How does ocean acidification affect coral reef growth?

• A. It enhances pigment production
• B. It accelerates reef growth
• C. It has no effect on calcification
• D. It reduces carbonate availability, hindering calcification

Answer: (D)

Ocean acidification changes the chemistry of seawater so corals have fewer carbonate ions to build their calcium carbonate skeletons. When CO2 dissolves in seawater, it forms carbonic acid, which increases hydrogen ions and shifts carbonate balance toward bicarbonate. This lowers the aragonite saturation state, making it harder for corals to precipitate CaCO3 and, if conditions are very undersaturated, can even cause dissolution of existing skeletons. Because coral growth relies on depositing calcium carbonate, calcification slows and reef construction is hindered. Pigment production isn’t driven by this chemical change, and the idea of no effect or faster growth doesn’t fit because the key process for reef expansion—calcification—is directly reduced by the lower availability of carbonate.