Card #23: Calcification difficulties
When the pH drops, it becomes harder for calcium carbonate seashells to grow.
The formation of limestone (calcification) follows the chemical reaction Ca++ + 2HCO3- ⇔ CaCO3 + H2O + CO2
It requires the presence of bicarbonate ions (HCO3-). However, the quantity of these ions in water depends on the pH: in water, carbon dioxide, carbonic acid, bicarbonate ions and carbonates ion are in equilibrium, depending on the pH :
CO2 + H2O ⇔ H2CO3 ⇔ H+ + HCO3- ⇔ 2 H+ + CO32-.
The addition of an acid shifts the equilibrium towards the left of the equation. In other words, if the pH drops, there are fewer bicarbonate ions, making it more difficult for organisms to build their shells.
- Marine biodiversity Calcification problems do not only affect pteropods and coccolithophores. Coral reefs are also affected, for example, so this link is acceptable.