Seawater--Thermal properties

Bleaching of reef corals and other cnidarians symbiotic with zooxanthellae can be attributed to the stress response of the host, algae or both. To determine if zooxanthellae are involved in the bleaching process, I infected a single strain of sea anemone, Aiptasia pallida with zooxanthellae from different hosts. I measured expulsion of the algae from the host during 24-hour incubations at 25, 32 and 34C, as well as photosynthetic rates at these temperatures. Photosynthesis and expulsion of zooxanthellae were inversely and directly proportional to elevated temperatures, respectively. Photosynthesis and expulsion of zooxanthellae isolated from Condylactis gigantea showed the greatest sensitivity to elevated temperature when compared to other zooxanthellae tested. These results suggest that zooxanthellae have a function in the bleaching process and that this function may be related to their photosynthetic response. Thus, the differential tolerance of zooxanthellae to stress could partly explain the spatial variability characteristic of coral-bleaching episodes.

Coral bleaching involves the loss of symbiotic algae (zooxanthellae) from reef corals and other cnidarians during periods of environmental stress, particularly elevated temperature. In this study the thermal bleaching responses of the zoanthid Palythoa caribaeorum from three populations along the southeast coast of Florida were compared. Winter (2002--2003) and summer (2003) samples from three geographically separate sites were exposed to increased temperatures and the loss of zooxanthellae was measured. Zooxanthellae populations were sampled and identified using PCR-DGGE. The results showed that samples of P. caribaeorum from reefs that experience smaller annual thermal ranges released the most zooxanthellae. Seasonal comparisons revealed winter samples experienced a greater amount of zooxanthellae loss than summer samples. P. caribaeorum was found to harbor two genetic strains of zooxanthellae, C1 and D1a. Colonies either contained monotypic populations of these, or a combination of both.