BLEACHING IN CORAL-REEF ANTHOZOANS - EFFECTS OF IRRADIANCE, ULTRAVIOLET-RADIATION, AND TEMPERATURE ON THE ACTIVITIES OF PROTECTIVE ENZYMENS AGAINST ACTIVE OXYGEN

Recent widespread bleaching of coral reef anthozoans has been observed on the Great Barrier Reef, the Pacific coast of Panama, and in the Caribbean Sea. Bleaching events have been correlated with anomalously high sea surface temperatures which are presumed to cause the expulsion of zooxanthellae from their hosts. Our experimental results show that increases in temperature significantly reduce the total number of zooxanthellae per polyp. At the same time temperature, irradiance (photosynthetically active radiation=PAR), and ultraviolet radiation (UV) independently increase the activities of the enzymes superoxide dismutase, catalase, and ascorbate peroxidase within the zooxanthellae of the zoanthid Palythoa caribaeorum. Enzyme activities within the host are only suggestive of similar changes. These enzymes are responsible for detoxifying active forms of oxygen, and their elevated activities indirectly indicate an increase in the production of active oxygen species by increases in these environmental factors. Historically, bleaching has been attributed to changes in temperature, salinity, and UV. Increases in temperature or highly energetic UV radiation can increase the flux of active forms of oxygen, particularly at the elevated oxygen concentrations that prevail in the tissues during photosynthesis, with oxygen toxicity potentially mediating the bleaching event. Additionally, the concentration of UV absorbing compounds within the symbiosis is inversely related to temperature, potentially increasing exposure of the host and zooxanthellae to the direct effects of UV. © 1990 Springer-Verlag.