Calcium carbonate (/KAL-see-əm KARB-ə-nate/) is a white compound (CaCO₃), occurring naturally as chalk, limestone, and marble. Many invertebrates, such as mollusksof man, corals, and echinoderms construct their exoskeletons from this mineral. Each individual coral polyp secretes it, and such secretions fuse together with those of other members of the colony to form coral reefs, which are really tremendous, rock-hard skeletons containing pits that surround and protect the individual polyps. Seashells, too, are composed of this mineral. It is also present in the tests of many microorganisms. All vertebrate eggs laid on land (amniotic eggs), except turtle eggs, have shells made of calcite, a form of CaCO₃. Turtles lay eggs made of aragonite, another form of the same mineral.
The currently increasing levels of acidity in the world's oceans are a serious problem for many marine animals, because lowering the pH affects the chemical equilibrium of seawater with respect to calcium carbonate, reducing the concentration of carbonate ions and making it harder for organisms such as corals to build and maintain structures composed of CaCO₃ that are essential to their continued existence. Thus, rising acidity threatens them with extinction.
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