Zooxanthellae expelled from bleached corals at 33 degrees C are photosynthetically competent

While a number of factors have been linked to coral bleaching, such as high light, high temperature, low salinity, and UV exposure, the best explanati on for recent coral bleaching events are small temperature excursions of 1 to 2 degreesC above summer sea-surface temperatures in the tropics which in duce the dinoflagellate. symbionts (zooxanthellae) to be expelled from the host. The mechanism that triggers this expulsion of the algal symbionts is not resolved, but has been attributed to damage to the photosynthetic mecha nism of the zooxanthellae. In the present investigation we addressed the qu estion of whether such expelled zooxanthellae are indeed impaired irreversi bly in their photosynthesis. We employed a Microscopy Pulse Amplitude-Modul ated (PAM) fluorometer, by which individual zooxanthellae can be examined t o study photosynthesis in zooxanthellae expelled when corals are subjected to a temperature of 33 degreesC. We show that the expelled zooxanthellae fr om Cyphastrea serailia were largely unaffected in their photosynthesis and could be heated to 37 degreesC before showing temperature-induced photosynt hetic impairment. These results suggest strongly that the early events that trigger temperature-induced expulsion of zooxanthellae involve a dysfuncti on in the interaction of the zooxanthellae and the coral host tissue, and n ot a dysfunction in the zooxanthellae per se.