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Les caractéristiques biochimiques et les propriétés des matrices organiques extraites de fragments squelettiqueséchantillonnés dans diverses zones de croissance, ont été établies pour une carotte de 25 cm de long, prélevée dans une colonie de Poritesvivante. Testées dans des conditions expériementales, ces matrices organiques ont un eflat inhibiteur sur la précipitation du CaCO3qui augmente considérablement tout au long des 15–20 premiers cm. Il diminue dans le prélèvement de la base de la carotte, à 25cm. Parallèlement les compositions relatives en acides aminés montrent une augmentation de la représentation des acides aspartique et glutamique avec la profondeur du prélèvement, jusqu'à la base de la carotte. Les masses moléculaires des composés glycoprotéiques de ces matrices organiques sont comparables à celles de la zone vivante dans l'ensemble de la carotte, sauf dans le prélèvement basal. La juxtaposition des résultats expérimentaux montre qu'une efficacité maximale d'inhibition résulte de la quantité de matrice organique présente, de sa composition (notamment sa richesse en Asp) et de la préservation des assemblages macromoléculaires de masses élevées. Des conséquences pour la cimentation dans les zones superfielles des recifs coralliens sont envisagées: l'état des matrices organiques intrasquelettiques pourrait influencer le développement des ciments au contact direct des substrats squelettiques, ainsi que la formation d'aragonite de diagenèse cristallographiquement superposée à l'aragonite biogénique à l'intérieur des structures squelettiques. Sur la base de cette hypothèse, les conditions régnant dans les zones cryptiques à spongiaires des récifs coralliens sont comparées à celles des zones photiques à scléractiniaires.

Summary: The modern hadromerid coralline spongeSpirastrella (Acanthochaetetes) wellsi exhibits a unique secondary high-Mg calcite (>19 mol % MgCO3) basal skeleton. The basal skeleton is constructed of bundles of elongated crystals more or less tangentially orientated. The initial formation of these crystals is controlled by soluble highly acidic aspartic and glutamic-rich (40%) macromolecules. The skeletal mineralization occurs in four different loci: in the top of the calicles, at the tabulae, on collagenous anchor fibres, and within closed spaces between the tabulae. The clicle walls are formed on the uppermost top of the basal skeleton as a continuous process. Based on long term stainings with Ca2+-chelating fluorochroms (calcein, chlorotetracyclines) the growth rate of this sponge is extremely low with ca. 50–100μm/a. The skeletal formation takes places outside the sponge, within a narrow zone (300–500 nm) between the basopinacoderm and the mature basal skeleton. The sponge produces thread-like folded templates (‘spaghetti fibres’) of 0,5–2 μm size, the shape controlling insoluble organic matrix. These templates become mineralized in a first step as MgCO3, then are stretched. A soluble organic matrix is also secreted, and remains are included inside the mineralized skeleton. This organic matrix consists of in a complex mixture containing small very acidic proteins (5, 13, 31 KD; 40% Asp and Glu and therefore most probably Ca2+-binding) and high molecular weight glycoproteins among several other organic compounds. The mature crystals are high-Mg calcites. During calcification large cells with large reserve granules (LCG) are always present in a tight connection with the basopinacoderm. These cells form also the collagenous anchor fibres. Primary tabulae are formed by a non-collagenous organic sheet. Calcification happens only when LCG cells are enriched on the organic sheet. Randomly oriented high-Mg calcite crystals are growing on the collagenous anchor fibres. The same type of the mineralization is observed within the spaces of the tabulae. This particular case of mineralization is controlled by decaying sponge tissue (ammonification). The δ13C values are in equilibrium with the ambient sea water and vary between +3.2 and +2.8 ‰. The mode of mineralization of the basal skeleton can be described as biologically induced resp. matrix mediated.

Summary: This research report contains nine case studies (part II to X) dealing with Palaeozoic and Mesozoic mud mounds, microbial reefs, and modern zones of active micrite production, and two parts (I and XI) summarizing the major questions and results. The formation of different types ofin situ formed micrites (automicrites) in close association with siliceous sponges is documented in Devonian, Carboniferous, Triassic, Jurassic and Cretaceous mounds and suggests a common origin with a modern facies found within reef caves. Processes involved in the formation of autochthonous micrites comprise: (i) calcifying mucus enriched in Asp and Glu, this type presumably is linked to the formation of stromatolites, thrombolites and massive fabrics; (ii) protein-rich substances within confined spaces (e.g. microcavities) result in peloidal pockets, peloidal coatings and peloidal stromatolites, and (iii) decay of sponge soft tissues, presumably enriched with symbiotic bacteria, lead to the micropeloidal preservation of parts of former sponge bodies. As a consequence, there is strong evidence that the primary production of micrite in place represents the initial cause for buildup development. The mode of precipitation corresponds to biologically-induced, matrix-mediated mineralization which results in high-Mg-calcites, isotopically balanced with inorganic cements or equilibrium skeletal carbonates, respectively. If distinct automicritic fabrics are absent, the source or origin of micrite remains questionable. However, the co-occurring identifiable components are inadequate, by quantity and physiology, to explain the enhanced accumulation of fine-grained calcium carbonate. The stromatolite reefs from the Permian Zechstein Basin are regarded as reminiscent of ancestral (Precambrian) reef facies, considered the precursor of automicrite/sponge buildups. Automicrite/sponge buildups represent the basic Phanerozoic reef type. Analogous facies are still present within modern cryptic reef habitats, where the biocalcifying carbonate factory is restricted in space.

Changes in ∂O18 values (measurements by two distinct laboratories) and composition of mineralizingorganic phases appear to be well correlated in a core made in a Porites skeleton from New Caledonia. This punctual result is discussed by comparison with the global characteristics of calcareous biominerals. Factors that can affect the stability of these organo-mineral architectures, particularly the presence of organic phases dispersed at the submicronic scale within the crystal-like microstructural units and also the presence of intracrystalline water associated to glyco-proteic macromolecular compounds, are briefly discussed. Consequences for the development of diagenetic processes and importance for accurate measurement or environmental data in ancient coral colonies are discussed.

A partir de spécimens recueillis dans le gisement type (Rhétien de Fischerwiese, Autriche-Prov. de Styrie), les modalités de biominéralisation chez Spongiomorpha acyclicasont étudiées. Les caractéristiques microstructurales des meilleurs spécimens permettent de reconstituer très précisément les étapes de la croissance du tissu fibreux chez cette espèce, et de restaurer la proposition d'affinité qui avait été initialement celle de Frech: la proximité avec Astraeomorpha crassiseptaFrech, 1890, un Madréporaire du même gisement. La proximité de ces deux formes est également établie par l'analyse des modalités de l'évolution diagénétique de leur tissu fibreux, et qui s'opposent à celui des Démosponges calcifiées présentes dans les mêmes gisements. En effet, les teneurs en éléments mineurs présents dans les tissus de deux Cératoporellides de la faune de Fischerwiese illustrent l'évolution différentielle des matériaux aragonitiques élaborés dans les différents taxons majeurs.

A partir d'une référence actuelle (Astrosclera willeyanaLister), Démosponge à squelette carbonaté principal, les microstructures de matériels triasiques et permiens sont comparées, de façon à apprécier leur évolution diagénétique.

Summary: Soluble organic matrices extracted from aragonitic skeletons produced by recent zooxanthellate and non-zooxanthellate scleractinian corals were studied after suitable hydrolyses, by HPLC chromatographies allowing characterization of their amino acid and monasaccharide compositions. Clear compositional differences can be correlated with the symbiotic or non-symbiotic character in both proteic (via Asp, Glu, Ala and Ser) and glucidic phases of soluble matrices (via GalN, GlcN and Gal), providing new criteria to assess the impact of photosynthetic metabolism on skeletal features of scleractinian corals.