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Concept
Marine biogenic calcification
Summary
Marine biogenic calcification is the process by which marine organisms such as oysters and clams form calcium carbonate. Seawater is full of dissolved compounds, ions and nutrients that organisms can use for energy and, in the case of calcification, to build shells and outer structures. Calcifying organisms in the ocean include molluscs, foraminifera, coccolithophores, crustaceans, echinoderms such as sea urchins, and corals. The shells and skeletons produced from calcification have important functions for the physiology and ecology of the organisms that create them.
The ocean is the largest sink or reservoir of atmospheric carbon dioxide (CO2), continually taking in carbon from the air. This dissolved CO2 initially reacts with water to form carbonic acid, before reacting further to generate carbonate (CO32−), bicarbonate (HCO3−), and hydrogen (H+) ions. Not only does this sequence of reactions regulate oceanic pH levels, it also determines the saturation state of seawater - how saturated (or unsaturated) the seawater is with said ions. This has a proportionate effect on the net calcification of marine calcifiers - dissolution versus calcification.
The saturation state for calcium carbonate (CaCO3) can be determined using the equation:
Ω = ([Ca2+][CO32−])/ Ksp
Where the numerator denotes the concentrations of calcium ions to carbonate ions, and the denominator Ksp refers to the stoichiometric solubility product for the mineral (solid) phase of calcium carbonate. When the saturation state is high, organisms can extract the calcium and carbonate ions from the seawater and form solid crystals of calcium carbonate.
Ca2+(aq) + 2HCO3−(aq) → CaCO3(s) + CO2 + H2O
The three most common calcium carbonate minerals are aragonite, calcite, and vaterite. Though these minerals have the same chemical formula (CaCO3), they are considered polymorphs because the atoms making up the molecules are stacked in different configurations. For example, aragonite minerals have an orthorhombic crystal lattice structure while calcite crystals have a trigonal structure.
Official source
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