A better understanding of coral skeleton growth suggests ways to restore coral reefs

Coral reefs are vibrant communities that are home to a quarter of all species in the ocean and are indirectly crucial to the survival of the rest. But they are slowly dying – some estimates say 30 to 50 percent of coral reefs have been lost – due to climate change.

In a new study, physicists from the University of Wisconsin-Madison looked at the corals that form the reef at the nanoscale and identified how they create their skeletons. The findings provide an explanation of how corals are resistant to ocean acidification caused by rising carbon dioxide levels and suggest that controlling water temperature, not acidity, is crucial to mitigate loss and restore coral reefs.

“Coral reefs are currently threatened by climate change. It’s not in the future, it’s in the present,” says Pupa Gilbert, professor of physics at UW-Madison and senior author of the study. “How corals deposit their skeletons is critically important in assessing and helping their survival.”

The corals that form the reefs are marine animals that produce a hard skeleton made up of aragonite, a form of the mineral calcium carbonate. But how the skeletons grow remained unclear. One model suggests that calcium and carbonate ions dissolved in the calcifying fluid of corals attach one at a time to the crystalline aragonite of the growing skeleton. A different model, proposed by Gilbert and colleagues in 2017 and based on a study of a coral species, suggests instead that the undissolved nanoparticles attach and then crystallize slowly.

In the first part of a new study, published on November 9 on Proceedings of the National Academy of Sciences, Gilbert and his research group used a spectromicroscopy technique known as PEEM to probe the growing skeletons of five newly harvested corals, including representatives of all four possible reef-forming coral forms: branched, massive , encrusted and table. PEEM chemical maps of calcium spectra allowed scientists to determine the organization of different forms of calcium carbonate at the nanoscale.

PEEM results showed amorphous nanoparticles present in coral tissue, on the growing surface and in the region between tissue and skeleton, but never in the mature skeleton itself, supporting the nanoparticle attachment pattern. However, they also showed that while the growth rim is not densely packed with calcium carbonate, the mature skeleton is a result that does not support the nanoparticle attachment pattern.

“If you imagine a group of spheres, you can never completely fill the space; there is always space between the spheres,” says Gilbert. “So this was the first indication that nanoparticle attachment may not be the only method.”

The researchers then used a technique that measures the exposed inner surface of porous materials. Large geological crystals of aragonite or calcite, formed from something non-living, have been found to have a surface area about 100 times smaller than the same amount of nanoparticle material. When they applied this method to corals, their skeletons gave almost the same value as large crystals, not nanoparticulate materials.

“Corals fill space as much as a single crystal of calcite or aragonite. Therefore, both ionic and particle attack must occur,” says Gilbert. “The two separate fields that hold up the particles against the ions are actually both right.”

This new understanding of coral skeleton formation can only make sense if one other thing is true: that seawater is not in direct contact with the growing skeleton, as has been commonly assumed. In fact, recent studies of coral calcifying fluid have found that it contains slightly higher concentrations of calcium and three times more bicarbonate ions than seawater, supporting the idea that the growing skeleton is effectively isolated from seawater. .

Instead, the researchers propose a model in which corals pump calcium and carbonate ions from seawater through coral tissue, which concentrates those minerals near the skeleton. Importantly, this control allows corals to regulate their internal ion concentrations, even as the oceans become acidified due to rising carbon dioxide levels.

“Up until this work, people had assumed that there was contact between seawater and the growing skeleton. We have shown that the skeleton is completely separated from seawater, and this has immediate consequences,” says Gilbert. . “If there are to be coral reef remediation strategies, they shouldn’t focus on fighting ocean acidification, they should focus on fighting ocean warming. To save coral reefs we should lower the temperature, not raise the pH of the water.”