Bleaching of Symbiotic Algae (Zooxanthellae) and their Invertebrate Hosts: Causes and Mechanisms

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Bleaching, the loss of symbiotic dinoflagellates)="zooxanthellae" hereafter) of their pigments, of reef corals and other invertebrates has become a world-wide problem in tropical marine ecosystem, linked by some researchers to global warming. The results of bleaching have potentially devastating environmental, ecological and economic effects in the Caribbean, IndoPacific, an other tropical marine areas. Though there is some experimental work showing involvement of both higher than average temperature and light, the mechanisms involved in bleaching are not well understood this project will test three hypotheses. 1. Bleaching in nature is caused by high temperature stress coupled with high energy blue light (and possibly UV-A between 380-400nm). Preliminary evidence shows that while high temperature alone will induce bleaching, natural light exposure during high temperature treatment exacerbates the effect by lowering the temperature threshold and time to bleaching at a given temperature. this study will determine which component of light is responsible for this effect and the mechanisms of action. Early theories on bleaching had light playing a major role, but experimental evidence has not yet supported this contention. Potentially harmful chemical alterations associated with high energy wavelengths of blue light (and possibly some near-blue wavelengths of UV-A, that are not adsorbed by UV-protecting pigments found in corals) are not only consistent with field observations of bleaching, but are also supported by both laboratory and field-based preliminary experiments. 2. The mechanisms of temperature-light induced bleaching involves the irreversible dissociation of the chlorophyll-protein associations in the chloroplast. The harmful effects of high temperatures and light on algae include the irreversible separation or inactivation of the chlorophyll-protein complexes associated with reaction centers in the chloroplast. Electron transport activity and eventually carbon fixation decrease markedly. 3. High light and temperatures cause decreases in "protective" pigments which absorb ultraviolet light. The role of different wavelengths of light in conjunction with high temperature in determining concentrations of UV-screening pigments will be determined as well as their relationship with photosynthetic rates. These hypotheses will be tested using cultured and freshly isolated zooxanthellae, and intact hosts both in the laboratory and in field-based experiments.