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

Abstract

Pulse–amplitude–modulation chlorophyll fluorometry was used to examine changes in dark-adapted F_v/F_m of endosymbiotic dinoflagellate microalgae within the tissues of the temperate coral Plesiastrea versipora exposed to elevated seawater temperature. The F_v/F_m was markedly reduced following exposure of corals to 28°C for 48 h. When corals were returned to ambient (24°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 first 2–3 days of the recovery period. After 14 days, F_v/F_m was still significantly 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 healthy algae and also the selective removal of photo-damaged dinoflagellates. Under field conditions, bleached corals sampled at Heron Island Reef during a bleaching event had significantly lower F_v/F_m than non-bleached colonies; 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 consistent with the hypothesis that an impairment of photosynthesis occurs during heat-stress, and is the underlying cause of coral bleaching.