Marine biogenic calcification is the production of calcium carbonate by organisms in the global ocean. Marine biogenic calcification is the biologically mediated process by which marine organisms produce and deposit calcium carbonate minerals to form skeletal structures or hard tissues. This process is a fundamental aspect of the life cycle of some marine organisms, including corals, mollusks, foraminifera, certain types of plankton, and other calcifying marine invertebrates. The resulting structures, such as shells, skeletons, and coral reefs, function as protection, support, and shelter and create some of the most biodiverse habitats in the world. Marine biogenic calcifiers also play a key role in the biological carbon pump and the biogeochemical cycling of nutrients, alkalinity, and organic matter.
Processes of Marine Biogenic Calcification
Biochemical mechanisms
Cellular and molecular processes of biogenic calcification Calcium carbonate plays a fundamental role in the skeletal formation of marine calcifiers. The skeletal structures of these organisms are predominantly composed of calcium carbonate minerals, specifically aragonite and calcite. These structures provide support, protection, and housing for marine calcifiers and are formed through the biochemical processes of biomineralization to precipitate the crystal structures that form the hard tissues of these organisms. The biogenic formation of calcium carbonate structures is the result of a combination of biological and physical processes such as genetics, cellular activity, crystal competition, growth in confined spaces, and self-organization processes. The composition of these structures, and the mechanisms involved in building them, are highly diverse. For example, some corals can incorporate both calcite and aragonite polymorphs into their skeletons. Some species, like corals and byrozoans, can incorporate other minerals to form complex protein matrices that perform specific functions. The key steps involved in marine biogenic calcification include the uptake of dissolved calcium ions (Ca2+) and carbonate ions (CO32-) from seawater, the precipitation of calcium carbonate crystals, and the controlled formation of skeletal structures through biomineralization processes. These organisms often regulate the calcification process through the secretion of organic molecules and proteins that influence the nucleation and growth of crystalline structures. A range of biochemical calcification (biocalcification) mechanisms exist, indicated by the fact that marine calcifiers use different forms of calcium carbonate minerals. Within this range of mechanisms, there are two broad categories of biogenic calcification in marine organisms: extracellular mineralization and intracellular mineralization. In particular, mollusks and corals use the extracellular strategy in which ion exchange pumps actively pump ions out of a cell into the extracellular space, where environmental conditions, such as pH, can be tightly controlled. In contrast, during intracellular mineralization the calcium carbonate is formed within the organism and can either be kept within the organism as an internal structure or is later moved to the outside while retaining the cell membrane covering. Broadly, the intracellular mechanism pumps ions into a vesicle within the cell. This vesicle can then be secreted to the outside of the organism. Often, cells will fuse their membranes and combine these vesicles in order to build very large calcium carbonate structures that would not be possible within a single cell.
Forms of calcium carbonate The three most common calcium carbonate minerals are aragonite, calcite, and vaterite. Although these minerals have the same chemical formula (CaCO3), they are considered polymorphs because the atoms that make up the molecule are stacked in different arrangements. For example, aragonite minerals have an orthorhombic crystal lattice structure, while calcite crystals have a trigonal structure. Some of the calcite polymorphs are further subdivided by relative magnesium content (Mg/Ca ratio), with calcite solubility increasing with increasing Mg. The solubility of various forms of CaCO3 differs in seawater; specifically, aragonite exhibits greater solubility compared to pure calcite.
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