Three species of Caribbean coral are hardy enough to survive the rising levels of ocean acidification expected to take place over the next century, according to the results of a recently-published study. However, whilst the corals were able to live and grow in an unusually hostile environment, their skeletons were seriously affected, leaving them vulnerable to erosion.

The diverse marine ecosystems of our Blue Planet are being dramatically changed by human beings. Plastic waste and other forms of anthropogenic pollution plague the once-pristine oceans, while entire species are being brought to the brink of extinction by overfishing.

Other forms of underwater life suffer from the insidious side effects of human-led climate change. Rising temperatures put massive amounts of stress on coral populations, causing them to expel symbiotic algae and thus to "bleach." Depending on the amount of time that the algae remain absent, this can result in the death of the coral, which in turn can have significant consequences for the ecosystem in which it lived.

Corals are also suffering from an increase in ocean acidity brought on by elevated carbon levels in Earth's atmosphere. A vast amount of atmospheric carbon dioxide is absorbed by Earth's oceans each day. When this happens, chemical reactions reduce the seawater's pH, making it more acidic. This in turn reduces the amount of calcium carbonate present in the water, which corals need to build their skeletons and so grow larger.

An international team of scientists set out to examine the effects that long-term exposure to high ocean acidity could have on three coral species native to the Caribbean. The researchers transplanted coral samples at sites along the coastline of Mexico's Yucatan Peninsula, where water issuing from submarine springs had lowered the pH of the surrounding seawater.

According to the team, the unusual environment surrounding the springs is even more acidic than Earth's oceans are predicted to be in the year 2100.

The researchers transplanted samples of Siderastrea siderea, also known as the massive starlet, Porites asteoides, also called the mustard hill coral, and Porites porites, or the finger coral. The first two are slow growing dome-shaped corals and are important for the long-term structure of a reef. The third species is a faster-growing coral that extends out into branching formations.

The samples were placed at two sites, one affected by the springs, and one not. They were then observed for two years as the team tracked their survival and growth rates alongside other physiological characteristics. Despite the high acidity of the environment surrounding the springs, the corals were able to survive and even grow their skeletons with calcium carbonate.

The massive starlet coral coped best with the adverse conditions, while the mustard hill coral had a survival rate 20 percent lower, but was also able to adapt. Only 33 percent of the finger corals were able to endure to the end of the two-year field study.

The researchers believe that the success of the two dome-shaped corals could be at least in part due to their algae. In the low-pH regions, the algae had a higher than normal chlorophyll concentration, which could indicate that they were creating more energy to help the corals fight the stress arising from their uncomfortable new environment. Furthermore, both species' offspring are fertilized internally, allowing them to settle in the immediate vicinity of their parent coral. This allows for multiple generations of offspring to genetically adapt to the conditions around the springs.

Whilst all of the corals that survived were able to grow, the researchers found that their skeleton density had reduced by 15 to 30 percent relative to their counterparts that were placed away from the springs. This lack of bone mass would undermine the strength of the reef, making it more susceptible to storm damage and natural erosion.

Whilst the survival of the coral transplants is encouraging, corals inhabiting Earth's oceans will have to contend with challenges beyond ocean acidity. Climate change is predicted to increase the temperature of our planet's seas, triggering wide-spread coral bleaching. Furthermore, the sea level is expected to continue rising. This could leave slow growing corals too deep in the water to allow their symbiotic algae to effectively photosynthesize, meaning that they would essentially die from light starvation.

The information collected in the study could be helpful in protecting Earth's reefs, or even aid in the creation of genetically modified coral species that are better equipped to cope with the threats posed by climate change.

"These corals are more robust than we thought," said Adina Paytan, a research professor at the University of California Santa Cruz, and corresponding author on the new paper. "They have the potential to persist with ocean acidification, but it costs them energy to cope with it, so we have to do all we can to reduce other stressors, such as nutrient pollution and sedimentation."

The paper has been published in the journal Proceedings of the Royal Society B.