What is the central mechanism by which ocean acidification affects marine organisms with calcium carbonate shells?

The central mechanism by which ocean acidification affects marine organisms with calcium carbonate shells is by reducing the availability of carbonate ions (CO3^2-) in seawater. Ocean acidification is the ongoing decrease in the pH of the Earth's oceans, caused by the uptake of carbon dioxide (CO2) from the atmosphere. As atmospheric CO2 levels increase, more CO2 dissolves into the ocean. When CO2 dissolves in seawater, it reacts with water (H2O) to form carbonic acid (H2CO3). Carbonic acid then dissociates, releasing hydrogen ions (H+). This increase in hydrogen ions lowers the pH of the seawater, making it more acidic. The increased concentration of hydrogen ions also reacts with carbonate ions (CO3^2-), which are essential building blocks for marine organisms that build shells and skeletons made of calcium carbonate (CaCO3). This reaction reduces the concentration of carbonate ions in the seawater. Calcium carbonate saturation state refers to the thermodynamic favorability for calcium carbonate minerals to either precipitate (form) or dissolve. Marine organisms, like corals, shellfish, and some plankton, need to extract calcium and carbonate ions from seawater to build their shells and skeletons. With fewer carbonate ions available, it becomes more difficult for these organisms to build and maintain their calcium carbonate structures. In severe cases, if the seawater becomes undersaturated with respect to calcium carbonate, existing shells and skeletons can begin to dissolve. This can weaken the shells, making the organisms more vulnerable to predators and environmental stresses. It can also impair their growth and reproduction, leading to declines in populations and disruptions of marine ecosystems.