Final answer:
Aquatic organisms use calcium carbonate to construct their shells, which form sediment and contribute to Earth's largest carbon reservoir in the form of limestone. Rising ocean acidity challenges this process by making it harder for organisms to absorb calcium and carbonate required for their shells. Therefore, the correct option is D.
Step-by-step explanation:
Some aquatic organisms use calcium carbonate primarily to make their shells. Calcium carbonate (CaCO3) is formed from dissolved CO2 in the water through a series of chemical reactions where carbonate ions CO32- are created. These carbonate ions combine with calcium ions in seawater to form CaCO3, which is the main component of marine organism shells. Over time, these shells contribute to sediment on the ocean floor and over geologic time scales, become limestone, which holds the largest carbon reservoir on Earth, acting as significant carbon sinks. This process is critical for the organisms that rely on it, such as shellfish, corals, and certain types of algae, as it provides structural support and protection. However, rising ocean acidity can hinder organisms' ability to absorb the necessary calcium and carbonate ions, making shell formation more difficult, indicating the importance of a stable pH in marine environments for these species to thrive.