How do coral reefs and seashells store carbon?

Coral reefs and seashells store carbon by utilizing calcium carbonate (${\text{CaCO}}_3$) to build their rigid structures.12

• Coral Reefs: Reef-building corals, such as the stony corals, secrete calcium carbonate to form a hard, calcareous skeleton. This process is supported by symbiotic, photosynthetic algae called zooxanthellae that live within the coral tissue and assist in building the solid matrix. Over time, these structures accumulate and can become limestone, effectively locking carbon into the Earth's crust.1324
• Seashells: Many marine organisms, including mollusks like clams and scallops, construct external calcareous shells. These shells grow through the addition of layers of calcium carbonate. When these organisms die, their shells may break down into fragments that contribute to marine sediments or consolidate into rock, further sequestering carbon.3

Because these organisms rely on carbonate ions in the water to build their structures, ocean acidification—caused by the absorption of atmospheric carbon dioxide—poses a significant threat to their ability to maintain these carbon-storing skeletons and shells.5