CRITICAL TRANSITIONS IN THE BIOFABRICATION OF ABALONE SHELLS AND FLATPEARLS

Analyses of biopolymer/calcium carbonate composites grown on inorganic abiotic substrates implanted between the shell and the shell-secretin g epithelium oflive red abalones (Haliotis rufescens) provide detailed spatial and temporal data on the in vivo assembly process that genera tes the shell. X-ray diffraction and scanning electron microscopy anal yses of the growth of these flat pearl composites reveal that biominer alization is initiated by the deposition of an organic sheet on the im planted substrate, followed by the growth of a calcite layer with pref erred {10.4} orientation and, finally, by the growth of nacreous arago nite. The calcite layer is structurally similar to the green organic/c alcite heterolayer of native shell nacre. It comprises 0.2-2.0-mu m-di ameter 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 aragoni te in both the shell and the flat pearl is abrupt. In vitro calcium ca rbonate growth experiments reveal that a similar calcite-to-aragonite transition is induced by the addition of soluble proteins isolated fro m the aragonitic nacre. The growth of flat pearls is highly sensitive to physical and chemical properties of the abiotic substrate. Either r oughened 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 org anic sheet. Insertion of flat pearls as substrates, however, results i n continued nacre growth without the deposition of an organic sheet an d a calcite layer.