Reefs get global warming lifeline
Certain algae can allow coral to withstand high temperatures.
Coral may be able to adapt to increased sea temperatures more easily than experts had thought. The finding allows cautious hope that the world's reefs will escape devastation at the hands of global warming.
Researchers have discovered that coral, tiny animals that forge alliances with algae to harness energy from the Sun, can team up with algae that are more tolerant of heat in response to warmer sea temperatures.
Experts had worried that warming could wipe out the world's reefs by 'bleaching' them, a process in which the coloured algae are destroyed to leave the corals in a bone-white, limbo state that can kill off a reef in weeks. But now it seems that some algae can survive higher temperatures, and can colonize bleached reefs, restoring them to life.
"Corals have a cunning ability to adapt to events because they're flexible in their associations," says Andrew Baker of the Wildlife Conservation Society in New York, one of the researchers who made the discovery. "This shows that the more dramatic predictions of coral-reef doom are simplistic."
Baker's team sampled coral of the genus Pocillopora from around the world between 1995 and 2001. As the researchers report in this week's Nature1, reefs subjected to previous bleaching were far more likely to contain coral hosting heat-tolerant algae than those that had remained intact.
Wildlife Conservation Society in New York
In Panama, corals equipped with Symbiodinium D resisted the high temperatures that caused much of the region's coral to bleach in 1997. As a result, the proportion of corals teamed with these algae increased from 43% in 1995 to 63% in 2001.
"It does sound like good news for corals," says Rob Rowan, who has studied coral bleaching on the Pacific island of Guam. In an accompanying study in Nature2, he shows that the species Symbiodinium D genuinely does withstand higher temperatures than related algae.
Rowan sampled species known as Symbiodinium C and Symbiodinium D from Guam's coral reefs. When transferred from 28.5ºC to 32ºC, Symbiodinium C's light-harnessing power was damaged, whereas Symbiodinium D survived unscathed.
If heat-tolerant Symbiodinium can team up with a wide range of corals, the world's reefs may be able to adapt faster to warmer temperatures, Rowan suggests. Rather than themselves evolving slowly to live in warmer waters, corals could simply swap one algal partner for another.
And there is evidence that this is possible, he adds. Many corals live on 'reef flats' in sunkissed shallows, where temperatures are often higher than in the surrounding deep water. Rowan suspects that many of these corals already harbour heat-friendly algae.
The discovery may mean that corals have more time than pessimists had feared, says Baker. Some experts had predicted that the world's reefs would all be gone in 20 to 30 years.
There is no cause for complacency, he adds. Corals are still threatened by factors such as water pollution and damage caused by fishing. But most of these factors are easier to reverse than climate change, Baker points out, especially if conservation efforts are spurred on by the idea that corals are not doomed by global warming.
"We may have more time than we thought to put policies in place," he says. "But this argues for us really getting on top of the factors that we can control."
- Rowan R., et al. Nature, 430. 742 (2004).
- Baker A. C, Starger C. J., McClanahan T. R. & Glynn P. W. Nature, 430. 741 (2004).