Goiran C., 1994. Symbiosis between scleractinian corals and dinoflagellates : zooxanthella Symbiodinium sp. of Galaxea fascicularis coral physiology, out of the symbiotic association. PhD from Nice-Sophia Antipolis University.

ABSTRACT

Although symbiosis between scleractinian corals and dinoflagellates (zooxanthellae) is fundamental to the existence of coral reefs, many of the participating physiological processes are poorly understood. To enable investigation of some of thèse processes, zooxanthellae were isolated from the coral Galaxea fascicularis. In scleractinians, zooxanthellae are located iside cells of the endoderm in membrane-bound vesicles. Each functional unit of zooxanthella and encapsulating vesicle is termed a symbiosome. Isolated zooxanthellae were produced by direct isolation from host tissue (freshly isolated zooxanthellae, FIZ) and in vitro culture (cultured zooxanthellae, CZ). Ultrastructural examination of FIZ revealed vestiges of the symbiosome membrane remaining after isolation, thus indicating disruption of the functional unit. During isolation, zooxanthellae were subjected to ionic shock caused by différences in the ionic composition of the cytosol of host cell and sea water. This shock caused intracellular sodium ion concentration to double. Subsequently, regulatory mechanisms, became active, most likely controled by Na+/K+-ATPases, that caused intracellular sodium concentration return to pre-isolation levels. Mechanisms of inorganic carbon uptake for photosynthesis were investigated in FIZ and CZ. At normal sea water pH, the concentration of HCO3" is about 200 times greater than that of CO2. The data support the interprétation that CZ absorb HCO3" by means of a Na+/HCO3" symport. FIZ, in contrast, appear to take up CO2 by diffusion. The data thus indicate that FIZ enhance conversion of HCO3" in CO2 at the exterior surface of the cell membrane by a process which involves H+-ATPase and a peripheral carbonic anhydrase. The results of this study demonstrates différences in the physiological adaptation of zooxanthellae to sea water and the coral host environment.

Key-words : Scleractinian, Dinoflagellate, symbiosis, zooxanthella, Symbiodinium, Galaxea fascicularis, photosynthesis, membrane transport, bicarbonate, CO2, inorganic carbon, sodium.