Bleached Zooxanthellae are Healthy

Bleached Zooxanthellae Are Healthy!


Background: Increased temperature and light stress causes coral bleaching. Bleaching is the loss of symbiotic microalgae (also known as zooxanthellae) from the host coral tissue. Once corals are exposed to elevated temperatures and light, the zooxanthellae are expelled, leaving the coral white in appearance, hence the term bleached. It has been assumed that if zooxanthellae are expelled from the host coral, then they must either be dead or represent a threat to the health of the host. It has been found that coral on the southern Great Barrier Reef, bleach at about 33^oC. Research aims: Determine the upper thermal limit of zooxanthellae isolated from healthy coral at 25^oC. Determine the upper thermal limit of zooxanthellae bleached at 33^oC. Does the condition of expelled zooxanthellae decline with increasing thermal exposure? Summary of results: The adjacent figure shows how zooxanthellae photosynthesise (linked to electron transport) at a range of light intensities. The control samples were collected from corals at room temperature (25^oC), and the rate of electrons being moved through the photosystems indicated the level of photosynthesis. At 100 µmol photons m^-2 s^-1 (about 20% of the normal light available to coral in a shallow reef lagoon), the electron transport rate is about 10. If we give the coral more light, photosynthesis is reduced. Zooxanthellae collected from a bleached coral at 33^oC are no different from the controls. If we warm those cells up to 37^oC, the electron transport rate (ETR) is only slightly lower (about 7.5). Only when we heat the cells to 38^oC does the photosynthesis actually cease. This shows us that the upper thermal limit for zooxanthellae is around 37-38^oC, not 33^oC, which is the temperature at which this coral usually bleaches in the wild. Future directions: Why is the upper limit of the zooxanthellae higher than the bleaching temperature of the coral? Can expelled zooxanthellae survive a bleaching event? Research team: Associate Professor Peter Ralph, University of Technology, Sydney Rolf Gademann, Wurzburg, Germany Professor Tony Larkum, Sydney University Funding support for this project: University of Technology, Sydney Internal funds Further information Can be found in the following manuscript: Ralph P.J. Gademann R, A.W.D. Larkum (2001) Zooxanthellae expelled from bleached corals at 33^oC are photosynthetically competent. Marine Ecology Progress Series 220, 163-168. [PDF]