Influence of energy flux and quality of light on the molecular organization of the photosynthetic apparatus in Scenedesmus.

The photosynthetic apparatus of the unicellular green alga Scenedesmus obliquus adapts to different levels and qualities of light as documented by the fluence-rate curves of photosynthetic oxygen evolution. Cultures adapted to low fluence rates of white light (5W·m(-2)) have more chlorophyll (Chl) per cell mass, a higher chlorophyll to carotenoid ratio and a doubling of the chlorophyll to cytochrome f ratio compared with cells adapted to high fluence rates of white light (20W·m(-2)). Only small differences can be observed in the halfrise time of fluorescence induction, the electrophoretic profile of the pigment-protein complexes and the Chl a/Chl b-ratio. Scenedesmus cells adapted to blue light of high spectral purity demonstrate, in comparison with those adapted to red light, a higher chlorophyll content, a higher ratio of chlorophyll to carotenoid and a much higher ratio of chlorophyll to cytochrome f. Regarding these parameters and the fluence-rate curves of photosynthesis, the blue light causes the same effects as low levels of white light. In contrast, the action of red light resembles rather that of high levels of white light. Blue-light-adapted Scenedesmus cells have a smaller Chl a to Chl b ratio, a faster half-rise time of fluorescence induction and more chlorophyll in the light-harvesting system than red-light-adapted cells, as shown in the electrophoretic profile of the pigment-protein complexes. Based on these results we propose a model for the adaptation of the photosynthetic apparatus of Scenedesmus to different levels and qualities of light. In this model low as compared with high levels of white light result in an increase in the number of photosystems per electron-transport chain, but not in an increase in the size of these photosystems. The same result is obtained by adaptation to blue light. The lack of sufficient Chl b formation in red-light-adapted cells results in a decrease in the light harvesting chlorophyll-protein complexes and a photosynthetic response like that found in cells adapted to high light levels. The findings reported here confirm our earlier results in comparing blue-and red-light adaptation of the photosynthetic apparatus with adaptation to low and high levels of white light, respectively.