Transformation mechanism of amorphous calcium carbonate into calcite in the sea urchin larval spicule

Abstract

Sea urchin larval spicules transform amorphous calcium carbonate (ACC) into calcite single crystals. The mechanism of transformation is enigmatic: the transforming spicule displays both amorphous and crystalline properties, with no defined crystallization front. Here, we use X-ray photoelectron emission spectromicroscopy with probing size of 40–200 nm. We resolve 3 distinct mineral phases: An initial short-lived, presumably hydrated ACC phase, followed by an intermediate transient form of ACC, and finally the biogenic crystalline calcite phase. The amorphous and crystalline phases are juxtaposed, often appearing in adjacent sites at a scale of tens of nanometers. We propose that the amorphous-crystal transformation propagates in a tortuous path through preexisting 40- to 100-nm amorphous units, via a secondary nucleation mechanism.

Mechanism of Calcite Co-Orientation in the Sea Urchin Tooth

“…Figure S6 shows the image of the forming tooth, and the pixels from which the spectra of Figure 3 were extracted. The three phases detected in the forming S. purpuratus tooth are identical to those observed by Politi et al 16 in the S. purpuratus larval spicules, also shown in Figure 3. It is important to note that the ACC spectra of Figure 3 were collected from solid portions of the forming sea urchin tooth, as shown in Figure S6.…”

“…8 The present study reveals for the first time that the forming sea urchin tooth contains not one but two amorphous precursor minerals. These precursor phases are identical to those reported by Politi et al 16 in S.…”

“…15 Or, does an amorphous precursor phase form first and crystallinity propagate though it subsequently? 16,17 Ma et al observed that the fibers, and possibly the plates, are initially composed of amorphous calcium carbonate (ACC). 8 The present study reveals for the first time that the forming sea urchin tooth contains not one but two amorphous precursor minerals.…”

“…In the study by Politi et al,. 16 a high-resolution and clear pattern of crystallinity propagation could not be imaged. The three-dimensional shape of the larval spicule makes it very difficult to generate that sort of data with X-PEEM.…”

Solid‐State Transformation of Amorphous Calcium Carbonate to Aragonite Captured by CryoTEM

“…Biogenic aragonite formation has been shown to form from an ACC precursor, and it has been suggested that it occurs by a non‐classical mechanism,7c, 13 similar to that demonstrated for biogenic calcite 4a, 9d, 14. To date, however, there is no direct confirmation for such pathway in vitro.…”