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By Donna Lu

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Corals get a double-whammy negative from heat – those that are bleached as a result of heat stress also become less resilient to ocean acidification.

Robert Eagle at the University of California, Los Angeles and his colleagues have analysed the effect of elevated temperatures on the growth of two species of stony coral when the corals are also exposed to ocean acidification.

The acidification of oceans occurs as result of carbon dioxide in the atmosphere being absorbed by seawater. The results are a decrease in the pH of the water, a decrease in its concentration of carbonate ions and a drop in the saturation states of calcium carbonate minerals.

Both carbonate ions and calcium carbonate minerals are essential for forming coral skeletons, and a drop in the saturation states of calcium carbonate minerals makes it more likely for the skeletons to start to dissolve. The pH of surface ocean waters has decreased by 0.1 units since the beginning of the industrial revolution.

To test the resilience of corals to acidification, the researchers exposed samples of cauliflower coral, Pocillopora damicornis, and hood coral, Stylophora pistillata, to different partial pressures of carbon dioxide at both 28° C, a near-optimal temperature, and 31°C, an elevated temperature.

Eagle and his team used two methods to measure the corals’ pH. Firstly, they used robotically controlled microelectrodes that were inserted directly into coral tissues to measure the pH in fluid pockets from which the coral skeleton grows. They also indirectly measured the pH using a boron-isotoping method.

The team found that both species of coral coped with ocean acidification conditions at 28°C. To compensate, the corals elevated their internal pH and also altered their internal chemistry to promote calcification, the process by which corals form their skeletons.

When in 31°C waters – enough heat stress that it caused the corals to bleach – the rates of calcification decreased for both coral species.

A better understanding of how different stressors interact is important for coral conservation, according to Eagle. “Conservation is not just about temperature, even though that is clearly the most important problem,” he says. “The scale of the effort that it would take for human intervention to preserve and restore these environments is clearly a huge effort.”

Journal reference: Science Advances, DOI: 10.1126/sciadv.aba9958