What was done
The authors carried out what they describe as the first field study to follow changes in Symbiodinium genotypes in coral colonies over a period of time that included a natural episode of bleaching. It was conducted on a reef flat adjacent to Miall Island (23°09'S, 150°54'E), which is part of the Keppel Island group located in the southern inshore Great Barrier Reef, where between September 2004 and March 2005 they tagged 460 colonies of Acropora millepora coral that subsequently experienced a high-temperature event in January and February of 2006 that led to the bleaching of 89% of the tagged colonies. In this study, the coral symbionts were identified by "a combination of single-stranded conformation polymorphism (SSCP) analysis, cloning and DNA sequencing," which was applied to all of the 460 colonies at the start of the study and to a subset of 79 colonies that were still alive three months post-bleaching, while mortality was assessed six months after bleaching by visually estimating the percentage of live and dead coral tissue on 159 randomly chosen tagged colonies using pre-bleaching photos of each colony as a reference.

What was learned
Prior to the bleaching event, Jones et al. report that "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%)." During the bleaching event, they further report that "the relative difference in bleaching susceptibility between corals predominated by C2 and D was clearly evident, with the former bleaching white and the latter normally pigmented," while corals harboring a mix of Symbiodinium C2 and D were "mostly pale in appearance." Then, three months after the bleaching event, they observed "a major shift to thermally tolerant type D and C1 symbiont communities ... in the surviving colonies," the latter of which types had not been detected in any of the corals prior to bleaching; and they report that "this shift resulted partly from a change of symbionts within coral colonies that survived the bleaching event (42%) and partly from selective mortality of the more bleaching-sensitive C2-predominant colonies (37%)." In addition, they report that all of the colonies that harbored low levels of D-type symbionts prior to the bleaching event survived and changed from clade C2 to D predominance.

What it means
The five researchers conclude 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," as they note that "a shift from bleaching-sensitive type C2 to clade D increased the thermal tolerance of this species by 1-1.5°C." Last of all, they say that "a recent study has shown that the majority of scleractinian corals are likely to harbor symbiont types at levels that are undetectable using electrophoretic genetic techniques (Mieog et al., 2007)," such as was the case with the thermo-tolerant clade C1 in their study, which they say suggests that "symbiont flexibility may also be more common than previously thought."

Reference
Mieog, J.C., van Oppen, M.J.H., Cantin, N., Stam, D.W.T. and Olsen, J.L. 2007. Real-time PCR reveals a high incidence of Symbiodinium clade D at low levels in four scleractinian corals across the Great Barrier Reef: implications for symbiont shuffling. Coral Reefs 26: 449-457.