Amorphous calcium carbonate particles form coral skeletons

Abstract

Do corals form their skeletons by precipitation from solution or by attachment of amorphous precursor particles as observed in other minerals and biominerals? The classical model assumes precipitation in contrast with observed “vital effects,” that is, deviations from elemental and isotopic compositions at thermodynamic equilibrium. Here, we show direct spectromicroscopy evidence in Stylophora pistillata corals that two amorphous precursors exist, one hydrated and one anhydrous amorphous calcium carbonate (ACC); that these are formed in the tissue as 400-nm particles; and that they attach to the surface of coral skeletons, remain amorphous for hours, and finally, crystallize into aragonite (CaCO3). We show in both coral and synthetic aragonite spherulites that crystal growth by attachment of ACC particles is more than 100 times faster than ion-by-ion growth from solution. Fast growth provides a distinct physiological advantage to corals in the rigors of the reef, a crowded and fiercely competitive ecosystem. Corals are affected by warming-induced bleaching and postmortem dissolution, but the finding here that ACC particles are formed inside tissue may make coral skeleton formation less susceptible to ocean acidification than previously assumed. If this is how other corals form their skeletons, perhaps this is how a few corals survived past CO2 increases, such as the Paleocene–Eocene Thermal Maximum that occurred 56 Mya.

Amorphous Phase Mediated Crystallization: Fundamentals of Biomineralization

“…Mass et al used direct photoemission electron spectromicroscopy (PEEM, PEEM-3 at the Advanced Light Source in Berkeley, CA, USA.) and X-ray absorption near-edge structure spectroscopy evidence in Stylophora pistillata corals to show that two amorphous precursors existed, including one hydrated and one anhydrous ACC [40]. This work reveals two advantages of ACC-mediated crystallization.…”

“…Amorphous calcium phosphate (ACP) was found to be the precursor for hydroxyapatite (HAP) crystallization in bio- [25,[27][28][29] and biomimetic [10,20,[30][31][32][33][34] CaP mineralization systems. Amorphous calcium carbonate (ACC) is the precursor phase for many invertebrate biomineralizations, such as in the formation of sea urchins [35,36], mollusk shells [23,[37][38][39], and coral skeletons [40]. The transformation of amorphous phases to crystalline phases (Table 1) can occur via several crystallization pathways [21,24,[41][42][43][44][45], such as surface-mediated heterogeneous nucleation [11,[46][47][48], dissolution/re-precipitation [49][50][51][52][53][54][55][56], and solid-solid phase transformation (structure reorganization) [19,33,[57][58][59][60][61]…”

Water: How Does It Influence the CaCO3 Formation?

“…This suggestion is in line with TGA results that reveal an n below 0.8 for ACCs amples that have been treated at temperatures around 100 8 8Ctoremove the mobile water. [73,91,97,98] More recently,t he short-range order of ACCp articles possessing different degrees of hydration was qualitatively characterized with OK -edge [7,99] and Ca L-edge [7,11,17,18,99,100] XANES techniques that are more sensitive towards structural changes. [73,74,89,91] During dehydration, H 2 Om olecules present in the first Ca-O coordination shell are replaced by CO 3 2À groups.Asaresult of this exchange,the values of CN in the first Ca-O coordination shell and the average distance between Oand Ca atoms, R,remain nearly constant, as shown in Figure 5B.…”

“…In many organisms such as the sea urchins, [8,9,[11][12][13][14] crayfish, [15,16] corals, [17] and gastropods, [18] these transient precursors are stored in designated reservoirs that serve as temporary storage of Ca 2+ ions.T he precursors are transported to the desired location before they are rapidly transformed into CaCO 3 crystals.This crystallization pathway usually results in CaCO 3 crystals that display aw ell-defined orientation. Forexample,the calcite crystals that form the sea urchin teeth all have the same orientation, as shown in Figure 1A.…”

Synchrotron X-ray Microdiffraction and Fluorescence Imaging of Mineral and Rock Samples

“…This method has a wide variety of applications and has been used extensively, particularly in materials science and engineering, to analyze everything from 3D-printed metals 1,2 , to solar panel deformation 3 , to strain in topological materials 4 , to memory alloy phase transitions 5 , to the high-pressure behavior of nanocrystalline materials 6,7 . Recent geoscience projects include the analysis of strain in various quartz samples 8,9 of volcanic cementitious processes 10,11 , and also of biominerals such as calcite and aragonite in shells and corals 12,13 or apatite in teeth ). The x-ray beam exits the storage ring and is directed using a toroidal mirror (M201), the purpose of which is to refocus the source at the entrance of the experimental hutch.…”