MEPS 220:163-168 (2001)  -  doi:10.3354/meps220163

Zooxanthellae expelled from bleached corals at 338C are photosynthetically competent

Peter J. Ralph1,*, Rolf Gademann2, Anthony W. D. Larkum3

1Department of Environmental Sciences, University of Technology, PO Box 123 Broadway, Sydney, New South Wales 2065, Australia
2Gademann Messtechnik, Würzburg, Germany
3School of Biological Sciences (A08), University of Sydney, New South Wales 2006, Australia

ABSTRACT: While a number of factors have been linked to coral bleaching, such as high light, high temperature, low salinity, and UV exposure, the best explanation for recent coral bleaching events are small temperature excursions of 1 to 2°C above summer sea-surface temperatures in the tropics which induce 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 mechanism of the zooxanthellae. In the present investigation we addressed the question of whether such expelled zooxanthellae are indeed impaired irreversibly in their photosynthesis. We employed a Microscopy Pulse Amplitude-Modulated (PAM) fluorometer, by which individual zooxanthellae can be examined to study photosynthesis in zooxanthellae expelled when corals are subjected to a temperature of 33°C. We show that the expelled zooxanthellae from Cyphastrea serailia were largely unaffected in their photosynthesis and could be heated to 37°C before showing temperature-induced photosynthetic impairment. These results suggest strongly that the early events that trigger temperature-induced expulsion of zooxanthellae involve a dysfunction in the interaction of the zooxanthellae and the coral host tissue, and not a dysfunction in the zooxanthellae per se.

KEY WORDS: Chlorophyll fluorescence · Coral bleaching · Zooxanthellae · PAM

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