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Imaging the Formation of Sea Urchin Spicules Print
Thursday, 16 August 2012 15:39

Sea urchin spicules are single-crystalline calcite structures with smooth, rounded shapes, significantly different from the flat faces and sharp edges of calcite formed geologically or synthetically. Though many other biominerals exhibit this morphological character, sea urchin spicules are an ideal system for studying biomineral formation mechanisms because they contain 99.9% calcite (CaCO3) by weight and only 0.1% intracrystalline proteins.

In principle, sea urchins could build spicules by depositing amorphous precursor phases, which can be morphed into any shape, and letting them slowly crystallize. The three CaCO3 phases present in sea urchin spicules (ACC•H2O, ACC, and calcite) are spectroscopically distinct, enabling two-dimensional component mapping. Using x-ray absorption near edge structure (XANES) spectroscopy and photoelectron emission microscopy (PEEM) at ALS Beamline 11.0.1, researchers have directly observed each of these three phases in cross-sections of sea urchin spicules caught in the act of crystallizing. These data provide the first experimental evidence that sea urchins form spicules by first depositing ACC•H2O onto the surface of the forming spicule, then ACC•H2O dehydrates to form ACC, and finally the ACC crystallizes to form calcite.

Unexpectedly, a small amount of ACC•H2O remains entrapped within crystallized spicules analyzed months after harvesting. This observation suggests the presence within the spicules of a powerful inhibitor of the transformation from ACC•H2O to ACC. Researchers performed a protein assay searching for a possible dehydration inhibitor protein, and found a potential candidate: matrix protein SM50, the most abundant protein in sea urchin spicules, teeth, spines, and tests, was found to stabilize ACC•H2O in vitro.

 

XANES spectra at the calcium L-edge extracted from sea urchin spicule cross-sections. The blue spectrum is crystalline calcite, red is (ACC·H2O), and green is anhydrous ACC. Each spectrum was obtained by first acquiring 6-10 single-pixel spectra from 20 nm pixels, from independent energy scans, then averaging all spectra to minimize experimental noise. These low-noise spectra were used to identify and localize these phases.

 

Component mapping of ACC•H2O, ACC, and calcite in spicules from the Californian sea urchin Strongylocentrotus purpuratus. (A) XANES-PEEM image of 3 spicules embedded in epoxy, polished to expose a cross-section, and coated with 1 nm of platinum. (B) Red, green, and blue (RGB) map displaying the results of component mapping, in which each component is color-coded. The box indicates the region magnified in (C). (C) Zoomed-in portion of the RGB map in (B), where each 15-nm pixel shows a different color. Pure phases are R, G, or B, while mixed phases are cyan, magenta, or yellow. The white line shows the positions of the 20 pixels from which the spectra in (D) were extracted. (D) Sequence of 20 XANES spectra extracted from 15-nm adjacent pixels along the white line in (C).

 


 

Work performed on ALS Beamline 11.0.1.

Citation: Y.U.T. Gong,  C.E. Killian,  I.C. Olson,  N.P. Appathurai,  A.L. Amasino,  M.C. Martin,  L.J. Holt,  F.H. Wilt,  P.U.P.A. Gilbert, “Phase Transitions in Biogenic Amorphous Calcium Carbonate,” Procs. Natl. Acad. Sci. USA 109,  6088-6093 (2012).