Ocean acidification happens when carbon dioxide is released into the air and absorbed by the ocean. The carbon molecules bind with the water to make carbonate, calcium carbonate is the building block for the skeletons and the shells of these sea creatures. Even though this causes an increase in the amounts of carbonate the rising pH levels make it so that some areas are undersaturated and the carbonate can't survive in those levels of acidity.

 This causes the bleaching and destroying of coral reefs, destroying many ecosystems, and it makes it so that shellfish and other organisms that rely on calcium carbonate can't survive. In most cases, their shells dissolve in acidic water. For example, sea urchins, their spines are made out of calcite, which is primarily made up of calcium carbonate. They help cut down on macroalgae which can help revive coral reefs, but with the increase in acidification, and their numbers are steadily dropping. They can't do their job unless we do ours...