The relationship between heat-stress and photobleaching in green and blue-green algae

Two characteristic temperatures were identified from measurements of the temperature dependence of O2 evolution by Chlorella vulgaris and Anacystis nidulans: T1, the threshold temperature for inhibition of O2 evolution under saturating light conditions, and T2, the upper temperature limit for O2 evolution. Measurement of delayed light emission from photosystem II (PSII) showed that it passed through a maximum at T1 and was virtually eliminated on heating the samples to T2. Related changes were observed in low-temperature (77K) fluoresence emission spectra. Heat-stress had little effect on the absorption properties of the cells at temperatures below T1 but incubation at higher temperatures, particularly under high-light conditions, resulted in extensive absorption losses. An analysis of these measurements suggests that this increased susceptibility to photobleaching is triggered by an inhibition of the flow of reducing equivalents from PSII that normally serves to protect the light-harvesting apparatus of the cells from photo-oxidation. Adaptation to higher growth temperatures resulted in increases in the values of T1 and T2 for Anacystis nidulans but not for Chlorella vulgaris.