Evidence for polyamorph calcium carbonate nanoparticles driving crystal growth in molluscan shells

Le résumé fourni par la source

Biomineralization produces crystalline materials with exceptional structural and functional properties, yet how organisms control crystal growth remains incompletely understood. Amorphous calcium carbonate (ACC) is widely considered a transient precursor to crystalline calcium carbonate. It has also been proposed that crystals form by non-classical growth mechanisms involving preexisting nanoparticles that attach to growing crystal surfaces. However, direct evidence that discrete ACC nanoparticles participate in crystal growth in vivo is lacking. Here, we investigate the outer calcitic and inner aragonitic mineralization compartments of the mussel Mytilus galloprovincialis using high-pressure freezing, high-resolution transmission electron microscopy (HRTEM), and monochromated scanning transmission electron microscopy–electron energy-loss spectroscopy (STEM-EELS). We identify abundant ~2 nm ACC nanoparticles within the mineralization compartments of both nacre and fibrous calcite. The nanoparticles are amorphous by HRTEM but crystallize under electron irradiation. STEM-EELS reveals spectral signatures compatible with calcite and aragonite in nanoparticles from the corresponding compartments, consistent with polymorph-specific amorphous precursors. Nanoparticles accumulate at crystal growth fronts, where nacre tablets and calcite fibers possess amorphous, organic-rich rims. Together, these observations provide direct in situ evidence that discrete ACC nanoparticles exhibiting short-range order contribute to biomineral growth by particle attachment. We propose that polymorph-specific nanoparticles are delivered through the extrapallial space, attach to growing crystal surfaces, and form a transient amorphous, liquid-like cortex that subsequently crystallizes. Our findings provide direct evidence for particle-mediated crystal growth in situ and establish ACC polyamorphism as a key factor in biological control over calcium carbonate polymorphism.

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