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

Abstract

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 Q A 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 S 2Q B − state consisted of two different components, the B 32 and the B 24 bands with the peak temperatures at 32 and 24 °C, respectively. The intensity of the B 32 band and the B 24 band decreased significantly with increasing temperature but the decrease of the intensity of the B 24 band was much greater than that of the B 32 band. There were no significant changes in the peak temperatures of the B 24 and B 32 bands. The intensity of the Q band corresponding to the recombination of the S 2Q A − 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 S 2Q A − and S 2Q B − states and that the different populations of the active PSII reaction centers show different sensitivity to heat stress in S. platensis cells.