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: Kashiwa and Komorous  demonstrated intra- and extracellular calcium- and P-rich spherules within fresh calcifying cartilage samples from regions preceding endochondral calcification. Kashiwa  also identified calcium- and P-rich granules within, and adjacent to, mature and hypertrophic calcifying chondrocytes when staining was performed on fresh samples to avoid the effects of sample preparation on unstable structures. Boonrungsiman et al.  observed Ca- and P-rich mitochondrial granules within mineralizing murine osteoblast cultures, and presented evidence of vesicle–mitochondrial interactions with high angle-annular dark-field scanning TEM of samples prepared with high-pressure freezing and freeze substitution (HPF-FS) (Fig. 3). Although the presence of these unstable, electron-dense, Ca- and P-containing granules has been identified by different groups, their specific composition is unknown. Fluorescence imaging has shown colocalization of polyPs within murine growth plate calcifying cartilage  but not at the resolution required to identify granules. How these granules are secreted into the ECM where they transform into carbonated apatite remains unknown. The realization of these phenomena must lie with the activity of matrix proteins.Fig. 3