Why do corals and shellfish need carbonate ions from seawater?

Corals, shellfish, and other marine calcifiers require carbonate ions from seawater because they are essential raw materials used to build and maintain their calcium carbonate (CaCO${}_3$) skeletons and shells.1

Through a process known as biomineralization, these organisms extract these minerals from the surrounding water to construct their protective structures. For example, reef-building corals secrete a solid calcium carbonate matrix that forms the foundation of coral reefs, while mollusks like clams and scallops use these materials to grow their external shells.1234

When the concentration of carbonate ions in seawater decreases—often due to ocean acidification—these organisms struggle to obtain the necessary components for their growth and structural integrity. This deficiency can lead to smaller, weaker shells and skeletons, leaving the organisms more vulnerable to predators and environmental stress.1