Figure 1.
Skeleton of the coral Balanophyllia europaea.
(A) Digital camera picture of the skeleton of the coral Balanophyllia europaea after digestion in a sodium hypochlorite solution to remove the soft organic tissues. (B) X-ray powder diffraction pattern from a powdered coral skeleton sample. Only the characteristic diffraction peaks from aragonite are observable. The main diffraction peaks of the Miller index are indicated according to the reference pattern PDF 98-006-0908 [60]. (C) FTIR spectrum from a powdered coral skeleton sample. Only the typical absorption bands from aragonite were detected. They were assigned as ν2 = 858 cm−1; ν3 = 1470 cm−1; ν4 = 713 cm−1 [42].
Figure 2.
FTIR spectra of intra-skeletal soluble (SOM) and insoluble (IOM) organic matrix from the Balanophyllia europaea aragonitic skeleton. Typical absorption bands from protein molecules (around 1600 cm−1), polysaccharides (around 1000 cm−1) and lipids (around 2900 cm−1) are indicated. The absorption due to the polysaccharidic regions appeared stronger than the one due to the proteic regions in both the IOM and SOM spectra.
Figure 3.
SDS-polyacrylamide gels electrophoresis of intra-skeletal insoluble (IOM) and soluble (SOM) organic matrix extracted from the Balanophyllia europaea skeleton. In the first lane the markers are reported. The arrows indicate the major proteic bands.
Table 1.
Amino acid compositions (relative mol %) of proteins extracted from the soluble (SOM) and insoluble (IOM) fractions of the Balanophyllia europaea skeleton intra-skeletal organic matrix.
Figure 4.
Calcium carbonate overgrowth experiments.
(A–D) SEM pictures of a fragment of coral skeleton after the calcium carbonate overgrowth experiment. The overgrowth of calcite and aragonite crystals was observed as shown in (B). In (B) the arrow indicates an aggregate of crystals of aragonite and the dashed square the area illustrated at higher magnification in the inset. The crystals of aragonite appear clustered in bunch of fibers (C), which locally exhibited preferential orientation (arrows in the inset in C). Organic matrix is visible in contact with the crystals of aragonite (arrows in inset in B and in C). The crystals of calcite showed an additional group of {hk0} faces other than the {104} set (D). These new faces showed as smooth surface, typical of interaction with macromolecules (inset in D).
Figure 5.
Calcium carbonate crystallization experiments.
Crystallization experiments of calcium carbonate from 10 mM CaCl2 solution in the absence of additives (Ctrl) and in the presence of soluble organic matrix (SOM), insoluble organic matrix (IOM) or both additives (IOM+SOM). The Miller indexes of calcite faces are indicated. left Scanning electron microscope images, the insets show sample details. right FTIR spectra of the precipitated. The main absorption bands of calcium carbonate are indicated.
Figure 6.
Calcium carbonate crystallization experiments.
Crystallization experiments of calcium carbonate from 10 mM CaCl2 solution with a Mg/Ca molar ratio equal to 3 (Ctrl3) and in the presence of soluble organic matrix (SOM), insoluble organic matrix (IOM) or both of these (IOM+SOM). The Miller indexes of calcite faces are indicated. left Scanning electron microscope images, the insets show sample's details right FTIR spectra of the precipitated. The main absorption bands of calcium carbonate are indicated.
Figure 7.
Calcium carbonate crystallization experiments.
Crystallization experiments of calcium carbonate from 10 mM CaCl2 solution with a Mg/Ca molar ratio equal to 5 (Ctrl5) and in the presence of soluble organic matrix (SOM), insoluble organic matrix (IOM) or both additives (IOM+SOM). The direction of the crystallographic c-axis of aragonite is indicated. left Scanning electron microscope images, the insets show samples details right FTIR spectra of the precipitated. The main absorption bands of calcium carbonate are indicated.
Table 2.
Crystalline phases precipitated at different amounts of magnesium ions in 10 mM calcium chloride solutions and in the presence of SOM (cs = 044 mg/mL), IOM (ci = 050 mg) or both of them, SOM+IOM (cs SOM; ci IOM).
Figure 8.
Calcium carbonate crystallization experiments.
(A) X-ray powder diffraction patterns of the precipitates obtained from a 10 mM calcium chloride solution (Ctrl), from a 10 mM calcium chloride solution containing SOM and magnesium ions in Mg/Ca molar ratio equal to 5 (SOM-Mg5). In (Ctrl) only calcite is present, while in (SOM-Mg5) a mixture of calcite and aragonite is present. Moreover, in (SOM-Mg5) the broad band around between 20° and 40° suggests the presence of amorphous material. The calcite diffraction peaks, (012), (104), (110), (018) and (116), were indexed according to the reference pattern PDF 98-000-5342 [61], while the aragonite diffraction peaks, (111), (102) and (211), were indexed according to the reference pattern PDF 98-006-0908 [60]. (B and C) SEM pictures of the precipitate obtained from SOM-Mg5. Crystals of magnesium calcite (see arrow in (A)) and aragonite deposited on, and into, a jagged layer of ACC. In (B) is shown a magnification of the ACC layer, in it small particles of about 100 nm in diameter are visible.