Changes in quantum efficiency of Photosystem II of symbiotic dinoflagellates of corals after heat stress, and of bleached corals sampled after the 1998 Great Barrier Reef mass bleaching event

Pulse-amplitude-modulation chlorophyll fluorometry was used to examine chan ges in dark-adapted F-v/F-m of endosymbiotic dinoflagellate microalgae with in the tissues of the temperate coral Plesiastrea versipora exposed to elev ated seawater temperature. The F-v/F-m was markedly reduced following expos ure of corals to 28 degrees C for 48 h. When corals were returned to ambien t (24 degrees C) conditions, F-v/F-m increased in an initial rapid and then secondary slower phase. Tissue discolouration (coral bleaching), caused by a significant decrease in the density of algae, was observed during the fi rst 2-3 days of the recovery period. After 14 days, F-v/F-m was still signi ficantly lower than in control corals. The recovery of F-v/F-m is discussed in terms of repair processes within the symbiotic algae, division of healt hy algae and also the selective removal of photo-damaged dinoflagellates. U nder field conditions, bleached corals sampled at Heron Island Reef during a bleaching event had significantly lower F-v/F-m than non-bleached colonie s; four months after the bleaching event, there were no differences in F-v/ F-m or algal density in corals marked as having bleached or having shown no signs of colour loss. The results of this laboratory and field study are c onsistent with the hypothesis that an impairment of photosynthesis occurs d uring heat-stress, and is the underlying cause of coral bleaching.