Some corals can survive in acidified ocean conditions

Out of many corals which face survival risk because of rising acidity of the oceans, Caribbean coral species can survive and grow under the rising carbon dioxide levels, found researchers.

A new study published in the journal of Biological Sciences showed that at least three Caribbean coral species can survive and grow under conditions of ocean acidification more severe than those expected to occur during this century, although the density of their skeletons was lower than normal.

The study took advantage of the unusual seawater chemistry found naturally at sites along the Caribbean coastline of Mexico's Yucatan Peninsula, where water discharging from submarine springs has lower pH than the surrounding seawater, with reduced availability of the carbonate ions corals need to build their calcium carbonate skeletons.

In a two-year field experiment, the international team of researchers transplanted genetically identical fragments of three species of corals to a site affected by the springs and to a nearby control site not influenced by the springs, and then monitored the survival, growth rates, and other physiological traits of the transplants.

"The good news is the corals can survive and deposit calcium carbonate, but the density of their skeletons is reduced, which means the framework of the reef would be less robust and might be more susceptible to storm damage and bioerosion," said Adina Paytan, the author of the study.

Of the three species tested, the one that performed best in the low-pH conditions was Siderastrea siderea, commonly known as massive starlet coral, a slow-growing species that forms large dome-shaped structures. Another slow-growing dome-shaped species, Porites astreoides (mustard hill coral), did almost as well, although its survival rate was 20 percent lower. Both of these species outperformed the fast-growing branching coral Porites porites (finger coral).

"The slow-growing, dome-shaped corals tend to be more tolerant of extreme conditions, and they are important in building up the permanent structure of the reef. We found that they have the potential for persistence in acidified conditions," said co-author, Donald Potts.

Corals will have to cope with more than ocean acidification, however. The increasing carbon dioxide level in the atmosphere is also driving climate change, resulting in warmer ocean temperatures and rising sea levels. Unusually warm temperatures can disrupt the symbiosis between coral polyps and the algae that live in them, leading to coral bleaching. And rapidly rising sea levels could leave slow-growing corals at depths where they would die from insufficient sunlight.

Nevertheless, Potts noted that several species of Caribbean corals have long fossil records showing that they have persisted through major changes in Earth's history.