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Calcifiers cover up changes in ocean pH

Shell shock Corals and molluscs are able to protect themselves from the affects of acidic oceans, but only to a point, say marine scientists who have measured individual species resilience to pH change.

Australian researcher Dr Ross Jeffree, one of the study's principal investigators, says as pH levels fall protective structures such as shells that are based on calcium carbonate dissolve.

But their loss is offset by calcification or rebuilding of the lost skeleton or shells.

Jeffree, who until recently headed a marine environment laboratory at the International Atomic Energy Agency in Monaco, explains that they wanted to distinguish the rate of loss and the rate of accrual from the net rate of calcification.

"We wanted to understand in greater detail the ability to possibly enhance calcification rates when exposed to acidified waters and also to see whether the exposed parts of the shell will still dissolve."

The study, which is published in Nature Climate Change, examined three types of marine calcifiers: mussels, limpets and two species of corals. One of the coral species is completely covered by tissue while the other has regions of its skeleton exposed.

Relocations

There were several parts to the experiments.

"The only way to really understand what is happening with the calcification process is to couple transplantation experiments in the field with studies in the lab," says Jeffree.

In the transplantation experiments, the scientists relocated live and dead animals to an area near underwater volcanic vents off the Italian island of Ischia.

Volcanic vents naturally release CO2 and therefore decrease the surrounding pH. The natural pH gradient was found to range from 7.3 to 8.1.

Jeffree and colleagues also kept live samples, shells and dead corals in aquaria at temperature and pH conditions similar to their collection sites, and in acidic conditions.

To measure the rate of calcium deposited by the animal, Jeffree says they dissolved radioisotope-labelled calcium in the aquariums.

"We periodically removed pieces of shell, dissolved it, and measured the incorporation of the radio isotope using radioactive counters," he explains.

The net calcification rates were based on weight.

The researchers found that the rate of calcification in all species was higher in the low pH waters, and that when the animals shell or skeleton was protected by organic matter or tissue it was better able to resist acid attack.

This meant the mussels, which have a periostracum (skin-like organic layer) were better able to cope with low pH than the limpets, and the coral that was completely covered in tissue was also more resilient.

Detrimental effects

Dr Zanna Chase, an oceanographer at the University of Tasmania, says it's an interesting study that could help reconcile inconsistent results from other studies that have also looked at the affect of low pH.

"Most of the studies have just looked at the net effect … but this study is unique in that it is actually measuring rates of calcification using an isotopic method."

"It is really useful to look at the individual components of calcification and the rate of dissolution independently because they are affected differently by the acidification," she says.

Chase also says the paper also raises some interesting questions about interactions with other environmental components such as the nutrient and algal levels that may affect an organism's resilience to changed conditions.

According to Jeffree, who is now at the University of Technology, Sydney, the transplanted animals found the combined environmental impacts of the warmer water during the summer and low pH even more difficult to contend with.

"The future marine waters will not only be acidified, they are going to be increasing in temperature. So when you combine the two, they are even more challenged and find it even more difficult to maintain their skeleton or to calcify. It's one of those situations that seem to be increasing in the Mediterranean," he says.

"I think more experimental investigations combining the temperature variable with the acidification variable will shed more light on what the future actually holds."