What role do carbonate ions play in seawater with respect to calcification?

• A. They have no role in calcification.
• B. They are used to build calcium carbonate for shells; decreasing availability reduces calcification.
• C. They prevent shells from dissolving.
• D. They only affect fish respiration.

Answer: (B)

Carbonate ions in seawater are the building blocks used by many marine organisms to create calcium carbonate shells and skeletons. When calcium ions meet carbonate ions, they form CaCO3, providing the material for shells. So, higher carbonate ion availability supports calcification, while lower availability slows or reduces it. In seawater with rising CO2, more hydrogen ions lower pH and push the carbonate system toward bicarbonate, reducing the concentration of carbonate ions. That makes calcification harder and can even lead to shell dissolution if the water becomes undersaturated with calcium carbonate. So decreasing carbonate availability directly reduces calcification.