Adaptation of the Photosynthetic Apparatus of Scenedesmus obliquus to Strong and Weak Light Conditions

Simulating ‘sun’ and ‘shade’ conditions of higher plants, homocontinuous cultures of Scenedesmus obliquus were grown under 28 W/m2 (∼28,000 lux) or 5 W/m2 (∼5000 lux). In the high light intensity grown culture the photosynthetic capacity per chlorophyll was three times higher than in the low light intensity grown cells. The activity of photosystem II changed parallel to the photosynthetic capacity, whereas photosystem I activity was influenced to a lesser amount by the light intensity under which Scenedesmus was grown. The molar concentration of all cytochromes and reactive plastoquinone were higher in cells grown under high light intensities. Since only plastoquinone activity represented changes equivalent to photosynthetic capacity, its activity is regarded as the rate limiting step. In spite of the changes in the photosystems and the electron transport chains, the size of the photosynthetic units is independent of the light intensity under which the cells are grown. The results reported for Scenedesmus represent a striking similarity to those published for higher plants grown under different light intensities.