Which statement describes the effect of atmospheric CO2 on carbonate ion availability in seawater?

• A. It increases carbonate ion availability, enhancing calcification.
• B. It has no effect on carbonate ion concentration.
• C. It reduces carbonate ion availability, hindering calcification.
• D. It converts carbonate to bicarbonate increasing algae growth.

Answer: (C)

The key idea is that adding CO2 to seawater changes its carbonate chemistry and reduces the building blocks for calcium carbonate. When CO2 dissolves in seawater, it forms carbonic acid, which lowers the pH. The extra hydrogen ions push the carbonate system toward bicarbonate, so carbonate ions (the form needed to make CaCO3 shells and skeletons) become less available. With fewer carbonate ions, the saturation state of calcium carbonate drops, making it harder for organisms like corals and shellfish to calcify. So, atmospheric CO2 reduces carbonate ion availability and hinders calcification. The other statements don’t fit because CO2’s addition actually shifts carbonate toward bicarbonate and lowers the carbonate available for calcification, not increases it or leave it unchanged.