Under Pressure: How Coral Skeletons Handle Ocean Acidification

Written by Sofia Perez

Edited by Manu Madhavan

We often see the world in terms of duality: right and wrong, black and white, man and woman. However, upon closer inspection, these dualities have an annoying habit of breaking down. Yes, I said it: there’s nuance. The same applies to the structures all around us. There is no clear divide between the mechanical properties of man-made buildings, statues, and sculptures versus those of reefs, trees, and caves. However, we find ourselves in a situation where, as human beings, we are only just arriving at the genius that the natural world has held all along. This is why I have become enchanted by biomechanics. 

For now, let’s just talk about coral reefs, the fascinating albeit enigmatic biological structures we find across the globe. These are structures surrounded by mystery, but also surrounded by life. There are many components to a successful reef, including environmental factors such as sea surface temperature, salinity, pH, and sunlight. Biotic factors like the plethora of flora and fauna that ensure ecological balance. There are also zooxanthellae, the algae that live in symbiosis with coral, providing food and energy as a byproduct of photosynthesis. Today though, I want to talk about the coral skeleton and the threat it faces from ocean acidification, which according to the National Oceanic and Atmospheric Administration, “refers to a reduction in the pH of the ocean over an extended period of time, caused primarily by uptake of carbon dioxide from the atmosphere”.

Why, you may ask? Well, the strength of a coral’s skeleton is a vital measure of its adaptability, which is a necessary feature to possess in a rapidly changing and erratic climate. The main reason is the interaction between carbon dioxide, seawater, and limestone (i.e., calcium carbonate, which is what reefs are made of). According to this post by MIT’s Earth, Atmospheric, and Planetary Sciences department, coral skeletons are made of aragonite, a commonly-occurring type of calcium carbonate crystal. To grow toward the sunlight, which is necessary to optimize photosynthesis in its symbiotic zooxanthellae, corals develop a network of aragonite crystals supported by more bundles of crystals. This makes reefs more resilient to waves, storms, boring and biting animals, and breakage caused by currents. 

Fig. 1 Kamelev, Egor. Pexels, http://www.pexels.com/photo/corals-920161/. Accessed 5 Mar. 2023.

However, the mechanical properties of these aragonite skeletons are currently under a plethora of threats, such as ocean acidification, caused by rising levels of carbon dioxide in the atmosphere. Usually, the carbon dioxide is absorbed into the seawater, producing bicarbonate and carbonate ions, which coral polyps bring into a “calcifying space”—called the calicoblastic ectoderm if you want to be fancy- alongside calcium ions. This “calcifying space” resides in between cells and the surface of the existing skeleton. The coral polyps then pump hydrogen ions out of this space to produce more carbonate ions, which bond with calcium ions to make calcium carbonate. Acidification, however, throws this process for a loop, as the decrease in seawater pH is said to cause these carbonate minerals to dissolve. 

Previous studies on species like Stylophora pistillata also support the claim that an increase in carbon dioxide concentration- which decreases pH, leading to acidification – leads to a change in physiology by reducing the amount of tissue on each organism, altering the chlorophyll (i.e. the part of the zooxanthellae carrying out photosynthesis), the density of the zooxanthellae celll, the, calcification rate, porosity, and skeletal density. But what about the coral skeleton?

According to this 2019 paper, empirical evidence supports the idea that in more acidic water, the coral’s tissue gets thicker and makes more organic material, as this helps the coral make its skeleton even in suboptimal conditions. Another experiment from this same study supported the idea that even as the acidity of the water changed, the coral was able to control the chemistry used to make its skeleton by increasing its alkalinity. The weight loss observed in the experiment suggests that the coral is producing more organic material (i.e. aragonite) to buffer against changes in acidity.  This is necessary to promote calcification under stressful conditions. 

However, the December 2022 study Evaluation of the current understanding of the impact of climate change on coral physiology after three decades of experimental research suggests that while there is broad consensus on the negative effects of heat-stress on coral reefs, the impacts of ocean acidification are not well-established. The paper then goes on to use a review of published studies and experimental analysis to confirm how different species of corals react to ocean acidification and global warming, concluding that while the effects vary across species, the impacts of global warming are more severe than acidification.

Ultimately, there are still a plethora of research gaps on the response of coral reefs to acidification, as we still don’t fully understand how coral and zooxanthellae work together to build the coral structure. As such, it is still critical to understand which parts of the coral are being affected by the changing climate so that we can develop conservation efforts  to preserve them. 


Chamberlain, J. A. “Mechanical Properties of Coral Skeleton; Compressive Strength and Its Adaptive Significance.” Paleobiology, vol. 4, no. 4, 1 Oct. 1978, pp. 419–435, pubs.geoscienceworld.org/paleobiol/article-standard/4/4/419/86628/Mechanical-properties-of-coral-skeleton. Accessed 5 Mar. 2023.

Chamberlain, John A. “Mechanical Properties of Coral Skeleton: Compressive Strength and Its Adaptive Significance.” Paleobiology, vol. 4, no. 4, 1978, pp. 419–435, https://doi.org/10.1017/s0094837300006163. Accessed 28 Feb. 2020.

Coronado, Ismael, et al. “Impact of Ocean Acidification on Crystallographic Vital Effect of the Coral Skeleton.” Nature Communications, vol. 10, no. 1, 1 July 2019, http://www.nature.com/articles/s41467-019-10833-6, https://doi.org/10.1038/s41467-019-10833-6. Accessed 28 Oct. 2019.

Krämer, Wiebke E., et al. “Evaluation of the Current Understanding of the Impact of Climate Change on Coral Physiology after Three Decades of Experimental Research.” Communications Biology, vol. 5, no. 1, 26 Dec. 2022, https://doi.org/10.1038/s42003-022-04353-1. Accessed 2 Feb. 2023.

Lippsett, Lonny. “How Do Corals Build Their Skeletons?” Woods Hole Oceanographic Institution, 2019, http://www.whoi.edu/oceanus/feature/how-do-corals-build-their-skeletons/. Accessed 5 Mar. 2023.

—. “How Do Corals Build Their Skeletons? | MIT Department of Earth, Atmospheric and Planetary Sciences.” Eapsweb.mit.edu, 16 Nov. 2018, eapsweb.mit.edu/news/2018/how-do-corals-build-their-skeletons. Accessed 5 Mar. 2023.

National Ocean Service. “What Is Ocean Acidification?” Noaa.gov, NOAA, 26 Feb. 2021, oceanservice.noaa.gov/facts/acidification.html. Accessed 5 Mar. 2023.

Rinkevich, Baruch. “Ecological Engineering Approaches in Coral Reef Restoration.” ICES Journal of Marine Science, 4 Mar. 2020, https://doi.org/10.1093/icesjms/fsaa022. Accessed 1 June 2020.

Viau, Elise. “Adoptez Un Corail.” Coral Guardian, 3 Feb. 2021, http://www.coralguardian.org/en/the-link-between-the-skeleton-of-corals-and-past-climatic-ocean-conditions/. Accessed 5 Mar. 2023.

Zhong, Yu, et al. “Changes of Physical and Mechanical Properties of Coral Reef Limestone under CO2–Seawater–Rock Interaction.” Applied Sciences, vol. 12, no. 9, 19 Apr. 2022, p. 4105, https://doi.org/10.3390/app12094105. Accessed 1 June 2022.

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