Known as the backbone of ocean biodiversity, corals are facing threats from climate change. However, recent research indicates that these organisms may possess greater resilience than we once believed. In a study featured in Science Advances, a researcher has demonstrated that, even with a steady rise in ocean acidity over the last 200 years, certain corals appear capable of adapting and maintaining 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, CU Boulder. “It’s an unexpected and hopeful signal; however, we need more long-term data to know what it really means.”
As corals develop, they create their skeletons by taking in ions from seawater into a space located between the existing skeleton and the soft tissue above, known as the coral calcifying fluid. The coral has mechanisms to control the chemistry of this fluid, ensuring optimal conditions for calcium and carbonate ions to merge and form calcium carbonate, the substance that constitutes coral skeletons. The ocean takes in approximately 30% of carbon dioxide emissions resulting from human activities. As more CO2 dissolves in the ocean, a chemical reaction occurs that increases the acidity of the seawater. Previous research indicates that ocean acidity has risen by 40% since the Industrial Revolution and is expected to continue increasing. This alteration affects the balance of carbon species in seawater, leading to a decrease in the availability of carbonate ions, which are essential for corals to construct their skeletons. Scientists have forecasted that ocean acidification will hinder corals’ ability to grow and sustain their skeletons, resulting in less dense structures that are more susceptible to damage. However, earlier experiments conducted in both laboratory and natural settings have produced ambiguous outcomes.
Hankins embarked on a study of long-lived coral skeletons utilising a cutting-edge imaging technology known as Raman spectroscopy. Raman spectroscopy employs lasers to uncover the chemical composition and molecular arrangement in various objects, including rocks, paintings, and proteins. This technique could provide intricate details about the chemistry of coral skeletons, according to Hankins, who also manages 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 favourable, corals seem to prioritise 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.
[image: The Great Barrier Reef is the world’s largest coral reef system. Credit: Jessica Hankins]
