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: PolyP granules were detected in marine sediment, where they were mixed with granules of similar size that have been identified as apatite by X-ray fluorescence (Fig. 2) . These apatite granules were theorized to originate from diatoms, as Diaz et al.  observed that the 0.5–3 μM polyP granules were similar in size to diatom granules. PolyP stores within diatoms are normally protected from the environment within their silica skeleton (frustule). Diaz et al.  proposed that after diatom death, bacteria consume the outer organic layer that protects the silica within the frustule, resulting in silica dissolution. After falling through the water column, polyP granules freed from or exposed within compromised frustules would be exposed to the sediment environment.Fig. 2