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.