THE INFLUENCE OF LIGHT-INTENSITY AND WAVELENGTH ON THE CONTENTS OF ALPHA-CAROTENE AND BETA-CAROTENE AND THEIR XANTHOPHYLLS IN GREEN-ALGAE

Chloroplasts of dark-grown cells of the green alga Scenedesmus obliquus contain both alpha- and beta-carotene and their respective major xanthophylls, i.e. (3R, 3'R, 6'R)-beta,epsilon-carotene-3,3'-diol (lutein) and (3R, 3'R, 6'R)-beta,epsilon-carotene-3,19,3'-triol (loroxanthin), and (3R, 3'R)-beta,beta-carotene-3,3'-diol (zeaxanthin), (3R, 5R, 6S, 5'R, 6'S)-5,6,5',6'-diepoxy-5,6,5',6'-tetradehydro-beta,beta-carotene-3,3'-diol (violaxanthin) and (3S,5R, 6R, 3'S, 5'R, 6'S)-5'6'-epoxy-6,7-didehydro-5,6,5',6'-tetrahydro-beta,beta-carotene-3,5,3'-triol (neoxanthin) respectively. When exposed to light major changes in the pool sizes of some of these carotenoids occur, the extent of which depends on both the intensity and wavelength of the light employed and the duration of exposure. The primary alterations noted during a 24 h illumination period (20 W m-2) were in alpha-carotene and its derivatives with only small changes occurring in the total beta-carotene pool. Principally, illumination produces an immediate and significant increase in the levels of lutein at the expense of alpha-carotene and trihydroxy-alpha-carotene (loroxanthin). Comparable studies were performed on mutant phenotypes of Scenedesmus which were (i) unable to synthesize chlorophyll b and the light-harvesting complexes (LHCs), and (ii) failed to develop chlorophyll and the total photosynthetic apparatus when grown in darkness, but greened when exposed to light. In the LHC-deficient strains, changes in alpha-carotene and its xanthophylls occurred, but were less dramatic than those seen in the wild type; this results from their inability to develop a stable LHC in vivo. The developmental strain, C-2A', showed light-induced changes in the carotenoids comparable with those observed in the wild type. Since the xanthophylls, such as lutein, are located primarily in both the proximal and distal LHCs of the two photosystems, the observed changes are probably associated with the adaptation of the antennae systems to increasing light intensities. There was no indication that the zeaxanthin/violaxanthin cycle operates in Scenedesmus at the intensities and exposure times utilized in these experiments.