Though coral reefs make up less than one percent of the ocean floor, these ecosystems are among the most biodiverse on the planet—with over a million species estimated to be associated with reefs. The coral species that make up these reefs with their bony skeletons are known to be differently sensitive or resilient to ocean acidification—the result of increasing atmospheric carbon dioxide levels. But scientists are not sure why.
“Many agencies keep putting out reports in which they say, ‘Yes, coral reefs are threatened,’ with no idea what to do,” said Pupa Gilbert, a physics professor at the University of Wisconsin–Madison and senior author of the study. “Finding solutions that are science-based is a priority, and having a quantitative idea of exactly what’s happening with climate change to coral reefs and skeletons is really important.”
Reef-forming corals are marine animals that produce a hard skeleton made up of the mostly insoluble crystalline material aragonite. Aragonite forms when precursors made up of a more soluble form, amorphous calcium carbonate, are deposited onto the growing skeleton and then crystallize.
The team studied three genera of coral and took an in-depth look at the components of their growing skeletons. At Advanced Light Source Beamline 18.104.22.168, they used a technique that Gilbert pioneered called photoemission electron microscopy (PEEM), which detects the different forms of calcium carbonate with the greatest sensitivity to date.
When they used these spectromicroscopy images to compare the thickness of amorphous precursors between different genera, they found that Acropora, which is more sensitive to acidification, had a much thicker band of amorphous calcium carbonate than Stylophora, which is less sensitive.
A third genus of unknown sensitivity, Turbinaria, had an even thinner amorphous precursor layer than Stylophora, suggesting it should be the most resilient of the three to ocean acidification.
“If the surface of the coral skeleton, where all this amorphous calcium carbonate is being deposited by the living animal, crystallizes quickly, then that particular species is resilient to ocean acidification; if it crystallizes slowly, then it’s vulnerable,” Gilbert says. “For once, it’s a really simple mechanism.”
C.A. Schmidt, C.A. Stifler, E.L. Luffey, B.I. Fordyce, A. Ahmed, G. Barreiro Pujol, C.P. Breit, S.S. Davison, C.N. Klaus, I.J. Koehler, I.M. LeCloux, C. Matute Diaz, C.M. Nguyen, V. Quach, J.S. Sengkhammee, E.J. Walch, M.M. Xiong, E. Tambutté, S. Tambutté, T. Mass, and P.U.P A. Gilbert, “Faster Crystallization during Coral Skeleton Formation Correlates with Resilience to Ocean Acidification,” J. Am. Chem. Soc. 144, 1332 (2022), doi:10.1021/jacs.1c11434.
Adapted from the UW–Madison press release, “Coral skeleton formation rate determines resilience to acidifying oceans.”