MEPS 272:117-130 (2004)  -  doi:10.3354/meps272117

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

D. J. Franklin1,3,*, O. Hoegh-Guldberg2, R. J. Jones2, J. A. Berges1,4

1School of Biology and Biochemistry, The Queen¹s University of Belfast, 97 Lisburn Road, Belfast BT9 7BL, Northern Ireland, UK
2Centre for Marine Studies, University of Queensland, Brisbane, Queensland 4072, Australia
3Present address: Centre for Marine Studies, The University of Queensland, Brisbane, Queensland 4072, Australia
4Present address: Department of Biological Sciences, University of Wisconsin-Milwaukee, 3209 N Maryland Avenue, Milwaukee, Wisconsin 53211, USA

ABSTRACT: Rising sea temperatures are increasing the incidences of mass coral bleaching (the dissociation of 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 sp.) within the tissue of the hard coral Stylophora pistillata (Esper) were assessed using a suite of techniques. Bleaching of S. pistillata was accompanied by declines in the maximum potential quantum yield of photosynthesis (Fv/Fm, measured using pulse amplitude modulated [PAM] fluorometry), an increase in the number of Sytox-green-stained algae (indicating compromised algal membrane integrity and cell death), an increase in 2¹,7¹-dichlorodihydrofluroscein diacetate (H2DCFDA)-stained algae (indicating increased oxidative stress), as well as ultrastructural changes (vacuolisation, losses of chlorophyll, and an increase in accumulation bodies). Algae expelled from S. pistillata exhibited a complete disorganisation of cellular contents; expelled cells contained only amorphous material. In situ samples taken during a natural mass coral bleaching event on the Great Barrier Reef in February 2002 also revealed a high number of Sytox-labelled algae cells in symbio. Dinoflagellate degeneration during bleaching seems to be similar to the changes resulting from senescence-phase cell death in cultured algae. These data support a role for oxidative stress in the mechanism of coral bleaching and highlight the importance of algal degeneration during the bleaching of a reef coral.

KEY WORDS: Symbiodinium · Coral bleaching · Temperature/light stress · Global climate change · Ultrastructure · Dinoflagellate · Symbiont

Full text in pdf format