutant strains of Scenedesmus obliquus (Chlorophyta) have been isolated which, when cultured heterotrophically, are pale green or yellow, in contrast to the dark green of the wild type. On the basis of their carotenoid compositions, four groups of pale-green strains have been delineated. These accumulate, respectively, no carotenoid, phytoene, mainly zeta-carotene and mainly zeta-carotene together with some neurosporene and lycopene. All these strains synthesized no chlorophyll b and only small amounts of chlorophyll a. A further group of yellow strains produced the normal Scenedesmus obliquus range of cyclic carotenes and xanthophylls, but no chlorophyll. Most of the pale-green strains were killed by exposure to light, but two strains, PG1 and 1E, which accumulated predominantly zeta-carotene when grown in the dark, survived exposure to the light and developed photosynthetically active chloroplasts containing the normal pigments. The possible biosynthetic implications of the carotenoid composition of these mutant strains, and the relationshp between the carotenoid composition and protection of the cells from photooxidative destruction are discussed.