BUXTON, L.; GUTOWSKA, M.; SAWNEY, S.; OWEN, R.; TRAPIDO-ROSENTHAL, H.*; Bermuda Biological Station for Research; Evergreen State College; Eckerd College; Bermuda Biological Station for Research; Bermuda Biological Station for Research: Increases in the Activity of Zooxanthellae Nitric Oxide Synthase are Associated with the Temperature-Induced Bleaching of the Coral Madracis mirabilis

Coral bleaching, in which corals lose their photosynthetic algal symbionts, is a phenomenon that is recognized as one of fundamental ecological importance. However, mechanisms by which bleaching occurs are not at all well understood. Zooxanthellae living in symbiotic association with cnidarians have been shown to possess nitric oxide synthase (NOS) activity, the enzyme that generates the multifunctional signal molecule nitric oxide (NO). In previous in vitro work, we demonstrated that in specimens of Madracis mirabilis subjected to temperature shock, algal NOS activity was dramatically elevated prior to the �emigration� of algae from the host. We have now extended this study to zooxanthellae isolated from corals collected from the field during February (when water temperature was as low as 16�C) and August (when water temperature was as high as 28�C). Our results indicate that stressed corals (many of which were visibly in the process of bleaching), contain zooxanthellae with elevated levels of NOS activity, relative to corals that are not showing signs of stress-initiated algal loss. In addition, in further in vitro assays, we show that in the presence of the NOS inhibitor 7-nitroindazole, bleaching in response to temperature stress does not occur. Taken together, we interpret these results as support for the hypothesis that zooxanthellae-generated NO is involved in temperature-induced coral bleaching. We further postulate that zooxanthellae NOS activity has the potential to be developed as a useful tool for monitoring the condition of symbiotic assemblages in coral reef ecosystems.