Why can ocean acidification harm corals and shell-forming animals?

Ocean acidification harms corals and shell-forming animals primarily by reducing the availability of carbonate ions in seawater.1

These marine organisms, known as calcifiers, rely on carbonate ions and aragonite to build and maintain their calcium carbonate skeletons and shells. When carbon dioxide is absorbed by the ocean, it reacts with water to form carbonic acid, which releases hydrogen ions and subsequently consumes carbonate ions.12

As the concentration of these essential building blocks falls, calcifiers struggle to grow, leading to smaller, weaker structures that increase their vulnerability to predators. In extreme cases, the shells of some organisms can even dissolve. Furthermore, the rising acidity can shift the "saturation horizon"—a boundary below which carbonate minerals dissolve—closer to the surface, threatening the survival of both cold-water and warm-water coral reefs that provide vital habitats for marine life. Protecting these ecosystems is an ethical imperative, as their decline threatens global biodiversity and the food security of human populations.1