Biomineralization of Calcium Carbonate. The Interplay with Biosubstrates

Organisms produce elaborate mineral parts that defy the common traits of "inorganic" crystallization in many ways. Calcium carbonate minerals are mainly used by invertebrates to build skeletal, protective, and storage mineralized constructs. Three crystalline polymorphs: calcite, aragonite, and vaterite, along with transient and stable amorphous calcium carbonate (ACC) are used for specific functions. In this chapter recent insights into the biomineralization processes of calcium carbonate are presented. The control over crystal nucleation, growth, shape and internal symmetry of the crystalline elements are mediated by specialized biomacromolecules. Studies of organisms from diverse phyla have yielded several common features: Accumulation of material into the mineralization site is done by aggregation of amorphous nanometer sized spherules. Usage of transient amorphous material as a precursor phase is a general phenomenon. It allows molding the shape of the crystal into a desired form. Specific, glutamic acid-rich proteins interact with ACC and possibly prevent it from crystallization. Another set of highly acidic, aspartic acid rich-proteins interact specifically with the crystalline phases. Subsets of these acidic proteins constitute the crystal nucleation sites and the intracrystalline proteins and are specific for calcite versus aragonite formation. Specifically occluded, intracrystalline proteins modify the intrinsic texture of the biogenic crystals, increase their toughness and reduce the natural brittleness. Some aspects of biological mineral formation have been reproduced and studied in artificially deposited calcium carbonate in the presence of macromolecules extracted from biominerals or in synthetic analogous systems. The simple crystal habit of calcite facilitates the interpretation of the directional interactions it undergoes with synthetic compounds in vitro and provides systematic clues for the control mechanisms exerted by the organisms on the minerals they deposit.