Many marine organisms have shells of calcium carbonate. There is a sensitive relationship between calcium carbonate CaCO3 and Ca2+ 2HCO3-. When CO2 is dissolved in water, HCO3- and H+ are formed, in other words acidification, which prevents CaCO3 from being formed. Formation of CaCO3 binds CO2, and over time leads to the formation of limestone. If the ocean becomes acidic, this effect diminishes.