CHANGES IN THE CAROTENOID PATTERN DURING THE SYNCHRONOUS LIFE-CYCLE OF SCENEDESMUS

The wild type (WT) of Scenedesmus obliquus and a mutant lacking chlorophyll b and the light-harvesting complexes (WT-LHC1) were synchronized by a light-dark regime. Both cultures contained the same type of carotenoids. However, concentrations and patterns of carotenoids were different during their synchronous life cycles. The concentration of total carotenoids followed more or less that of chlorophyll. The WT contained more carotenoids per cell mass, but slightly less per chlorophyll. It is discussed that part of the carotenoids of the mutant, lacking the peripheral antenna of PSII, might be located in the chlorophyll b-less apoprotein or in an enlarged core antenna of PSII. During the life cycle of Scenedesmus the carotenes are initially synthesized and most of the alpha-carotene is immediately oxidized to lutein which is inserted in the antennae systems of PSII and PSI. The further oxidation of lutein to loroxanthin seems to depend on both the change from dark to light, and on stages of the life cycle itself. Although the major part of beta-carotene appears to be inserted in the reaction centers, a fraction of the total pool is rapidly converted to violaxanthin, following the onset of illumination. The conversion may serve to protect against photooxidation. Further conversion of violaxanthin to neoxanthin occurs to a greater extent in the mutant, WT-LHC1. The results demonstrate (1) the close connection between the carotenoid pattern and the modulation of the photosynthetic apparatus during the life cycle of Scenedesmus and (2) the flexibility of the organism in compensating for the absence of the light-harvesting complexes of photosystems II by adjusting the carotenoid distribution.