Background
Over the past quarter-century many corals around the world periodically experienced the significant - and sometimes deadly - stress associated with episodes of unseasonably high temperatures that caused them to "bleach" and lose their complements of symbiotic photosynthetic algae or zooxanthellae, with some corals subsequently recovering but with others dying. Climate alarmists contend that this unfortunate state of affairs will only worsen with the passage of time, and that the world's corals are doomed to extinction, if CO2-induced global warming continues unabated.

What was done
Berkelmans and van Oppen (2006) investigated the thermal acclimatization potential of Acropora millepora - a common and widespread Indo-Pacific hard coral species - to rising temperatures through transplantation and experimental manipulation. Working within Australia's Great Barrier Reef system, they transplanted multiple colonies of the coral from a cool (mean summer water temperature of 27.0°C) southern inshore reef (North Keppel Island), as well as from a cool (mean summer water temperature of 28.3°C) central offshore reef (Davies Reef), to a warm (mean summer water temperature of 29.2°C) inshore bay (Magnetic Island). At the same time, they also maintained colonies from each site in tanks maintained at 27.5°C (a non-bleaching control treatment), 30.0°C, 31.0°C and 32.0°C in indoor aquariums. In addition, each coral's zooxanthellae were identified, both before and after transplantation and what followed it, based on "the nuclear ribosomal DNA internal transcribed spacer 1 (ITS1) region using single stranded conformation polymorphism and sequencing analysis (van Oppen et al., 2001; Ulstrup and van Oppen, 2003)."

What was learned
As a result of their experimental manipulations and measurements, the two researchers found that adult corals, as they describe the situation, "are capable of acquiring increased thermal tolerance and that the increased tolerance is a direct result of a change in the symbiont type dominating their tissues from Symbiodinium type C to D," and they report that "the level of increased tolerance gained by the corals changing their dominant symbiont type to D is around 1-1.5°C."

What it means
In describing the importance of their findings, which they say are of "huge ecological significance for many coral species," Berkelmans and van Oppen state that their results indicate that "autonomous thermal acclimatization/adaptation, mediated by zooxanthella change, is a real and naturally operating process in reef corals," and they aver that their findings "support the adaptive bleaching hypothesis as originally proposed (Buddemeier and Fautin, 1993) and later refined (Buddemeier et al., 2004)." With respect to a question that yet remains unresolved, however, they add that whether this phenomenon of symbiont shuffling, as it is more commonly called, "was brought about simply by a shuffling of the zooxanthella types already contained in the coral tissues, or was the result of taking up new types from the environment is uncertain." Nevertheless, and whichever the case may be, it is clear that the phenomenon provides what the two marine biologists call an important "nugget of hope" for coral reefs "in an era of climate change."

References
Buddemeier, R.W. and Fautin, D.G. 1993. Coral bleaching as an adaptive mechanism. BioScience 43: 320-326.

Buddemeier, R.W., Baker, A.C., Fautin, D.G. and Jacobs, J.R. 2004. The adaptive hypothesis of bleaching. In: Rosenberg, E. and Loya, Y. (Eds.), Coral Health and Disease, Springer-Verlag, Berlin, Germany.

Ulstrup, K.E. and van Oppen, M.J.H. 2003. Geographic and habitat partitioning of genetically distinct zooxanthellae (Symbiodinium) in Acropora corals on the Great Barrier Reef. Molecular Ecology 12: 3477-3484.

Van Oppen, M.J.H., Palstra, F.P., Piquet, A.M.-T. and Miller, D.J. 2001. Patterns of coral-dinoflagellate associations in Acropora: significance of local availability and physiology of Symbiodinium strains and host-symbiont selectivity. Proceedings of the Royal Society B 268: 1759-1767.