Heat stress induces a reversible inhibition of electron transport at the acceptor side of photosystem II in a cyanobacterium Spirulina platensis

The effects of high temperature (30-52.5 °C) on electron transport at the acceptor side of photosystem II (PSII) in a cyanobacterium Spirulina platensis grown at 30 °C were studied by measuring the relaxation of flash-induced variable chlorophyll fluorescence. In order to investigate the reversibility of the effects of heat stress on electron transport at the acceptor side of PSII, heat-induced effects were measured either immediately after the cells were treated at high temperature for 5 min or after the heat-treated cells were cooled down to the growth temperature and incubated for another 5 min. The maximal efficiency of PSII photochemistry (Fv/Fm) decreased significantly with increasing temperature when measured immediately after the temperature treatment. Fv/Fm showed only a partial recovery (5-15%) when the heat-treated cells were cooled down to the growth temperature and incubated for another 5 min. The decrease in Fv/Fm was a result of a considerable increase in the minimal fluorescence yield and a significant decrease in the maximal fluorescence yield. Heat stress affected considerably the relaxation of flash-induced variable chlorophyll fluorescence. The amplitudes of the fast (≈160 μs) and the middle (≈2 s) components, assigned to electron transfer from QA− to QB site and the diffusion of PQ molecules to an empty QB site, respectively, decreased significantly with increasing temperature. On the other hand, the slow component (≈4 s) due to the charge recombination from the S2QA− state of the water oxidation to the S1QA state, increased significantly. These results suggest that heat stress leads to the inhibition of electron transport from QA− to QB, the binding affinity of a PQ molecule to the QB-binding site, and the increased probability of back electron transfer from QA− to S2 state at the expense of forward electron flow. However, these changes in the function of the acceptor side of PSII were mostly recovered after 5 min incubation at the growth temperature.