The Transient Phase of Amorphous Calcium Carbonate in Sea Urchin Larval Spicules: The Involvement of Proteins and Magnesium Ions in Its Formation and Stabilization

Abstract

Amorphous calcium carbonate (ACC) is a precursor phase of calcite in the formation of the sea urchin larval spicule. The goal of this research is to study the formation and stabilization mode of this transient phase. We first characterized the mineralogy of the spicules from the sea urchin Strongylocentrotus purpuratus. We then examined the role of the macromolecules extracted from the spicules at different growth stages in the formation of transient ACC in vitro.The biogenic amorphous transient phase is shown to be both structurally and compositionally different from the known stable ACC phases. It does not contain bound water, and is thus the first dehydrated ACC phase to be detected. The macromolecules that were extracted at early stages of spicule growth, when the amorphous content of the biogenic mineral is high, induced the formation of transient ACC in vitro in the presence of magnesium ions. In contrast, the macromolecules extracted at a later stage, when the spicules are completely crystalline, induced the formation of single crystals of low magnesian calcite. We therefore deduce that the macromolecules from the sea urchin larval spicules together with magnesium ions, mediate the transient formation of ACC as a precursor to calcite. These observations may well provide novel ideas for improved materials synthesis.

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

“…Extended X-ray absorption fine structure (EXAFS) spectroscopy at the Ca K-edge showed that even at early stages, when the mineral is still predominantly amorphous, it already has a nascent short-range order around the calcium ions similar to that in calcite (18). In contrast to stable biogenic ACC, which contains 1 water molecule per CaCO 3 , the amorphous phase in the spicules is mostly anhydrous when the spicules are extracted at an advanced developmental stage (12,19). Macroscopically, therefore, the spicule displays both amorphous and crystalline qualities.…”

“…The mature larval spicule is composed of a single crystal of magnesium-bearing calcite (10,11). Small amounts of organic macromolecules (0.1 wt%) are incorporated within the mineral and are known to play a role in the transient stabilization of the amorphous phase (12).…”

The dynamics of secretion during sea urchin embryonic skeleton formation

“…Furthermore, a prevalent spicule matrix protein, SpSM30B, is slowly modified at the time of delivery to the spicule and this modification continues after incorporation of protein into the mineralized spicule. We should note that the discovery of distinct pathways for calcium versus SpSM30B and SpSM50 does not exclude the possibility that some other matrix protein (s) are associated with precipitated calcium, and one candidate would be the matrix protein(s) shown by Raz et al [34] to stabilize amorphous calcium carbonate.…”

Acidified seawater impacts sea urchin larvae pH regulatory systems relevant for calcification

“…When larvae are chronically exposed to elevated seawater pCO 2 of >0.1 kPa, e.g., as is predicted to occur during the next century in response to anthropogenic CO 2 emissions or through upwelling of low-pH deep water, this sensitivity is reflected in reduced growth and developmental rates (5,6). Echinoderm larvae are considered to be especially vulnerable to seawater pH reduction and to the associated changes in calcium carbonate saturation state of seawater (Ω Cal ) because their internal skeleton is composed of high magnesium calcite, a highly soluble form of CaCO 3 (7,8). However, long-term reductions in growth and development might just as well be evoked by other physiological mechanisms that are also sensitive to hypercapnia and the related acid-base disturbances.…”