Evidence for a Liquid Precursor to Biomineral Formation

Abstract

The crystals in animal biominerals, such as sea urchin spines, mollusk shells, and coral skeletons, form by attachment of amorphous particles that subsequently crystallize. Do these solid amorphous precursor particles have liquid precursors? Polymer-induced liquid precursors (PILP) or prenucleation clusters coalescing into a liquid precursor to calcium carbonate crystallization have been observed extensively in synthetic systems. Molecular dynamics simulations also predict liquid–liquid phase separation. However, evidence for liquid precursors in natural biominerals remains elusive. Here, we present Scanning or PhotoEmission Electron Microscopy (SEM or PEEM) evidence consistent with a dense liquid-like precursor in regenerating sea urchin spines. The observed precursor originates in tissue and ultimately transforms into a single crystal of calcite (CaCO3) with complex stereom morphology.

Formation, Chemical Evolution, and Solidification of the Calcium Carbonate Dense Liquid Phase

“…The fact that carboxyl-rich proteins, which are common constituents of biomineralizing systems, produce the same stabilizing effect, provides further evidence that the DLP serves as a precursor to crystalline CaCO 3 biominerals as suggested by recent reports on their ultramicrostructure 15 . Moreover, the ability of the PILP process to create mineralized collagen that closely resembles t of trabecular bone 37,38 , as well as reports of hollow calcium phosphate particles 39 , suggests a DLP consisting of condensed Ca 2+ and H 2 PO 4 ions with their solvation shells may well exist in the calcium phosphate system and likewise evolve to amorphous calcium phosphate and subsequent crystalline phases through a process similar to that reported here.…”

Biomineralization: Integrating mechanism and evolutionary history

“…For example, in foraminifera, vacuoles contain chemically modified seawater (35,36), as well as smaller Mg-and Ca-rich vesicles (43). Elevated Ca in intracellular vesicles is observed in coral cells (44), sea urchin embryos (45)(46)(47) and spines (48), and coccolithophorids (49).…”

“…Vesicles can also be locations where initial biomineral-forming ACC nanoparticles are nucleated, as first hypothesized by Cohen and McConnaughey ( 50 ) for the case of coral skeleton formation. Now, evidence for such intracellular precursor nanoparticles is overwhelming in the tissue adjacent to the forming surface of coral skeletons ( 25 , 51 – 53 ), in sea urchin embryonic cells forming spicules ( 47 ), and in the tissue regenerating adult sea urchin spines ( 48 ). Cells extracted from coral polyps cannot form a tissue-bounded privileged space with an ECF, yet they are able to form carbonate crystals, likely explained by calcification in intracellular vesicles ( 54 ).…”

“…Many previous models for biomineral formation described two contrasting mechanisms: IA from a CF ( 63 – 65 ) or PA of ACC precursors from intracellular vesicles ( 15 , 25 , 26 , 45 , 47 , 48 , 53 , 66 ). IA is unavoidable in the presence of a CF that is rich in ions and supersaturated with respect to the final biomineral ( 31 , 64 , 67 , 68 ), yet evidence for PA of ACC precursors is abundant ( 15 , 25 , 26 , 45 , 47 , 48 , 53 , 66 , 69 ). Here, we propose a mechanistic model that includes both PA and IA.…”

“…A weakness of the model is that ICF is only reasonably deduced to exist in most organisms; it has only been observed and measured directly during the formation of sea urchin spicules ( 47 ) and indirectly during sea urchin spine ( 48 ) and coral skeleton ( 53 ) formation. Thus far, the best characterized CF is the ECF in one coral species, S. pistillata ( 31 , 67 , 111 , 112 ).…”