Pascale Gautret

Nature and environmental significance of microbialites in Quaternary reefs: the Tahiti paradox

Drill cores from the bamer reef-edge of Tahiti exhibit 85-93-m-thick coralgal sequences recording at least 13,500 years of continuous reef growth in optimal environmental conditions. The cored reef sequences form an overall shallowingupward succession with assemblages of branching or massive colonies of Porires at the base overlain by a robust-branching community (Acropom gr. dmmi/robirstn) heavily encrusted by coralline algae, sessile vermetid gastropods and arborescent foraminifers, which grew at depths less than 6 m. Microbialites generally form the last stage of encrustation of coral colonies, or more commonly, of related encrusting organisms, thus appearing as a major structural component of the reef sequence where they may locally form 809 of the rock by volume. They developed in an open cavity system of the reef framework with freely circulating normal-marine water. Microbialites include laminated crusts and clotted micritic masses, commonly associated in compound crusts, probably feflecting differences in the composition of the involved biological communities and in biomineralization processes that controlled the accretion of the crusts. The isotopic composition of the microbialites (+2.05 to +3.92% I they underwent also indivitlual processes of biominrlali2ation. Besicles the overall decrease in lizht and enqy conditions rttlecting progressive burial by conil growth, the widespread development of microbialites within the reef framework may be related to increased alkalinity and niitrient availability in interstitial waters due to temstrial groundwwr seepage and puiodic runoffs. The tlevelopment of micrubialites in the cryptic niches of the reef framework ceased about 6000 years

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.

Composition of soluble mineralizing matrices in zooxanthellate and non-zooxanthellate scleractinian corals: Biochemical assessment of photosynthetic metabolism through the study of a skeletal feature

SummarySoluble 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.

Skeletal formation in the modern but ultraconservative chaetetid spongeSpirastrella (Acanthochaetetes) wellsi (demospongiae, porifera)

SummaryThe 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.

Organic and Biogeochemical Patterns in Cryptic Microbialites

Cryptic non-cyanobacterial microbialites from reef caves at Lizard Island (Great Barrier Reef, Australia) and the Salt River Canyon, St.Croix (US Virgin Islands) exhibit very distinctive organic patterns. The Lizard Island microbialites consist of various types of automicrite. The biofacies is mainly characterized by highly diverse biofilms, foraminifers, sponges, brachiopods, and bryozoans. Biomarker characteristics of various microbes and sponges (e.g. demosponge acids and mid-chain branched, monomethylalkanoic acids) support histological observations. The microbialite sequence exhibits a light-decreasing facies succession — in this particular case a transgressive sequence with an age of ~3500 years B.P. Macromolecular analyses of the automicrites reveal an excess of acidic macromolecules, which are a feature of calcifying systems. Microbialites from the Salt River Canyon consist of peloids and thin automicritic layers (calcified biofilms) and also exhibit a light-decreasing sequence. The peloids are automicrites that are formed in acidic mucilage in the presence of bacterial remains and mucus. Biomarkers indicate a significant input of terrestrial plant remains and autochthonous heterotrophic bacteria, particularly anoxygenic ones. Sponge and other metazoan biomarkers were not observed in microbialites of the Salt River Canyon. In neither case cyanobacteria were noted, neither in histological sections nor by using biochemical methods.

Matrices organiques intrasquelettiques des scléractiniaires récifaux : évolution diagénétique précoce de leurs caractéristiques biochimiques et conséquences pour les processus de cimentation

Resume Les caracteristiques biochimiques et les proprietes des matrices organiques extraites de fragments squelettiquesechantillonnes dans diverses zones de croissance, ont ete etablies pour une carotte de 25 cm de long, prelevee dans une colonie de Porites vivante. Testees dans des conditions experiementales, ces matrices organiques ont un eflat inhibiteur sur la precipitation du CaCO3 qui augmente considerablement tout au long des 15–20 premiers cm. Il diminue dans le prelevement de la base de la carotte, a 25cm. Parallelement les compositions relatives en acides amines montrent une augmentation de la representation des acides aspartique et glutamique avec la profondeur du prelevement, jusqua la base de la carotte. Les masses moleculaires des composes glycoproteiques de ces matrices organiques sont comparables a celles de la zone vivante dans lensemble de la carotte, sauf dans le prelevement basal. La juxtaposition des resultats experimentaux montre quune efficacite maximale dinhibition resulte de la quantite de matrice organique presente, de sa composition (notamment sa richesse en Asp) et de la preservation des assemblages macromoleculaires de masses elevees. Des consequences pour la cimentation dans les zones superfielles des recifs coralliens sont envisagees: letat des matrices organiques intrasquelettiques pourrait influencer le developpement des ciments au contact direct des substrats squelettiques, ainsi que la formation daragonite de diagenese cristallographiquement superposee a laragonite biogenique a linterieur des structures squelettiques. Sur la base de cette hypothese, les conditions regnant dans les zones cryptiques a spongiaires des recifs coralliens sont comparees a celles des zones photiques a scleractiniaires.

Recherche sur les affinités desSpongiomorphidae Frech, 1890: 2 — Révision des caractéristiques microstructuralesdes espèces initialement attribuées aux Spongiomorphidae

Resume Sur la base dune revision des caracteristiques squelettiques, et essentiellement microstructurales, deSpongiomorpha acyclica Frech les criteres auxquels doivent repondre les especes attribuees aux Spongiomorphes sont etablis. Les autres formes triasiques de Fischerwiese (Zlambach beds, Sud-Autriche) initialement integrees dans ce taxon par Frech sont egalement reexaminees et leurs microstructures decrites. Letude met en evidence lhomogeneite des microstructures et des modalites de la croissance squelettique chez 5 de ces especes, qui trouvent dailleurs leur homologue dans une famille de Scleractiniaires triasiques, les Astraeomorphidae dont la diagnose a ete recemment emendee en tenant compte des caracteristiques microstructurales. Il est propose dintroduire les Spongiomorphides dans cette famille de Madreporaires, excepte S. (Heptastylopsis) ramosa, dont lorganisation microstructurale est typiquement trabeculaire.

Les Démosponges calcifiées des biohermes du Jurassique supérieur du Sud tunisien (Oxfordien de la région de Tataouine)

Resume Letude de Spongiaires provenant de biohermes oxfordiensdu Sud-Tunisien permet detablir la presence dun groupe bien diversifie sur le plan architectural, et nettement distinct, par ses caracteristiques mineralogiques et microstructurales, de lensemble des Demosponges permo-triasiques. Lexistence de traces de spicules inclus dans le squelette carbonate de plusieurs de ces especes montre clairement leur appartenance a la classe des Demosponges, mais limportance des phenomenes evolutifs quon commence a discerner a la limite Trias-Jurassique rend prematuree leur affectation a une des familles actuellement definies. En correlant la differentiation de ce type microstructural apres le Trias, avec la disparition a la fin de cette periode, des faunes de Spongiaires qui y avaient connu un grand developpement, on peut suggerer un rapprochement entre les conditions de formation des structures squelettiques dans ces groupes dorganismes et les phenomenes cycliques concernant la precipitation des carbonates sedimentaires. La reapparition des taxons permo-triasiques au cours du Cenozoique (et la disparition apparente des types structuraux decrits dans ce travail), illustre la possibilite dune secretion intermittente de ce type de structures squelettiques carbonatees chez les Demosponges.

Recherche sur les affinitésdes Spongiomorphidae Frech, 1890: 1-Caractéristiques microstructurales et minéralogiques de Spongiomorpha acyclica FRECH, 1890

Resume A partir de specimens recueillis dans le gisement type (Rhetien de Fischerwiese, Autriche-Prov. de Styrie), les modalites de biomineralisation chez Spongiomorpha acyclica sont etudiees. Les caracteristiques microstructurales des meilleurs specimens permettent de reconstituer tres precisement les etapes de la croissance du tissu fibreux chez cette espece, et de restaurer la proposition daffinite qui avait ete initialement celle de Frech: la proximite avec Astraeomorpha crassisepta Frech , 1890 , un Madreporaire du meme gisement. La proximite de ces deux formes est egalement etablie par lanalyse des modalites de levolution diagenetique de leur tissu fibreux, et qui sopposent a celui des Demosponges calcifiees presentes dans les memes gisements. En effet, les teneurs en elements mineurs presents dans les tissus de deux Ceratoporellides de la faune de Fischerwiese illustrent levolution differentielle des materiaux aragonitiques elabores dans les differents taxons majeurs.

Comparaison des modalités de diagenèse du squelette de Spongiaires carbonatés dans le Trias de Turquie et le Permien de Tunisie

Resume A partir dune reference actuelle ( Astrosclera willeyana Lister ), Demosponge a squelette carbonate principal, les microstructures de materiels triasiques et permiens sont comparees, de facon a apprecier leur evolution diagenetique. Dans ce but, les caracteristiques microstructuraleshabituelles, portant essentiellement sur la morphologie des fibres minerales, sont completees par des donnees relatives a leur composition chimique elementaire et une approche chromatographique de la composante organique intrasquelettique. Pour accroitre autant que possible le caractere significatif de cette comparaison, seuls des specimens a microstructure spherolitique ont ete pris en compte, quelle que soit leur organisation architecturale (Sphinctozoaires, Chaetetides et Inozoaires).