3D Calcium carbonate polymorphs imaging with stimulated Raman scattering in biominerals

Area of Science:

• Biomineralization research
• Marine biology
• Materials science

Background:

• Calcium carbonate (CaCO3) biomineralization involves complex transformations of amorphous precursors and crystalline phases.
• Understanding the physico-chemical temporal sequence of these transformations is crucial for marine organism shell and coral formation.
• Existing methods lack the resolution to identify intermediate phases in situ.

Purpose of the Study:

• To develop and apply a highly sensitive coherent Raman microscopy approach for tridimensional (3D) imaging of CaCO3 polymorphs.
• To identify the temporal sequence of CaCO3 crystallization during biomineralization in marine organisms.
• To reveal intermediate crystalline phases involved in shell and coral formation.

Main Methods:

• Utilized coherent Raman microscopy for high-sensitivity 3D imaging.
• Exploited distinct vibrational signatures of various calcium carbonate polymorphs (amorphous, aragonite, calcite, Mg-calcite, hemi-hydrated CaCO3).
• Applied the technique to the growing edge of *Pinctada margaritifera* (pearl oyster) shell and *Stylophora pistillata* (coral) samples.

Main Results:

• Achieved spatially and spectrally resolved imaging of different CaCO3 compounds.
• Inferred a temporal crystallization sequence in both pearl oyster shells and coral skeletons.
• Identified intermediate crystalline phases, including magnesian calcite in oysters and hemi-hydrated calcium carbonate in corals.

Conclusions:

• The developed coherent Raman microscopy approach enables detailed investigation of biomineralization pathways.
• The study highlights the existence of previously unidentified intermediate crystalline phases in marine CaCO3 biomineralization.
• This provides new insights into the dynamic processes governing shell and coral formation.