Coral bleaching—caused by the loss of the coral's photosynthetic algae symbiotic partners that live within each polyp—has been linked to global climate change and causes long-term damage to reef ecosystems. The ability of corals to adapt to the effects of global warming is uncertain, but two short papers published this week in Nature claim to provide evidence that corals might be able to evolve their way out of trouble.

In the first study, Andrew Baker of the Wildlife Conservation Society, together with colleagues from Columbia University and the University of Miami, present the results of molecular surveys of coral reefs from around the world, before, during, and after El Niño Southern Oscillation bleaching events, which tend to have warming effects and are thus considered a good model for climate change. Studying the zooxanthellae that give the coral its color, they discovered that one group of symbionts—"clade D"—which is usually relatively rare, occurred much more frequently in coral reefs that had recently escaped bleaching, when others nearby had succumbed.

In the second report, Rob Rowan, a marine biologist at the University of Guam Marine Laboratory, lends support to Baker et al.'s findings by showing that clade D Symbiodinium algae are better at withstanding higher temperatures. An increase of less than 4 degrees centigrade was enough to cause a persistent reduction in the photosynthetic activity of clade C zooxanthellae, whereas corals containing clade D symbionts actually performed better at the higher temperature.

Both sets of findings suggest that coral reefs might adapt to future increases in sea temperature by switching their symbionts: adopting clade D zooxanthellae could enable them to avoid bleaching. If true, the authors suggest, it would be great news for the long-term health of coral reef ecosystems and would demand the revision of models predicting the effects of climate change.

"By identifying what makes some corals more thermally tolerant than others it might become possible to predict which populations are likely to be affected most by global warming," Rowan told The Scientist. "This would allow conservation programs to target those coral reefs that are more likely to survive."

"Even though Rowan's findings are limited to a single species of coral, they are encouraging," said marine biologist Clay Cook at the Harbor Branch Oceanographic Institution, who was not involved in the studies. "Combined with Baker et al.'s findings, they give us some hope that corals can recover from bleaching by having symbionts that can tolerate warmer temperatures."

However, some researchers reacted cautiously to the findings. For example, Ove Hoegh-Guldberg, director of the University of Queensland's Centre for Marine Studies, is concerned that the sampling methods used by Baker et al. might not have revealed the true nature of the problem.

"Clade D is also correlated with light levels and water turbidity, which might have little or nothing to do with water temperature," Hoegh-Guldberg said. Showing an increase in the proportion of corals using clade D symbionts isn't necessarily evidence of increasing resistance to climate change, he said; instead, they could suggest a reduction in genetic variation, signaling a less positive outcome for corals.

"The findings might simply be part of a 'going, going, gone' phenomenon. As the climate shifts, we are left with the toughest survivors, but they too might be doomed in the long run." Oceanic alkalinity is also currently falling, said Hoegh-Guldberg, who was not involved in the studies, and who warned that this decrease might be even more important than rising water temperatures.

Steve Coles, a coral biologist at the Bernice P. Bishop Museum, said he was optimistic, however. "Coral reefs have been a feature of the world's oceans for at least 65 million years, during which they have undergone numerous episodes of global warming," he told The Scientist. However, because corals tend to be tightly adapted to their local habitat, they frequently show bleaching stress whenever the normal temperature maximum is exceeded.

"On a worldwide level, though, corals show a wide range of thermal tolerance," said Coles, who was not involved in the studies. "What we need to discover is how rapidly they can adapt within a given geographical area."

"The reality is that only the next 30–50 years will reveal the truth about this issue," Coles said.