DYNAMICS OF SOLAR BLEACHING IN THE INTERTIDAL REEF CORAL GONIASTREA-ASPERA AT KO-PHUKET, THAILAND

The bleaching response in intertidal Goniastrea aspera (Verrill) was i nvestigated immediately at the onset of a natural bleaching event caus ed primarily by elevated solar irradiance. Bleaching was found to invo lve a significant decrease in zooxanthellae density and a significant reduction in chlorophyll c content of zooxanthellae. In contrast chlor ophyll a content of zooxanthellae appeared to increase in bleached com pared to normally pigmented tissues, an effect which may have been the result of interference of chlorophyll breakdown products with the pig ment analysis. Bleaching damage was restricted at the cellular level t o those host endodermal cells harbouring zooxanthellae, both of which exhibited cellular degradation. In bleached tissues, in situ degradati on of zooxanthellae, prior to their loss from host tissues, appeared t o be the principal factor associated with reduction in algal density. The results highlight that the mechanism of bleaching is a complex res ponse involving changes in the symbiotic association between specific host and zooxanthella cells, zooxanthella densities and pigment conten t.