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Corals can survive acidic waters

March 29, 2007 By Daemon Fairless This article courtesy of Nature News.

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

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 years1.

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 Science2.

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.

On their own

No one expected that corals could survive such low pH.
Maoz Fine, Bar-Ilan University.
Equally remarkable is that the coral colony transformed from an interdependent collective into completely self-reliant organisms. Under normal conditions, coral polyps are connected by a tissue called the coenosarc, which enables them to share nutrients and spread out energy demands throughout the colony. "Corals are like a kibbutz," explains Fine. "All members are equal and share everything; if one polyp hunts plankton, it shares it among the community."

But in the most acidic environments, the polyps withdrew their coenosarcs and proceeded to fend for themselves.

It is the first time researchers have seen this kind of response. Fine says the acid-induced changes are so radical that several of his colleagues were unable to identify the two relatively common species. "Our students — everyone we showed — thought we were joking."

Once pH levels were returned to normal, the polyps quickly reverted back to their original state, rebuilding their skeletons, shrinking to their original size and restoring their kibbutz-like colony.

Losing the reefs

Fine's observations may also shed some light on the fossil record of coral. Skeleton-building corals seem to have suddenly appeared in great abundance during the Triassic, about 237 million years ago; there is little fossil evidence for them beforehand, during periods of high atmospheric carbon dioxide3. Daphne Fautin, a marine zoologist at the University of Kansas, suggests that the calcium carbonate skeleton, which we see as a defining characteristic of stony corals, may in fact be an ephemeral trait that waxes and wanes in response to environmental changes.

The difference with contemporary climate change Fautin warns, "is the rate with which CO2 is accumulating — organisms may not be able to keep up."

Still, the work provides some hope. "It is comforting to think that all corals won't immediately die off as a result of decreasing pH in the ocean," says Allen Collins, an invertebrate zoologist with the US National Oceanic & Atmospheric Administration in Washington DC. But there are hundreds of other coral species, Collins points out, and no one yet knows what the impact of ocean acidification will be on them.

Fine is quick to point out that his findings contain no good news for the future of coral reefs, or the fish that depend on them for protection and food. "We have to remember that while coral polyps may survive, reefs won't," he says.


  1. Caldeira M. & Wickett M. E. Nature, 425. 365 (2003).
  2. Maoz F. & Maoz F. Science, 315 . 1811 (2007).
  3. Stanley G. D. & Fautin D. G. Science, 291 . 1913 - 1914 (2001).

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