Excitation pressure regulates the activation energy for recombination events in the photosystem II reaction centres ofChlamydomonas reinhardtii

Abstract

Using in vivo thermoluminescence, we examined the effects of growth irradiance and growth temperature on charge recombination events in photosystem II reaction centres of the model green alga Chlamydomonas reinhardtii. We report that growth at increasing irradiance at either 29 or 15 °C resulted in comparable downward shifts in the temperature peak maxima (T_M) for S₂Q_B^–charge pair recombination events, with minimal changes in S₂Q_A^–recombination events. This indicates that such growth conditions decrease the activation energy required for S₂Q_B^–charge pair recombination events with no concomitant change in the activation energy for S₂Q_A^–recombination events. This resulted in a decrease in the ΔT_Mbetween S₂Q_A^–and S₂Q_B^–recombination events, which was reversible when shifting cells from low to high irradiance and back to low irradiance at 29 °C. We interpret these results to indicate that the redox potential of Q_Bwas modulated independently of Q_A, which consequently narrowed the redox potential gap between Q_Aand Q_Bin photosystem II reaction centres. Since a decrease in the ΔT_Mbetween S₂Q_A^–and S₂Q_B^–recombination events correlated with growth at increasing excitation pressure, we conclude that acclimation to growth under high excitation pressure narrows the redox potential gap between Q_Aand Q_Bin photosystem II reaction centres, enhancing the probability for reaction center quenching in C. reinhardtii. We discuss the molecular basis for the modulation of the redox state of Q_B, and suggest that the potential for reaction center quenching complements antenna quenching via the xanthophyll cycle in the photoprotection of C. reinhardtii from excess light.