Ocean Acidification

Coccolithophores in different pH level. The left one in normal pH level.
The right one in a lower pH level.
http://www.awi.de/en/research/young_investigators/helmholtz_university_young_investigators_groups/
future_marine_carbon_cycle/topics/calcification_of_coccolithophores/

Ocean Acidification is a result of ocean absorbing carbon dioxide from the atmosphere. When carbon dioxide react with water, they will form carbonic acids and decrease the pH of water. Carbonic acids will later dissociate into bicarbonate and hydrogen ions. These excess hydrogen ions will bond with free carbonate ions in the ocean to form bicarbonate ions. This will limit the amount of carbonate ions available for marine organisms to build their shells and skeletons. Moreover, the decrease in pH can also reduce iron's availability, a nutrient critical for phytoplankton growth. This will affect shells of coccolithophores species and therefore reduce the primary productivity. Fewer phytoplankton will lead to less food for consumers across the food chain.

Ocean Acidification

Phytoplankton like coccolithophores play a major role in carbon cycle. After taking up carbon dioxide from the atmosphere, they will decay and take the organic carbon with them. Those carbon will be locked in the ocean bottom for decades. Changes in the chemistry of ocean will decrease the amount of carbon stored in the ocean.