A review of phosphate mineral nucleation in biology and geobiology.
Bottom Line: Subsequent release of these P reserves into the local marine environment as Pi results in biologically induced phosphorite nucleation.Polyphosphates may be associated with biologically controlled apatite nucleation within vertebrates and invertebrates.Enzymatic polyphosphate depolymerization would increase apatite saturation to the level required for mineral nucleation, while matrix proteins would simultaneously control the progression of new biological apatite formation.
Relationships between geological phosphorite deposition and biological apatite nucleation have often been overlooked. However, similarities in biological apatite and phosphorite mineralogy suggest that their chemical formation mechanisms may be similar. This review serves to draw parallels between two newly described phosphorite mineralization processes, and proposes a similar novel mechanism for biologically controlled apatite mineral nucleation. This mechanism integrates polyphosphate biochemistry with crystal nucleation theory. Recently, the roles of polyphosphates in the nucleation of marine phosphorites were discovered. Marine bacteria and diatoms have been shown to store and concentrate inorganic phosphate (Pi) as amorphous, polyphosphate granules. Subsequent release of these P reserves into the local marine environment as Pi results in biologically induced phosphorite nucleation. Pi storage and release through an intracellular polyphosphate intermediate may also occur in mineralizing oral bacteria. Polyphosphates may be associated with biologically controlled apatite nucleation within vertebrates and invertebrates. Historically, biological apatite nucleation has been attributed to either a biochemical increase in local Pi concentration or matrix-mediated apatite nucleation control. This review proposes a mechanism that integrates both theories. Intracellular and extracellular amorphous granules, rich in both calcium and phosphorus, have been observed in apatite-biomineralizing vertebrates, protists, and atremate brachiopods. These granules may represent stores of calcium-polyphosphate. Not unlike phosphorite nucleation by bacteria and diatoms, polyphosphate depolymerization to Pi would be controlled by phosphatase activity. Enzymatic polyphosphate depolymerization would increase apatite saturation to the level required for mineral nucleation, while matrix proteins would simultaneously control the progression of new biological apatite formation.
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Mentions: Assuming that matrix proteins are closely associated with Ca–polyP within precursor granules, it is proposed that selective removal of Ca–polyP within the precursor granule would leave behind the organic components, which may describe the “crystal ghosts” (Fig. 4). Enzymatic initiation of apatite nucleation, by polyP depolymerization and increased Pi and calcium concentrations, would nucleate an ordered Ca–Pi structure within the amorphous granule. This nucleation of an ordered mineral would be expected to exclude the granular proteins from the nucleating apatite crystal lattice. This is because crystallization processes offer the phenomenon of purifying materials from an impure starting material . As the apatite nucleus grows with more available calcium and Pi, it could exclude the associated granular proteins, eventually displacing them to the apatite mineral surface. When the granular polyP is consumed, the final product could be an apatite crystal now coated by the proteins that were secreted within the granule. This mechanism could provide one explanation for the transport of some proteins to skeletal mineral surfaces.Fig. 4