Abstract
In various mineralizing marine organisms, calcite or aragonite crystals form through the initial deposition of amorphous calcium carbonate (ACC) phases with different hydration levels. Using X-ray PhotoEmission Electron spectroMicroscopy (X-PEEM), ACCs with varied spectroscopic signatures were previously identified. In particular, ACC type I and II were recognized in embryonic sea urchin spicules. ACC type I was assigned to hydrated ACC based on spectral similarity with synthetic hydrated ACC. However, the identity of ACC type II has never been unequivocally determined experimentally. In the present study we show that synthetic anhydrous ACC and ACC type II identified here in sea urchin spines, have similar Ca L2,3-edge spectra. Moreover, using X-PEEM chemical mapping, we revealed the presence of ACC-H2O and anhydrous ACC in growing stereom and septa regions of sea urchin spines, supporting their role as precursor phases in both structures. However, the distribution and the abundance of the two ACC phases differ substantially between the two growing structures, suggesting a variation in the crystal growth mechanism; in particular, ACC dehydration, in the two-step reaction ACC-H2O → ACC → calcite, presents different kinetics, which are proposed to be controlled biologically.
Highlights
Adult sea urchins possess mineralized appendages called spines, which project from the test and are used for defense and locomotion
In order to test these hypotheses, here we studied the spatial distribution of amorphous calcium carbonate (ACC) phases in freshly re-grown micro-spines and older portions of stereom and septa using X-ray PhotoEmission Electron spectroMicroscopy (X-PEEM) with high spatial resolution
The micro-architecture of the skeleton of fully developed and regenerated sea urchin spines has been described thoroughly for a number of species (Drozdov et al, 2016; Dubois and Ameye, 2001; Gorzelak et al, 2011; Heatfield, 1971c; Vinnikova and Drozdov, 2011). Following from these previous studies, we decided to limit the portions of the adult spine we examined with X-PEEM analyses, to the micro-spines, the thickening stereom, the thickened stereom and the septa of the S. purpuratus sea urchin (Fig. 2)
Summary
Adult sea urchins possess mineralized appendages called spines, which project from the test (the body shell) and are used for defense and locomotion. The mineralized structure of the spines is composed of calcite, small amounts of stable amorphous calcium carbonate (ACC), water, and intra-crystalline organic molecules (Aizenberg et al, 1997; Albéric et al, 2018b; Ameye et al, 2001; Berman et al, 1990; Kanold et al, 2015; Seto et al, 2012). Spines serve as sacrificial appendages that constantly break and regenerate during the lifetime of sea urchins (Dubois and Ameye, 2001). The spines regenerate rapidly after fracture; with growth rates of 125 μm/day in length and 1 μm/day in width for the Paracentrotus lividus species (Gorzelak et al, 2011), and 160 μm/day in length for the Strongylocentrotus purpuratus (Heatfield, 1971c)
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