# 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

## TL;DR

This study explores how the Noah’s Ark shell forms aragonite structures using a nonclassical growth mechanism and identifies proteins involved in the process.

## Contribution

The paper revisits the nonclassical colloidal crystal growth mechanism on the nanoscale in biomineralization and identifies TPR proteins in mussels.

## Key findings

- The Noah’s Ark shell contains nanogranular and idiomorphic aragonite structures with orthorhombic symmetry.
- A small amount of soluble organic matrix (1.5%) connects the aragonite structures.
- TPR proteins from bacteria are proposed to play a role in mussel biomineralization.

## Abstract

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.