The skeletons of corals are made of calcium carbonate by biomineralization process, in the form of aragonite or calcite. To understand the characteristics of coral skeletons, especially mineralogy, crystal phases, organization and structure in individual species, X-ray powder diffraction techniques have gained increased interest in recent years as useful non-destructive tools. This review provides an overview on the recent progress in this field and briefly introduces the related experimental approach. The application of X-ray diffraction (XRD) to elucidating the structural and mechanical properties of mineral crystals in corals is reviewed in terms of characterization of CaCO3 crystal orientation. In addition, we discuss how this technique has increased our understanding of the function of the organic matrix proteins of calcified coral skeletons during mineral formation. Such information is helpful in deducing the mechanical and structural model of corals with respect to biomineralization system of skeletons.
One of the most important events in animal evolution has been the development of a hard skeleton within many independent stocks. In the subsequent diversification of these lines, skeletons have been adapted to fulfill a wide variety of functions, based on tissue support, protection, and locomotion. Skeletons or parts thereof may be variously classified into more or less distinct categories on the basis of position with respect to soft parts (exoskeleton versus endoskeleton), composition (organic versus inorganic, agglutinated versus precipitated, etc.), and permanence. Although the mechanism by which the skeleton forms is unclear, the coral probably removes calcium directly from sea water and combines it with inorganic carbon, consisting of carbon dioxide from respiration and bicarbonate from sea water [
Recent reports have focused on the characterization of proteins in the soluble matrix of soft coral sclerites (endoskeletons) [1,3-6,11-14] and stony corals [
X-ray diffraction (XRD) is an effective method for determination of the phase composition of unknown crystalline and amorphous materials. The scattering of X-rays by crystal atoms, produce a diffraction pattern that yields information about the structure of the crystal. This data is represented in a collection of single-phase X-ray powder diffraction patterns for the three most intense D values in the form of tables of interplanar spacings (D), relative intensities (I/Io), and mineral name. The techniques of X-ray diffraction analysis are used to study, for example, metals, alloys, minerals, inorganic and organic compounds, polymers, amorphous materials, liquids, gases, and the molecules of proteins and nucleic acids [1,20-26].
As we mentioned above, X-ray powder diffraction analysis could one of the most powerful tools in order to identify the crystal structure and mineral phases in the bio-mineralized protein induced coral skeletons. Crystallization plays a key role in the bio-calcification process and ultimately in the growth of coral skeletons [1,27]. Skeletal architecture and microstructure of the calcifying corals were characterized using XRD [
actinian coral skeletons indicated that, although some strontium substitutes for calcium in the aragonite structure, at concentrations of about 7500 parts per million, as much as 40 percent of the strontium resides in strontianite (SrCO3) [
From the last few years [2,11,12,30], acidic proteins were purified from the organic matrices of corals. In the present review, the crystallization of corals in the presence of these proteins was discussed in terms of XRD analysis. One widely used approach for studying the functions of these acidic proteins is to examine their effect on crystal growth, in vitro. Combinations of matrix components have been used to detect a collaborative effect [
The major polymorphisms involved in CaCO3 crystallization of marine organisms were identified and subsequently, the functions of specific organic matrix proteins in the bio-calcification process were determined [1,3,10, 14,28,32-37]. Here we showed the polymorphism of crystals in the endoskeletons of a soft coral in presence of matrix proteins by XRD. However, prior to isolate organic matrix proteins from the endoskeletons, there were shown the structural shape and the location of them in the soft coral colony (
How do calcite crystals form in soft corals? Very recently [
cated by arrows in (E), (F) and (G)). The transition of minerals from aragonites to calcites was confirmed by XRD, which showed that all phases of the crystals were calcites and that no aragonites were present (
As we observed, soft corals have special characters because the organic matrices themselves are highly aspartic acid-rich proteins [11,28]. In addition, previous studies on molluscan shells indicate that acidic amino acid residues may actually inhibit crystal nucleation [
The information from the basic structural unit of coral skeleton, i.e. mineral-crystal phases in which the organic matrix proteins are primarily responsible for the control of CaCO3 polymorphisms would provide a better understanding of the bulk composition and fragility of the tissue in the crystallization process. In this regard, X-ray diffraction has proved to be an effective technique to examine the structural and mechanical characteristic of coral mineral and provide reliable experimental data. This technique is highly potential in understanding the function of the acidic proteins in the matrices of calcified coral skeletons during biomineralization. This knowledge provides a way to expedite the coral formation and biocalcification processes. Furthermore, the idea about mineral crystals and their organization helps understand the structural integrity in coral and the overall skeletal characteristics. There is still a lot to be achieved; however, recent progress in this field has provided new hopes for future diffraction studies to be used as an effective nondestructive technique and reveal new aspects in the formation of CaCO3 in coral skeletons.