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Evidence of Symbiont Shuffling in Great Barrier Reef Corals in Response to Warming-Induced Bleaching
Reference
Jones, A.M., Berkelmans, R., van Oppen, M.J.H., Mieog, J.C. and Sinclair, W. 2008. A community change in the algal endosymbionts of a scleractinian coral following a natural bleaching event: field evidence of acclimatization. Proceedings of the Royal Society B 275: 1359-1365.

What was done
On a reef flat adjacent to Miall Island of the Keppel Island group located in the southern inshore Great Barrier Reef, the authors tagged 460 colonies of the stony coral Acropora millepora between September 2004 and March 2005, after which they assessed their Symbiodinium (algal endosymbiont) composition via a combination of single-stranded conformation polymorphism (SSCP) analysis, cloning, and DNA sequencing. Then, three and six months after a severe warming-induced bleaching event that occurred in January and February of 2006 (when 89% of the corals bleached), they assessed the Symbiodinium composition of a 79-member subset of the coral colonies that survived the bleaching.

What was learned
"Before the bleaching in 2006," in the words of Jones et al., "A. millepora at Miall reef associated predominantly with Symbiodinium type C2 (93.5%) and to a much lesser extent with Symbiodinium clade D (3.5%) or mixtures of C2 and D (3.0%)." When bleaching was at its most intense, they report that the corals hosting predominantly C2 endosymbionts were "bleaching white," while those predominantly hosting type D were "normally pigmented." Then, six months after the bleaching event, they observed that "colonies that were originally C2 predominant suffered high mortality (37%) compared with D-predominant colonies (8%)," while "71% of the surviving tagged colonies that were initially C2 predominant changed to D or C1 predominance."

What it means
The five marine biologists say that "as a direct result of the shift in symbiont community, the Miall Island A. millepora population is likely to have become more thermo-tolerant," based on the findings of Berkelmans and Van Oppen (2006), whose work in the same region suggested that "a shift from bleaching-sensitive type C2 to clade D increased the thermal tolerance of this species by 1-1.5C." As a result, they say their results "strongly support the reinterpreted adaptive bleaching hypothesis of Buddemeier et al. (2004), which postulates that a continuum of changing environmental states stimulates the loss of bleaching-sensitive symbionts in favor of symbionts that make the new holobiont more thermally tolerant." In fact, they state that their observations "provide the first extensive colony-specific documentation and quantification of temporal symbiont community change in the field in response to temperature stress, suggesting a population-wide acclimatization to increased water temperature," a finding that bodes especially well for earth's corals in a warming climate.

References
Berkelmans, R. and van Oppen, M.J.H. 2006. The role of zooxanthellae in the thermal tolerance of corals: a 'nugget of hope' for coral reefs in an era of climate change. Proceedings of the Royal Society B 273: 2305-2312.

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, Berlin, Germany, p. 427-444.

Reviewed 9 July 2008