Heat-mediated micro- and nano-pore evolution in sea urchin biominerals

Biomineralized structures with intricate shapes and morphologies, such as sea urchin skeletal elements, grow via the deposition of hydrated amorphous calcium carbonate (ACC) particles that subsequently crystallizes into single-crystalline calcite. This process is accompanied by volume changes due to density differences between the initial and final mineral state as well as variations in hydration levels. For this reason, the presence of macroporosity in synthetic systems was shown to be pivotal in the formation of large single crystals through ACC precursors. However, the role of macroporosity down to nanoporosity in the formation of biogenic minerals remains unknown. Here, we investigate the micro- and nano-porosity as well as the evolution of internal interfaces in the spines and test plates of Paracentrotus lividus sea urchins during the heat-mediated crystallization of remnant ACC and the destruction of intracrystalline organic molecules, using SEM, FIB-SEM, and in situ heating synchrotron SAXS measurements. We show the presence of nanopores likely filled with hydrated organics and visualize the evolution of nano- to micro-pores induced by heating, which may serve to accommodate the volume changes between amorphous and crystalline phases. The obtained results analyzed using thermodynamical considerations suggest that the growth in size of the nanopores is controlled by Ostwald ripening and is well described in the framework of classical pore coarsening theories. The extracted activation energies manifest that nanopore coarsening in the test plates is governed by surface diffusion, whereas in the spines by bulk diffusion. We suggest that such striking differences in diffusion mechanisms are caused by dissimilar levels of macroporosity and distributions of nano- and micro-internal interfaces in pristine biominerals

Gradients of orientation, composition and hydration of proteins for efficient light collection by the cornea of the horseshoe crab

The lateral eyes of the horseshoe crab, Limulus polyphemus, are the largest compound eyes within recent Arthropoda. While this visual system has been extensively described before, the precise mechanism allowing vision has remained controversial. Correlating quantitative refractive index (RI) mapping and detailed structural analysis, we demonstrate how gradients of RI in the cornea result from the hierarchical organization of chitin-protein fibers, heterogeneity in protein composition and bromine doping, as well as spatial variation in water content. Combining the realistic cornea structure and measured RI gradients with full-wave optical modelling and ray-tracing approaches, we show that the light collection mechanism depends on both refraction-based graded index (GRIN) optics and total internal reflection. The optical properties of the cornea are governed by different mechanisms at different hierarchical levels, demonstrating the remarkable versatility of arthropod cuticle.One-sentence summary Structural hierarchy and protein hydration determine the optical performance of the cornea of L. polyphemus.Competing Interest StatementTS and MAKAR are employed by TELIGHT. All other authors declare that they have no competing interests