Background
The authors write that "coral reefs are among the most biologically diverse and economically important ecosystems on the planet, providing ecological services that are vital to human society and industries through fisheries, coastal protection, pharmaceutical compounds and tourism," but they say that "if the thermal sensitivity of corals does not change, coral reefs face serious problems in scenarios where tropical seas may be as much as 2°C-4°C warmer by the end of this century."

What was done
In a study designed to further explore this subject, Bellantuono et al. "tested the response of Acropora millepora to thermal preconditioning by exposing coral nubbins to 28°C (3°C below bleaching threshold) for 10 days, prior to challenging them with water temperatures of 31°C for 8 days," while "in another treatment (non-preconditioned), corals were exposed to 31°C without prior exposure to the 28°C treatment," all of which procedures were conducted in a set of "transparent tanks plumbed into flowing sea water, with four replicate tanks for each treatment."

What was learned
The three researchers discovered that short-term preconditioning to higher-than-ambient temperatures (but still 3°C below the experimentally determined bleaching threshold) for a period of ten days provided thermal tolerance for the scleractinian coral and its symbionts. And based on various genotypic analyses they conducted, they determined that "the acclimatization of this coral species to thermal stress does not come down to simple changes in Symbiodinium symbiont shuffling and/or the bacterial communities that associate with reef-building corals bacterial shuffling."

What it means
Bellantuono et al. say their findings suggest that "the physiological plasticity of the host and/or symbiotic components appears to play an important role in responding to ocean warming," and they go on to describe some real-world examples of where this phenomenon may have played a crucial role in preserving corals exposed to extreme warm temperatures in the past (Fang et al., 1997; Middlebrook et al., 2008; Maynard et al., 2008), all of which suggests, of course, that earth's corals may be much better adapted to dealing with global warming than has long been believed.

References
Fang, L.S., Huang, S.P. and Lin, K.L. 1997. High temperature induces the synthesis of heat-shock proteins and the elevation of intracellular calcium in the coral Acropora grandis. Coral Reefs 16: 127-131.

Maynard, J.A., Anthony, K.R.N., Marshall, P.A. and Masiri, I. 2008. Major bleaching events can lead to increased thermal tolerance in corals. Marine Biology 155: 173-182.

Middlebrook, R., Hoegh-Guldberg, O. and Leggat, W. 2008. The effect of thermal history on the susceptibility of reef-building corals to thermal stress. Journal of Experimental Biology 211: 1050-1056.