What is the role of the ocean in the carbon cycle and how does acidification affect calcifying organisms?

• A. The ocean sink absorbs CO2.
• B. Acidification reduces carbonate ion availability, harming calcifying organisms.
• C. The ocean sink absorbs CO2; acidification reduces carbonate ion availability, harming calcifying organisms.
• D. The ocean does not interact with atmospheric CO2.

Answer: (C)

The ocean plays a dual role in the carbon cycle: it acts as a large sink that takes up a significant amount of atmospheric CO2, and the CO2 that dissolves in seawater changes the chemistry of the ocean. As CO2 dissolves, it forms carbonic acid, which releases hydrogen ions and lowers the seawater’s pH. This acidification shifts the balance of carbonate chemistry, reducing the availability of carbonate ions that calcifying organisms need to build their calcium carbonate shells or skeletons. With fewer carbonate ions and a lower saturation state of calcium carbonate minerals, calcification slows and shells can even begin to dissolve, threatening organisms like corals, mollusks, and some plankton. So the best answer captures both the ocean’s role as a CO2 sink and the chemical consequences of acidification on calcifying life.