Corals get a double blow negative from heat – those that are bleached as a result of heat stress are also less resistant to ocean acidification.
Robert Eagle of the University of California at Los Angeles and his colleagues analyzed the effects of elevated temperatures on the growth of two types of hard corals when the corals are also exposed to ocean acidification.
Ocean acidification occurs through the uptake of carbon dioxide in the atmosphere by seawater. The result is a decrease in the pH of the water, a decrease in its concentration of carbonate ions, and a decrease in the states of saturation of calcium carbonate minerals.
Both carbonate ions and calcium carbonate minerals are essential for the formation of coral skeletons, and a drop in the saturation states of calcium carbonate minerals increases the likelihood that the skeletons will disintegrate. The pH of surface ocean waters has decreased by 0.1 units since the beginning of the industrial revolution.
To test the acidity of corals, the researchers released samples of cauliflower corals. Pocillopora damicornisand crested coral, Stylophora pistillatato different partial pressures of carbon dioxide both at 28 ° C, an almost optimal temperature, and at 31 ° C, an elevated temperature.
Eagle and his team used two methods to measure coral pH. First, they used robotic microelectrodes inserted directly into coral tissue to measure the pH in pockets of fluid from which the coral skeleton grows. They also measured pH indirectly using a borisotopic method.
The team found that both types of coral could cope with ocean acidification conditions at 28 ° C. To compensate, the corals increased their internal pH and also changed their internal chemistry to encourage calcification, the process by which corals form their skeletons.
In waters with 31 ° C – enough heat stress to bleach the corals – the calcification rates decreased for both types of coral.
According to Eagle, a better understanding of the interaction between different stressors is important for coral protection. “Conservation isn’t just about temperature, although that’s clearly the most important problem,” he says. “The scale of the effort that would require human intervention to maintain and restore these environments is clearly a tremendous effort.”