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Seawater absorbs CO2 to produce carbonic acid (H2CO3), bicarbonate (HCO3--) and carbonate ions (CO32-).
These carbonate ions are essential to the calcification process that allows certain marine organisms to build their calcium carbonate shells and skeletons (e.g. hard tropical corals, cold water corals, molluscs, crustaceans, sea urchins, certain types of plankton, lobsters, etc).
However, increases in atmospheric CO2 levels lead to decrease in pH level, increase in the concentration of carbonic acid and bicarbonate ions, causing a decrease in the concentration of carbonate ions.
Thus carbonate ions are less available and calcification is therefore harder to achieve, and may be prevented altogether. Imagine trying to build a house while someone keeps stealing your bricks.
This impact of ocean acidification may therefore have potentially catastrophic consequences for ocean life and many marine species of economic importance.