text: Carbonate ions (CO) are essential in marine calcifying organisms, like plankton and shellfish, as they are required to produce their calcium carbonate () shells and skeletons. As the ocean acidifies, the increased uptake of CO by seawater increases the concentration of hydrogen ions, which lowers the pH of the water. This change in the chemical equilibrium of the inorganic carbon system reduces the concentration of these carbonate ions. This reduces the ability of these organisms to create their shells and skeletons.
  The two polymorphs of calcium carbonate that are produced by marine organisms are aragonite and calcite. These are the materials that makes up most of the shells and skeletons of these calcifying organisms. Aragonite, for example, makes up nearly all mollusc shells, as well as the exoskeleton of corals. The formation of these materials is dependent on the saturation state of CaCO in ocean water. Waters which are saturated in  are favorable to precipitation and formation of  shells and skeletons, but waters which are undersaturated are corrosive to  shells. In the absence of protective mechanisms, dissolution of calcium carbonate will occur. As colder arctic water absorbs more , the concentration of CO is reduced, therefore the saturation of calcium carbonate is lower in high-latitude oceans than it is in tropical or temperate oceans.
  The undersaturation of CaCO causes the shells of calcifying organisms to dissolve, which can have devastating consequences to the ecosystem. As the shells dissolve, the organisms struggle to maintain proper health, which can lead to mass mortality. The loss of many of these species can lead to intense consequences on the marine food web in the Arctic Ocean, as many of these marine calcifying organisms are keystone species. Laboratory experiments on various marine biota in an elevated  environment show that changes in aragonite saturation cause substantial changes in overall calcification rates for many species of marine organisms, including coccolithophore, foraminifera, pteropods, mussels, and clams.
  Although the undersaturation of arctic water has been proven to have an effect on the ability of organisms to precipitate their shells, recent studies have shown that the calcification rate of calcifiers, such as corals, coccolithophores, foraminiferans and bivalves, decrease with increasing p, even in seawater supersaturated with respect to . Additionally, increased p  has been found to have complex effects on the physiology, growth and reproductive success of various marine calcifiers.