Acclimation to the growth temperature and the high-temperature effects on photosystem II and plasma membranes in a mesophilic cyanobacterium, Synechocystis sp PCC6803

High-temperature effects on Photosystem. II and plasma membranes, temperatu re dependence of growth, and acclimation to the growth temperature were stu died in a mesophilic cyanobacterium, Synechocystis sp. PCC6803. The followi ng results were obtained. (1) Small but distinct temperature. acclimation o f the PSII reaction center activity was shown for the first time when the a ctivity was measured at inhibitory high temperatures. However, the reaction center activity showed no apparent acclimation when it was measured at gro wth temperatures after heat stress. (2) Oxygen-evolving activity and the pe rmeability of plasma membranes showed higher resistance to heat when PCC680 3 cells were grown at higher temperatures. (3) Acclimation of photosynthesi s to the growth temperature seemed to occur so as to maintain photosynthesi s activity not at a maximum level but in a certain range at the growth temp eratures. (4) Neither sensitivity to high-temperature-induced dissociation of phycobilisomes from the PSII reaction center complexes nor degradation o f phycocyanin were altered by changes in the environmental temperature. (5) A close relationship between the viability of cells and the structural cha nges of plasma membranes (but not the inactivation of photosynthesis) was o bserved. The denaturation process of PSII complexes and the relationship be tween the temperature dependence of the growth of Synechocystis PCC6803 cel ls and that of the photosynthetic activity are also discussed.