Critical Transitions in the Biofabrication of Abalone Shells and Flat Pearls

Analyses of biopolymer/calcium carbonate composites grown on inorganic abiotic substrates implanted between the shell and the shell-secreting epithelium of live red abalones ( Haliotis rufescens ) provide detailed spatial and temporal data on the in vivo assembly process that generates the shell. X-ray diffraction and scanning electron microscopy analyses of the growth of these flat pearl composites reveal that biomineralization is initiated by the deposition of an organic sheet on the implanted substrate, followed by the growth of a calcite layer with preferred {10.4} orientation and, finally, by the growth of nacreous aragonite. The calcite layer is structurally similar to the green organic/calcite heterolayer of native shell nacre. It comprises 0.2−2.0-μm-diameter elongated crystallites of typical geological habits in various aggregate arrangements. The shell also contains an external layer of (00.1)-oriented prismatic calcite, which is deposited on one edge of a flat pearl and has a morphology similar to that of the {10.4}-oriented calcite layer. The transition from {10.4}-oriented calcite to aragonite in both the shell and the flat pearl is abrupt. In vitro calcium carbonate growth experiments reveal that a similar calcite-to-aragonite transition is induced by the addition of soluble proteins isolated from the aragonitic nacre. The growth of flat pearls is highly sensitive to physical and chemical properties of the abiotic substrate. Either roughened or hydrophobic substrates result in abnormal arrangements of the basal calcite layer, which are corrected for by a reinitiation of the biomineralization process, beginning with the deposition of an organic sheet. Insertion of flat pearls as substrates, however, results in continued nacre growth without the deposition of an organic sheet and a calcite layer.