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Corals, the foundation of ocean biodiversity, are threatened by climate change. But new research suggests that these organisms might be more resilient than previously thought.

In a study in Science Advances, a CU Boulder researcher showed that despite a gradual increase in ocean acidity levels over the past 200 years, some corals seem to be able to adjust and continue to generate their hard, stony skeleton structures.

"We found that corals were able to regulate the mechanism they use to build and maintain their skeletons despite the ocean becoming more acidic," said Jessica Hankins, the paper's first author and a Ph.D. student in the Department of Geological Sciences. "It's an unexpected and hopeful signal; however, we need more long-term data to know what it really means."

As corals grow, they form their skeletons by absorbing ions from seawater into a space between the existing skeleton and the soft tissue above called the coral calcifying fluid. The coral has ways to regulate the chemistry of this fluid to make conditions ideal so that calcium and carbonate ions can combine to form calcium carbonate, the material that forms coral skeletons.

The ocean absorbs about 30% of carbon dioxide emissions from human activities. As more CO₂ dissolves in the ocean, the seawater undergoes a chemical reaction that makes the ocean surface more acidic. Previous studies suggest that ocean acidity has increased by 40% since the Industrial Revolution and is likely to rise further.

This shifts the balance of carbon species in seawater, resulting in fewer carbonate ions available in seawater, something that corals need to build their skeleton.

Scientists have predicted that ocean acidification would make it harder for corals to grow and maintain their skeletons, leading to less dense structures that are more prone to breakage. But prior experiments in the laboratory and in the wild have yielded unclear results.

Hankins set off to study long-lived coral skeletons using an advanced imaging technology called Raman spectroscopy.

Raman spectroscopy uses lasers to reveal the chemical makeup and molecule arrangement in objects like rocks, paintings and proteins. This method could show detailed information about coral skeleton chemistry, said Hankins, who is also the manager of CU Boulder's Raman Microspectroscopy lab.

When corals rapidly form the calcium carbonate mineral that composes their skeletons, which typically occurs when more carbonate ions are available, the resulting structures tend to contain other minerals extracted from seawater. These "impurities" affect the molecular arrangement and structure of calcium carbonate, showing an increase in the chaos of the coral skeleton under Raman spectroscopy.

"When conditions are favorable, corals seem to prioritize growth, even if that means producing skeletons that are a bit more disordered at the molecular level," Hankins said.

Hankins studied two pieces of coral skeleton, one nearly 200 years old and one 115 years old, from the Great Barrier Reef and the Coral Sea located off the northeastern coast of Australia. Using Raman spectroscopy, she found that both corals were able to regulate their internal fluid chemistry to maintain growth of their skeleton, despite an ongoing increase in ocean acidity due to ocean acidification. The corals appeared to be able to sustain the production of calcium carbonate even as the chemistry of the surrounding seawater grew less hospitable.

While it remains unclear how the corals adapted to the changing environment, Hankins said the secret might lie in their calcifying fluid.

"It could be that the processes corals use to modify and regulate their calcifying fluid are more complex than we've been able to constrain previously," said Hankins. "More studies are needed to determine if different species, or if the same species in a different location, have similar responses," she said.

In addition to ocean acidification, corals still face increasing stress from warming sea surface temperatures, human-induced pollution, and unsustainable fishing practices, Hankins said. Between 2023 and mid-May 2024, scientists have confirmed mass coral bleaching in at least 62 countries and territories worldwide. Coral bleaching occurs when corals expel the algae living in their tissues under stressful conditions, such as high ocean temperatures, causing them to turn completely white.

Coral reefs are the backbone of one of the largest ecosystems in the world. They protect shorelines threatened by erosion and storm damage, while providing marine organisms with a habitat, nursery, and spawning grounds.

"Corals provide the physical framework that reef ecosystems depend on. If they consistently grow weaker, less dense skeletons, it could trigger a domino effect that leads to a broader ecological collapse," Hankins said. "The ocean may feel distant from Colorado, but it's not separate. Every system on Earth is connected. What happens to coral reefs reverberates far beyond the shore."