Seawater absorbs some of the CO2 from the atmosphere, and it is thought that by 2100, this will increase the acidity of surface ocean waters by 0.3-0.5 pH units.
Acidity reduces the amount of available carbonate used by some marine organisms, such as corals and molluscs, to form shells and skeletons out of calcium carbonate.
Previous studies suggest different species of marine organisms that form shells and skeletons vary in their sensitivity to ocean acidification.
It is thought that an outer layer of living tissue on these organisms protects the skeleton or shell from dissolving in more acidic seawater.
Partly funded under the EU MedSeA project1, the researchers compared the rates at which they form shells and skeletons (a process known as calcification) and lost (a process known as dissolution) in samples of the corals Balanophyllia europaea (which has a protective outer layer of tissue) and Cladocora caespitose (which does not have a protective layer); and in samples of the molluscs Mytilus galloprovincialis (a mussel with an outer layer protecting the shell) and Patella caerulea, (a limpet with no protective layer).
Samples of the corals and molluscs were transplanted into water off the Island of Ischia in Italy.
Volcanic activity from nearby Mount Vesuvius causes CO2 to bubble up from the ocean floor, creating naturally occurring acidified seawater with a range of different pH conditions.
Currently, normal seawater is pH 8.1, but by 2100, it is projected to be 7.8 (lower pH denotes more acidity). In waters of pH 7.8 at the test site, corals and molluscs were able to continue calcifying, in some cases at faster than normal rates.
However, as the water became more acidic, the rate at which shells and skeletons dissolved increased. How much dissolved depended on the amount of protective tissue covering the shells or skeletons.
Living molluscs and limpets transplanted to the acidic waters continued to calcify at pH 8.1 (normal seawater) and pH 7.4. Mussels were able to increase the rate of calcification even at pH 7.2. Limpets living in highly acidic areas of the sea (pH 6.5) were able to increase their rate of calcification, possibly in response to the higher rates of dissolution of the shells.
Both species of coral in the test area were able to continue calcifying, although this rate decreased by 30% at pH 7.4 for C. caespitose, in contrast to B. europaea, which exhibited an increased rate of calcification at higher levels of acidity. The corrosive action of the seawater was evident on C. caespitose, whereas B. europaea was unaffected and protected by an outer layer of tissue.
However, coral and molluscs were more susceptible to the effects of ocean acidification under higher temperatures. Under unusually high temperatures in September 2009, for example, B. europaea samples in water of pH 8.0 continued to calcify normally, but almost stopped at pH 7.4. A warming Mediterranean Sea is likely to worsen the impact of ocean acidification, affecting even those organisms that were resistant to higher levels of acidity.