Corals more resilient to climate change than earlier believed

Reef-building corals may be more resilient against climate change than previously thought and 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, a new study by an Israeli zoologist has revealed.

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

Researchers estimate 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.

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

He subjected specimens in the lab to increasingly acidic conditions, and found that that 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, Fine and a colleague wrote in their study in Science.

Findings further revealed that 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,” said Fine, adding that equally remarkable were findings that showed that the coral colony transformed from interdependent collective organisms into completely self-reliant organisms.

Under normal conditions, coral polyps are connected by a tissue called the coenosarc. This enables them to share nutrients and spread out energy demands throughout the colony.

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

Fine said this is the first time they have seen this kind of response.

Interestingly, 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, Nature quoted Fine as saying.

“Corals are like a kibbutz. All members are equal and share everything; if one polyp hunts plankton, it shares it among the community,” said Fine.