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Bleaching of reef corals, involving loss of symbiotic algae (= zooxanthellae), loss of algal pigments, or both, has been linked to temperature stress. In this study the effects of high temperature and light on zooxanthellae living in the Caribbean reef corals Montastrea annularis, M. cavernosa, Agaricia agaricites, and A. lamarcki were studied. Pieces of coral colonies were incubated at ambient seawater temperature (26{deg} +/- 1{deg}C), and at 30{deg}, 32{deg}, and 34{deg}C. Symbiotic algae from M. annularis, a species of coral from the forereef that commonly bleaches, showed the following sequence of events when exposed to natural light at 32{deg}C; loss of photosynthetic potential measured as fluorescence yield, corresponding reduction of both oxygen production per zooxanthella and P:R (photosynthesis:respiration) ratio, and subsequent reduction in density of algae in relation to surface area of the coral. These parameters were not significantly reduced and no deaths occurred for M. annularis or any other coral species maintained at 26{deg} or 30{deg}C. However, the sequence of events was condensed to less than 24 h when M. annularis was subjected to 34{deg}C seawater, except that there was little if any reduction in algal density before tissue-sloughing and death occurred between 10 and 24 h. Loss of significant amounts of chlorophyll a per alga was not evident for any corals except those maintained at 34{deg}C longer than 10 h. In contrast, symbiotic algae in M. cavernosa, a species that rarely bleaches in nature, showed only slight reductions in photosynthesis and fluorescence yield, and no significant loss of algal cells or chlorophyll a, when maintained in seawater at 32{deg}C for 2 days. Thus zooxanthellae in M. cavernosa appeared to be less affected by sublethal high-temperature stress. Similar contrasting patterns of bleaching were seen in zooxanthellae from the plating coral Agaricia lamarcki, which often bleaches during the late summer and fall, compared with zooxanthellae from A. agaricites, a coral which bleaches less frequently. In addition, M. annularis exposed to sublethal high temperatures and ambient light bleached faster than those kept in dimmer light, supporting past field observations suggesting that light energy is an important component of bleaching in nature. When M. annularis was exposed to different wavelengths of natural light at 32{deg}C, the fluorescence yield declined more quickly in the presence of higher energy UV-A and blue light than with other photosynthetically active radiation. Natural levels of UV-B had little effect in this study. These data suggest that the patterns of bleaching seen in nature may be at least partially explained by different tolerances of the symbiotic algae in the corals, and that light plays a significant role in bleaching.

Bleaching Patterns of Four Species of Caribbean Reef Corals