Morphogenesis of Aragonite Biomineral Structures by the Nonclassical Colloidal Crystal Growth Mechanism Revisited on the Nanoscale: The Noah's Ark Shell (Arca noae, L.) Case Study

Characterization and formation of the biomineral aragonite structures of the Noah's Ark shell (Arca noae L.,1758) were studied from structural, morphogenetic, and biochemical points of view. Structural and morphological features were examined using X-ray diffraction, field-emission scanning electron microscopy, and atomic force microscopy, while thermal properties were determined by thermogravimetric and differential thermal analyses. Proteins from the soluble organic matrix (SOM) were analyzed by Edman degradation. The results showed that the Noah's Ark shell exhibits several distinct biomineral structures characterized by complex morphologies and different forms of aragonite. The inner shell of the Ark is characterized by a combination of nanogranular surfaces and micron-sized, idiomorphically developed aragonite crystals indicative of orthorhombic symmetry. The formation of these structures is discussed in terms of the nonclassical crystal growth route considering the colloidally mediated mechanism based on the initial particle-particle interaction of the nanosized and metastable precursor aragonite phase and their dissolution and recrystallization processes. These structures contained a small amount of connecting organic material, SOM, assessed at 1.5% of the total mass. Edman degradation revealed the partial amino acid sequence that is present also in the tetratricopeptide repeat (TPR) protein 8 from diverse mussels. Bacterial TPR-containing protein was found to be involved in the biomineralization process, so we propose such a function for these proteins also in mussels.