Ocean acidification is the ongoing decrease in the pH value of the Earth's oceans, caused by the uptake of carbon dioxide (CO 2) from the atmosphere. ... When carbon dioxide dissolves into seawater, it forms carbonic acid (H2CO3). Ocean acidification impacts many species, especially organisms like oysters and corals. Ocean acidification reduces the amount of carbonate, a key building block in seawater. This makes it more difficult for marine organisms, such as coral and some plankton, to form their shells and skeletons, and existing shells may begin to dissolve. ... The impacts of ocean acidification are not uniform across all species.