Steady State of Photosynthesis in Cyanobacterial Photosynthetic Systems Before and After Regulation of Electron Transport Composition: Overall Rate of Photosynthesis and PSI/PS II Composition

The effect of regulation of photosystem (PS) composition on the photosynthetic steady state was examined using the cyanobacterium Synechocystis PCC 6714. Photosynthetic rates under orange light absorbed by phycobiliprotein (PBP) (PBP light) and under red light absorbed mainly by chlorophyll a (Chi a light) were compared for the cells before and after adaptation to the respective light regimes. Results were as follows: (1) Photosynthetic rates per absorbed light quantum became higher after adaptation than those before adaptation. (2) Under Chi a light, the low turnover rate of PS I before adaptation was markedly enhanced after adaptation (decrease in PS I content), but in the case of adaptation to PBP light (increase in PS I content), a marked enhancement of PS II turnover occurred after adaptation. (3) In the former case, a low turnover rate of PS I before adaptation was due to the occurrence of a large number of closed PS I complexes, but in the latter, limited excitation of PS I caused a large number of closed PS II complexes before adaptation. Results for the latter case indicate that the energy transfer from phycobilisome (PBS) to one PS I complex is far smaller than that from PBS to one PS II complex, and that the imbalance of energy distribution from PBS to the two photosystems is compensated for by the increase in the number of PS I complexes.