Diurnal changes in the photochemical efficiency of the symbiotic dinoflagellates (Dinophyceae) of corals: photoprotection, photoinactivation and the relationship to coral bleaching
The photochemical efficiency of symbiotic dinoflagellates within the tissues of two reef-building corals in response to normal and excess irradiance at water temperatures < 30 °C were investigated using pulse amplitude modulated (PAM) chlorophyll fluorescence techniques. Dark-adapted Fv/Fm showed clear diurnal changes, decreasing to a low at solar noon and increasing in the afternoon. However, Fv/Fm also drifted downwards at night or in prolonged darkness, and increased rapidly during the early morning twilight. This parameter also increased when the oxygen concentration of the water holding the corals was increased. Such changes have not been described previously, and most probably reflect state transitions associated with PQ pool reduction via chlororespiration. These unusual characteristics may be a feature of an endosymbiotic environment, reflective of the well-documented night-time tissue hypoxia that occurs in corals. Fv/Fm decreased to 0·25 in response to full sunlight in shade-acclimated (shade) colonies of Stylophora pistillata, which is considerably lower than in light-acclimated (sun) colonies. In sun colonies, the reversible decrease in Fv/Fm was caused by a lowering of Fm and Fo suggesting photoprotection and no lasting damage. The decrease in Fv/Fm, however, was caused by a decrease in Fm and an increase in Fo in shade colonies suggesting photoinactivation and long-term cumulative photoinhibition. Shade colonies rapidly lost their symbiotic algae (bleached) during exposure to full sunlight. This study is consistent with the hypothesis that excess light leads to chronic damage of symbiotic dinoflagellates and their eventual removal from reef-building corals. It is significant that this can occur with high light conditions alone.