Jean-Pierre Cuif

XANES mapping of organic sulfate in three scleractinian coral skeletons

The presence and localization of organic sulfate within coral skeletons are studied by using X-ray absorption near edge structure spectroscopy (XANES) fluorescence. XANES spectra are recorded from four reference sulfur-bearing organic molecules: three amino acids (H-S-C bonds in cysteine; C-S-C bonds in methionine; one disulfide bond C-S-S-C bonds in cystine) and a sulfated sugar (C-SO4 bonds in chondroitin sulfate). Spectral responses of three coral skeletons show that the sulfated form is extremely dominant in coral aragonite, and practically exclusive within both centres of calcification and the surrounding fibrous tissues of coral septa. Mapping of S-sulfate concentrations in centres and fibres gives us direct evidence of high concentration of organic sulfate in centres of calcification. Additionally, a banding pattern of S-sulfate is visible in fibrous part of the coral septa, evidencing a biochemical zonation that corresponds to the step-by-step growth of fibres.

Distribution of magnesium in coral skeleton

Ion micro-probe imaging of the aragonite skeleton of Pavona clavus, a massive reef-building coral, shows that magnesium and strontium are distributed very differently. In contrast to strontium, the distribution of magnesium is strongly correlated with the fine-scale structure of the skeleton and corresponds to the layered organization of aragonite fibers surrounding the centers of calcification, which have up to ten times higher magnesium concentration. This indicates a strong biological control over the magnesium composition of all structural components within the skeleton. Magnesium may be used by the coral to actively control the growth of the different skeletal crystal components.

Suppression of skeletal growth in scleractinian corals by decreasing ambient carbonate-ion concentration: a cross-family comparison.

Biogenic calcification is influenced by the concentration of available carbonate ions. The recent confirmation of this for hermatypic corals has raised concern over the future of coral reefs because [CO32−] is a decreasing function of increasing pCO2 in the atmosphere. As one of the overriding features of coral reefs is their diversity, understanding the degree of variability between species in their ability to cope with a change in [CO32−] is a priority. We cultured four phylogenetically and physiologically different species of hermatypic coral (Acropora verweyi, Galaxea fascicularis, Pavona cactus and Turbinaria reniformis) under ‘normal’ (280 mu;mol kg−1) and ‘low’ (140 μmol kg−1) carbonate–ion concentrations. The effect on skeletogenesis was investigated quantitatively (by calcification rate) and qualitatively (by microstructural appearance of growing crystalline fibres using scanning electron microscopy (SEM)). The ‘low carbonate’ treatment resulted in a significant suppression of calcification rate and a tendency for weaker crystallization at the distal tips of fibres. However, while the calcification rate was affected uniformly across species (13–18%reduction), the magnitude of the microstructural response was highly species specific: crystallization was most markedly affected in A. verweyi and least in T. reniformis. These results are discussed in relation to past records and future predictions of carbonate variability in the oceans.

Microstructural and physico-chemical characterization of ‘centers of calcification’ in septa of some Recent scleractinian corals

KurzfassungFür eine Beurteilung des Konzeptes der ‘Kalzifikationszentren’ bei Korallen wurden von diesen Strukturen mikrostrukturelle, physikalische und chemische Daten gesammelt. Für jede der untersuchten 15 Arten wurden diese Daten mit ähnlichen Merkmalen, die in dem umgebenden fibrösen Hartteilgewebe auftreten, verglichen. Die Ergebnisse führen zu einer paradoxen Schlußfolgerung. Obwohl die Existenz von Kalzifikationszentren manchmal bestritten wird, lassen sie sich mittels der verschiedenen Techniken in den Septen aller untersuchter Arten, die zu verschiedenen Familien gehören, nachweisen. Somit scheinen Kalzifikationszentren ein Grundbaustein für die Korallensepten-Architektur darzustellen. Wenn man jedoch ihre mikrostrukturellen und chemischen Besonderheiten in Betracht zieht, führt dies zu einigen Änderungen in den gegenwärtigen Ansichten übei ihre Rolle bei der Skelettbildung von Scleractiniern.AbstractIn order to define the value of the concept of ‘center of calcification’, an attempt has been made to collect microstructural, physical, and chemical data from these particular structures. In each of the fifteen species studied, these data are compared with similar characteristics observed in the surrounding fibrous tissue. Results lead to a paradoxical conclusion. Although the existence of centers of calcification is sometimes denied, they have been evidenced by various techniques in septa of all the studied species, that belong to various families. Thus, ‘centers of calcification’ appear to be a basic component in the development of corallian septal architecture. But taking into account their microstructural and chemical peculiarities allows to introduce some changes in the current view concerning their role in skeletogenesis of Scleractinia.

Vital effects in coral skeletal composition display strict three-dimensional control

Biological control over coral skeletal composition is poorly understood but critically important to paleoenvironmental reconstructions. We present microanalytical measurements of trace-element abundances as well as oxygen and carbon isotopic compositions of individual skeletal components in the zooxanthellate coral Colpophyllia sp. Our data show that centers of calcification (COC) have higher trace element concentrations and distinctly lighter isotopic compositions than the fibrous components of the skeleton. These observations necessitate that COC and the fibrous skeleton are precipitated by different mechanisms, which are controlled by specialized domains of the calicoblastic cell-layer. Biological processes control the composition of the skeleton even at the ultrastructure level.

Biological forcing controls the chemistry of reef‐building coral skeleton

[1] We present analyses of major elements C and Ca and trace elements N, S, Mg and Sr in a Porites sp. exoskeleton with a spatial resolution better than similar to 150 nm. Trace element variations are evaluated directly against the ultrastructure of the skeleton and are ascribed to dynamic biological forcing. Individual growth layers in the bulk fibrous aragonite skeleton form on sub-daily timescales. Magnesium concentration variations are dramatically correlated with the growth layers, but are uncorrelated with Sr concentration variations. Observed (sub) seasonal relationships between water temperature and skeletal trace-element chemistry are secondary, mediated by sensitive biological processes to which classical thermodynamic formalism does not apply.

Structure and composition of the nacre-prisms transition in the shell of Pinctada margaritifera (Mollusca, Bivalvia)

A microstructural, mineralogical, and chemical study of the nacre–prisms boundary in the shells of Pinctada margaritifera shows that this boundary is not an abrupt transition, but that there exists a distinct fibrous layer with clear topographic structures and evidence of growth lines. A three-step biomineralization process is proposed that involves changes in the chemical and biochemical composition of the last growth increments of the calcite prisms, formation of the fibrous layer, and development of regular tablets in the nacreous layer.

Biochemical markers of zooxanthellae symbiosis in soluble matrices of skeleton of 24 Scleractinia species

Soluble skeletal organic components were isolated from coral skeletons belonging to 24 species, both zooxanthellate (13 species) and non-zooxanthellate (11 species). Statistical study of analytical data shows that four amino acids and five monosaccharides show distinct differences between species. Using this method of analysis, it possible to discriminate between symbiotic or non-symbiotic coral metabolism through the biochemical compositions of their mineralizing matrices.

Crystallization of biogenic Ca-carbonate within organo-mineral micro-domains. Structure of the calcite prisms of the Pelecypod Pinctada margaritifera (Mollusca) at the submicron to nanometre ranges

Abstract Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to investigate the fine structure of the calcite prisms from the pearl-oyster shell Pinctada margaritifera. The AFM analysis shows that the prisms are made of densely packed circular micro-domains (in the 0.1 μm range) surrounded by a dense cortex. The TEM images and diffraction patterns allow the internal structure of the micro-domains to be described. Each of them is enriched in Ca-carbonate. Hosted in distinct regions of each prism, some are fully amorphous, and some others fully crystallized as subunits of a large calcite single crystal. At the border separating the two regions, micro-domains display a crystallized core and an amorphous rim. Such a border probably marks out an arrested crystallization front having propagated through a previously bio-controlled architecture of the piling of amorphous micro-domains. Compared to recent data concerning the stepping mode of growth of the calcite prisms and the resulting layered organization at the μm-scale, these results give unexpected views regarding the modalities of biocrystallization.

Microstructure, nanostructure and composition of the shell of Concholepas concholepas (Gastropoda, Muricidae)

Among the gastropods, some muricid shells are composed of an inner aragonitic crossed lamellar layer and a calcitic prismatic outer layer. The analysis of the structure and composition of the two layers of Concholepasshows that the crossed lamellar layer is similar to those of other Mollusc taxa. Sr, Mg and S contents are low in both layers. According to infrared spectrometry, the organic content of the outer calcitic layer is higher than that of the aragonitic crossed lamellar layer. The study of the nanostructure allows for the proposal of a new 3D interpretation of the fine structure of the crossed lamellar layer. The calcitic prismatic layer is compared with the outer calcitic prismatic layer of an archaeogastropod genus: Haliotis.