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 tissue s of two reef-building corals in response to normal and excess irradiance a t wafer temperatures < 30 <degrees>C were investigated using pulse amplitud e modulated (PAM) chlorophyll fluorescence techniques, Dark-adapted F-v/F-m showed clear diurnal changes, decreasing to a low at solar noon and increa sing in the afternoon. However, F-v/F-m also drifted downwards at night or in prolonged darkness, and increased rapidly during the early morning twili ght. This parameter also increased when the oxygen concentration of the waf er holding the corals was increased. Such changes have not been described p reviously, and most probably reflect state transition's associated with PQ pool reduction via chlororespiration. These unusual characteristics may be a feature of an endosymbiotic environment, reflective of the well-documente d night-time tissue hypoxia that occurs in corals. F-v/F-m decreased to 0.2 5 in response to full sunlight in shade-acclimated (shade) colonies of Styl ophora pistillata, which is considerably lower than in light-acclimated (su n) colonies. In sun colonies, the reversible decrease in F-v/F-m was caused by a lowering of F-m and F-o suggesting photoprotection and no lasting dam age. The decrease in F-v/F-m, however, was caused by a decrease in F-m and an increase in F-o in shade colonies suggesting photoinactivation and long- term cumulative photoinhibition. Shade colonies rapidly lost their symbioti c algae (bleached) during exposure to full sunlight. This study is consiste nt with the hypothesis that excess light leads to chronic damage of symbiot ic dinoflagellates and their eventual removal from reef-building corals. It is significant that this can occur with high light conditions alone.