EFFECT OF PHOTOACCLIMATION ON THE ENERGY PARTITIONING BETWEEN CYCLIC AND NONCYCLIC PHOTOPHOSPHORYLATION

The effect of photoacclimation on cyclic (CPP), non-cyclic (NCPP), and oxidative phosphorylation was investigated in three algal species: Isochrisis galbana, Scenedesmus quadricauda and Synechococcus leopoliensis. The relative contribution of the three phosphorylative pathways to the incorporation of P-32 into soluble and particulate cellular pools was assessed. To block NCPP we used 1 muM dichlorophenyl-dimethylurea (DCMU) which does not interrupt CPP. Oxidative phosphorylation was blocked by purging oxygen with N2. In all three species the ratio of CPP to the total photophosphorylation (PP) increased in the process of photoacclimation to high light. We suggest that the observed increase in CPP in high-light-adapted cells allows bypassing the relatively slow enzymatic reactions in NCPP while channelling excess electrons to the faster CPP. This redistribution of energy increases ATP supply for the augmented metabolic requirements of fast growing, high-light-adapted cells.