http://www.nature.com/news/2007/070326/full/070326-13.html

Published online: 29 March 2007; | doi:10.1038/news070326-13

Corals can survive acidic waters

Mediterranean corals could strip, but not die, in response to climate change.

Daemon Fairless

All dressed up: the hard skeleton of a coral is essential for reefs, but maybe not for the coral itself. Punchstock

Reef-building corals may be more resilient against climate change than scientists had previously thought. Researchers have discovered that some species are able to survive an increase in seawater acidity, even though it strips the individual coral polyps of their protective calcium carbonate skeletons. This may be good news for individual polyps, but it doesn't change the gloomy outlook for reef ecosystems.

As atmospheric carbon dioxide levels continue to rise, so do the levels of dissolved carbon dioxide in sea water. This leads to an increase in ocean-borne carbonic acid, which is capable of dissolving calcium carbonate. "This is a major problem for corals," says Maoz Fine, a marine zoologist at Bar-Ilan University in Israel. "Essentially, acidification leads to naked coral."

Researchers estimates that ocean surface pH could decrease from 8.2 to 7.8 by the end of this century - more acidic than it has been for the past 20 million years.

Fine set out to study the effects of this ocean acidification on two species of Mediterranean coral, Oculina patagonica and Madracis pharencis.

He subjected specimens in the lab to increasingly acidic conditions. It didn't take long for the colonies in the most acidic environments - those with pH levels as low as 7.3 - to show remarkable changes; within a few weeks, their calcium carbonate skeletons had started to dissolve and the polyps became entirely exposed, he and a colleague report in Science.

Surprisingly, the polyps seemed to fare well under these conditions, growing up to three times their original size and reproducing unhindered. "No one expected that corals could survive such low pH," says Fine.