Award: OCE-2049407

Corals build reefs by creating skeletons made of calcium carbonate, a process known as calcification. These reefs protect shorelines and support about 25% of all marine life. Yet, how corals form these vital structures has remained somewhat of a mystery. As oceans absorb more carbon dioxide (CO₂) from the atmosphere, seawater chemistry changes. This acidifies the water and reduces the carbonate ions corals need to grow. At the same time, the process of building skeletons produces waste in the form of hydrogen ions, which may also slow growth. Therefore, this research aimed to investigate what limits coral calcification under changing ocean conditions, whether it's the lack of the carbonate ion or the buildup of the waste products (i.e., protons). By uncovering the main barrier to calcification, we can better predict how coral reefs will respond to climate change and develop more effective strategies to protect them.

We conducted 30-day experiments to investigate how changes in carbonate and proton concentrations affect coral calcification. Corals were exposed to different combinations of these conditions during both summer and winter seasons. Our results revealed that responses varied not only between coral species but also across seasons. For example, in summer, Montipora capitata showed a strong positive relationship between calcification and carbonate levels, meaning it built more skeleton when the carbonate was high. In contrast, during winter, its calcification declined as proton levels increased, suggesting greater sensitivity to increasing proton concentrations. Pocillopora acuta, on the other hand, responded differently: in winter, its calcification increased with carbonate, but in summer, it showed no clear relationship with either carbonate or proton levels.

We also performed short-term (1-hour) incubations during the day and night using two coral growth forms: branching and plating colonies of M. capitata and the branching P. acuta. Interestingly, branching M. capitata maintained steady calcification, while the plating form of M. capitata and P. acuta experienced reduced calcification at night. These findings highlight the importance of coral morphology and skeletal structure in determining sensitivity to ocean chemistry. For instance, M. capitata has tissue that is six times thicker than P. acuta, which may help it buffer against low pH conditions. The reduced nighttime calcification suggests that daytime photosynthesis plays a critical role in raising pH near the coral surface, known as the boundary layer, and supporting skeletal growth. Together, these results suggest that species-specific traits like shape, skeletal porosity, and tissue thickness influence how corals regulate their internal environment and cope with changing ocean conditions.

We used microsensors to measure hydrogen ions and oxygen near the coral's surface to understand how seawater chemistry affects coral growth. Coral shape turned out to be important; different shapes create different "boundary layers", the thin layer of water around the coral that controls how chemicals like carbonate and protons move in and out. In our study, Pocillopora acuta had trouble releasing hydrogen ions at night and absorbed less oxygen during the day, while Montipora capitata showed no major changes. Gene expression data revealed no changes in internal chemical regulation, suggesting that M. capitata's resilience is due more to its structure, especially its shape and thick tissues, than to internal physiological responses.

Field measurements were also made to assess the variability in oxygen and hydrogen ions in seawater as it flows across a natural coral reef. Large changes in oxygen and carbon chemistry were closely tied to coral density as high-resolution measurements were made while drifting across the reef. These findings illustrate the short-term time and space scales of measurable impacts that corals can have on the overlying seawater as it passes over the reef and the need for high-resolution measurements when trying to use these chemistry changes to quantify coral health and calcification rates.

 

Last Modified: 07/14/2025
Modified by: Christopher L Sabine