Changes in the Reactivity of the Photosynthetic Apparatus in Heterotrophic Ageing Cultures of Scenedesmus obliquus

Abstract Dark grown cultures of Scenedesmus aged for 2, 10 and 30 days, when transferred into light for 2 hours showed about 1.5, 10 and 80 fold increases, respectively, of their oxygen evolving capacity. The presence of Mg2+ and Mn2+ accelerated the reactivation; neither cell growth nor cell division was apparent during reactivation. An action spectrum for this reactivation process showed essentially the same absorption spectrum of that of chlorophyll in vivo which indicates the energy dependence of the process on photosynthesis. Kinetics of cytochrome f photo‐oxidation, 520 nm absorbance changes and fluorescence induction demonstrate that the immediate recovery of the photosynthetic capacity is mainly due to a reestablishment of a balanced electron transport system. Light periods as short as 30 minutes were found to be sufficient to reestablish an efficient coupling between the two photosystems.