Biocrystallization involves complex biochemical processes where living organisms control the crystalline form of their organo-mineral components. Only considering calcium carbonate polymorphs, a variety of millimetric structures and shapes are observed. These controlled biocrystallization pathways result from the synthesis of otherwise unstable crystalline forms. Mollusc shells, one of the most striking representatives, exhibit different crystalline polymorphs forming various architectures, perfectly illustrated by the famous Pinctada margaritifera pearl oyster.
This microscale architectural diversity contrasts with the quasi-systematic occurrence, in calcareous biominerals, of crystalline spheroid units (granules) of apparent diameter in the 50-500 nm range, coated by a visco-elastic (organic) cortex. This observation does not provide morphological evidence for a crystalline organization. However, the generic characteristic of the granular structure justifies the need of studying the hierarchical arrangement of these 'building-blocks'. The complete understanding of the biomineralization mechanisms could be proven by our capability to fully reproduce the observed structures.
Several bio-crystallization scenarios, able at reproducing some of the biomineral crystalline features, have been proposed. Among them, the polymer-induced liquid-precursor (PILP) process, which could explain most of the observed or inferred crystalline mesoscale properties (crystalline coherence iso-oriented crystalline domains, granular structure with absence of porosity) is gaining more importance. However, a clear demonstration of the relevancy of the PILP process for biomineralization is still lacking.
The question of the biological control on the mineral growth is highly intriguing as well and must be considered simultaneously. It has been recognized that the crystallization process occurs extra cellularly under the control of organic molecules. This active process requires cellular and molecular machinery, producing a leading organic matrix (OM), incorporated within the biomineralized structure itself. Understanding how the OM, and more specifically the proteins, control the mineralization requires identifying the organic components and characterizing their functions. Recently IFREMER has shown that the association between high throughput genomic and proteomic methods enables a real leap forward: 78 matrix proteins have been identified in the pearl oyster. However, their role is speculative. The PILP-based crystallization has been demonstrated for mineralizing systems containing acidic polyelectrolytes. The timely combination of genomic-proteomic results with PILP-based biomimetic crystallization should now provide decisive advances. The ultimate evidences still rely on our capability to analyze the biomimetic crystals at the relevant length scale, at which the granules appear, i. e., the mesoscale.
In this context, the recent breakthrough achieved by the PI and her group, should bring a relevant solution to this problem. In 2011, the PI’s group has proposed a new x-ray crystal-dedicated microscopy, 3D x-ray Bragg ptychography. This approach goes beyond the existing microscopy methods: It gives access to the 3D crystalline properties (crystalline coherence, tilts and strain fields) of a complex, nanostructured and extended crystal, which none of the existing methods is able to provide. In order to fully exploit the specificities of Bragg ptychography for the biomineralization problem, the major limits encountered in the former version have to be pushed back.
Hence, 3D-BioMat aims at deciphering the calcareous biomineralization mechanisms. Using a high-throughput crystalline nano-resolved microscopy, we expect to reveal sets of structural signatures related to the biomineralization pathways. A relevant biomineralization model should be proposed, demonstrating mesoscale structural properties in full agreement with the ones of the biominerals, a decisive proof that can only be brought with 3D Bragg ptychography.
