Sea Urchin Biomineralization – Formation of Intricate Single Calcite Crystals via Amorphous Precursors

Biomineralization of sea urchin skeletal elements results in complex structures with smooth, curved surfaces that diffract as single calcite crystals 1. This is achieved by a crystallization pathway involving amorphous precursor phases 2,3. Similar processes are now recognized in various other organisms. Yet, the dynamics of mineral rearrangement and the energetic landscape of this transformation are still poorly understood. We addressed these questions studying the crystallization of biogenic and synthetic ACCs by state-of-the-art calorimetric, spectroscopic and scattering methods. The skeletal elements of the sea urchin Paracentrotus lividus are composed of ACC, organics, a small amount of water and calcite 4 . Insight into the interplay between these components is gained by HR-XRPD of skeletal elements annealed at elevated temperatures. Complementarily, by mapping the distribution of ACC . H2O, ACC and calcite in growing spines by X-PEEM we demonstrated variable transformation kinetics across the spine. In-vitro experiments show that the effect of water, as well as organic and inorganic additives, on the stability of synthetic ACC is primarily kinetic [5]. A key finding with relevant to
biomineralization is that although water drastically changes the crystallization enthalpy, the overall ACC thermodynamic stability is independent of the hydration level due to enthalpy-entropy compensation. The rate of the transformation, on the other hand, is critically dependent on the water content; water increases ion mobility, resulting in higher kinetic instability [5].

1. Berman a, et al. (1993) Science 259, 776–9
2. Politi Y, Arad T, Klein E, Weiner S, Addadi L (2004) Science 306, 1161–4
3. Politi Y, et al. (2008) Proc Natl Acad Sci U S A 105, 17362–6
4. Albéric M, et al. (2018) Cryst Growth Des 18, 2189–201
5. Albéric M, et al. (2017) Adv Sci 1701000. doi:10.1002/advs.201701000
6. Jensen ACS, et al. (2018) J Phys Chem C 122, 3591–8
7. Zou Z, et al. (2018) J Mater Chem B 6, 449–57