Effects of heat stress on PSII photochemistry in a cyanobacterium Spirulina platensis

The effects of heat stress (30–50 °C) on photosystem II (PSII) photochemistry in a cyanobacterium Spirulina platensis grown at 30 °C were studied by measuring chlorophyll fluorescence induction kinetics and thermoluminescence. Heat stress inhibited significantly the maximum efficiency of PSII photochemistry. An investigation of the kinetics of flash-induced fluorescence yield decay revealed that heat stress caused a shift of the equilibrium towards QA but showed no effect on the kinetics of flash-induced fluorescence yield decay in the presence of DCMU. An analysis of polyphasic rise of fluorescence transients indicated that heat stress induced a decrease in the total number of PSII active reaction centers. Thermoluminescence measurements demonstrated that the B band in Spirulina cells corresponding to the S2QB− state consisted of two different components, the B32 and the B24 bands with the peak temperatures at 32 and 24 °C, respectively. The intensity of the B32 band and the B24 band decreased significantly with increasing temperature but the decrease of the intensity of the B24 band was much greater than that of the B32 band. There were no significant changes in the peak temperatures of the B24 and B32 bands. The intensity of the Q band corresponding to the recombination of the S2QA− charge pair decreased significantly with increasing temperature. The period-four oscillation of the B band was significantly damped at temperatures higher than 45 °C. The contents of D1 and PsbO proteins decreased with increasing temperature. The results in this study suggest that heat stress shows no effects on the stability of the S2QA− and S2QB− states and that the different populations of the active PSII reaction centers show different sensitivity to heat stress in S. platensis cells.