Cell death and degeneration in the symbiotic dinoflagellates of the coral Stylophora pistillata during bleaching

Rising sea temperatures are increasing the incidences of mass coral bleaching (the dissociation the coral–algal symbiosis) and coral mortality. In this study, the effects of bleaching (induced by elevated light and temperature) on the condition of symbiotic dinoflagellates (Symbiodinium. ) within the tissue of the hard coral Stylophora pistillata (Esper) were assessed using a suite techniques. Bleaching of S. pistillata was accompanied by declines in the maximum potential yield of photosynthesis (Fv/Fm, measured using pulse amplitude modulated PAM fluorometry), increase in the number of Sytox-green-stained algae (indicating compromised algal membrane and cell death), an increase in 2’, 7’-dichlorodihydrofluroscein diacetate (H2DCFDA) - algae (indicating increased oxidative stress), as well as ultrastructural changes (vacuolisation, of chlorophyll, and an increase in accumulation bodies). Algae expelled from S. pistillata a complete disorganisation of cellular contents; expelled cells contained only amorphous. In situ samples taken during a natural mass coral bleaching event on the Great Barrier Reef February 2002 also revealed a high number of Sytox-labelled algae cells in symbio. Dinoflagellate during bleaching seems to be similar to the changes resulting from senescence-phase death in cultured algae. These data support a role for oxidative stress in the mechanism of coral and highlight the importance of algal degeneration during the bleaching of a reef coral.