Hydrogen ions in seawater interfere with the shell formation because they tend to bond with carbonate ions and reduce the carbonate available for shell building. So organisms must expend energy to pump hydrogen out and increase carbonate.
Carbon Dioxide, Shell Building, and Ocean Acidification
All these ions are in seawater surrounding shell-building marine life.
According to a hypothesis, at the molecular level, seawater containing ions pass through the organisms' tissue into the “calcifying spaces” next to their shells.
As rising atmospheric CO2 causes hydrogen ions (H+) in seawater to increase, it may require more energy for organisms to pump them out. Possibly impacting some organisms’ ability to make shells.
Carbon dioxide (CO2) dissolves in the ocean and reacts with water to form carbonic acid—which in turn generates bicarbonate, carbonate, and hydrogen ions.
Calcium and carbonate ions combine to form the solid calcium carbonate crystals in their shells and skeletons.
calcium carbonate crystals
Seawater is a soup of dissolved substances, chemicals, and ions. To build shells and skeletons, marine organisms, such as this hypothetical clam, extract calcium ions (Ca+2) and carbonate ions (CO3-2) from seawater, combining them into solid crystals of calcium carbonate (CaCO3) that are laid down to make shells.