Scleractinian corals with photoprotective host pigments are hypersensitive to thermal bleaching

Recent episodes of mass coral bleaching, the loss of symbiotic dinoflagellates or photosynthetic pigment from hermatypic corals, have been triggered by elevated sea temperatures. Photosynthetic irradiance is an important secondary factor. Host based pigments (pocilloporins or Green Fluorescent Protein homologues) have been proposed to reduce the impact of elevated temperature by shading the dinoflagellate symbionts of corals, thereby reducing light stress. This study investigates this phenomenon in the reef-building coral Acropora aspera from Heron Island Research Station (Great Barrier Reef, Australia), which occurs as 3 distinct colour morphs. Experimental data showed that the host pigments are photoprotective at normal temperatures or <32°C; however, the loss of symbionts and reduction in the quantum yield of photosynthesis (dark adapted Fv/Fm) observed after exposure to elevated temperature was most severe in the heavily pigmented blue morph, eventually resulting in death of most experimental colonies. The results suggest that the protection offered by pocilloporins and other GFP-homologues is reduced by thermal stress, potentially leaving the shade-acclimated symbionts of heavily pigmented corals exposed to high light levels. Mature host pigments are thermally stable, but they are potentially vulnerable to heat during mRNA and protein synthesis and/or maturation. It is clear, however, that the broad assumption that host pigmented corals are less vulnerable to thermal stress is incorrect.