Organic geochemical and petrographic study have been carried out on the Messinian Calcare di Base formation cropping out in northern Calabria and in Sicily. The main aim of this research concerns the deposition causes of this peculiar formation, up to now interpreted as essentially evaporitic limestone. Thin section observations put in evidence that carbonate layers are characterized by a peloidal fabric and the absence of any kind of metazoan skeletons. The prevailing fabric is characterized by dark peloid clusters, cylindrical or subcylindrical in shape, patchily dispersed into a lighter matrix. The shape, mineral composition, dimensions and context suggest that many elongate bodies can be interpreted as fecal pellets of unknown organisms. In addition carbonate layers also show two other facies types: i) a detrital, very finely gradated layer, and ii) a microbioalitic, sometimes more or less stromatolitic fabric. The bright UV-epifluorescence suggests a high content of organic matter in both fecal pellets and stromatolitic fabric. The study of carbonaceous remains emphasized a great variety of the organic input. Geochemical data (Rock-Eval pyrolysis) indicate a mixed (marine and continental) organic input. These data have been confirmed by organic petrographic observations (palynofacies) which revealed the presence of phytoclasts derived from continental plant tissues, amorphous organic matter, and variable proportions of zooclasts, pollens, spores, phytoplanktonic organisms and filaments presumably attributable to cyanobacteria. Preliminary results from organic geochemistry and petrography could suggest that the depositional environment became more and more restricted, allowing the survival of organisms adapted to extreme conditions, only. These enigmatic organisms have not been observed yet, however their biological signatures in the sediments are testified by the geochemistry data and palynofacies observation. Moreover, the presence of well preserved and bright-fluorescent spores and pollens indicate that these elements did not undergo degradation and oxidation, suggesting a sedimentary environment characterized by a stratified water column with bottom anoxic conditions. Such context, combining a good preservation state of organic matter and a typified character (nearly extreme conditions) of the environment, reveals highly favorable for undertaking the biochemical study of organic compounds associated with these sedimentary deposits. The amino-acid analysis of peptide remains may help to understand the influence of soluble macromolecules (mainly derived from microbial EPS) in the formation of authigenic carbonates. We can also expect the GC-MS detection of preserved lipidic biomarkers, providing the molecular signature of microscopically non-identifiable (non-preserved) organisms.
Morphologically diverse organo-sedimentary structures (including microbial mats and stromatolites) provide a palaeobiological record through more than three billion years of Earth history. Since understanding much of the Archaean fossil record is contingent upon proving the biogenicity of such structures, mechanistic interpretations of well-preserved fossil microbialites can reinforce our understanding of their biogeochemistry and distinguish unambiguous biological characteristics in these structures, which represent some of the earliest records of life. Mechanistic morphogenetic understanding relies upon the analysis of geomicrobiological experiments. Herein, we report morphological-biogeochemical comparisons between micromorphologies observed in growth experiments using photosynthetic mats built by the cyanobacterium Coleofasciculus chthonoplastes (formerly Microcoleus) and green anoxygenic phototrophic Chloroflexus spp. (i.e., Coleofasciculus–Chloroflexus mats), and Precambrian organo-sedimentary structures, demonstrating parallels between them. In elevated ambient concentrations of Cu (toxic to Coleofasciculus), Coleofasciculus–Chloroflexus mats respond by forming centimetre-scale pinnacle-like structures (supra-lamina complexities) associated with large quantities of EPS at their surfaces. µPIXE mapping shows that Cu and other metals become concentrated within surficial sheath-EPS-Chloroflexus-rich layers, producing density-differential micromorphologies with distinct fabric orientations that are detectable using X-ray computed micro-tomography (X-ray µCT). Similar micromorphologies are also detectable in stromatolites from the 3.481 Ga Dresser Formation (Pilbara, Western Australia). The cause and response link between the presence of toxic elements (geochemical stress) and the development of multi-layered topographical complexities in organo-sedimentary structures may thus be considered an indicator of biogenicity, being an indisputably biological and predictable morphogenetic response reflecting, in this case, the differential responses of Coleofasciculus and Chloroflexus to Cu. Growth models for microbialite morphogenesis rely upon linking morphology to intrinsic (biological) and extrinsic (environmental) influences. Since the pinnacles of Coleofasciculus–Chloroflexus mats have an unambiguously biological origin linked to extrinsic geochemistry, we suggest that similar micromorphologies observed in ancient organo-sedimentary structures are indicative of biogenesis. An identical Coleofasciculus–Chloroflexus community subjected to salinity stress also produced supra-lamina complexities (tufts) but did not produce identifiable micromorphologies in three dimensions since salinity seems not to negatively impact either organism, and therefore cannot be used as a morphogenetic tool for the interpretation of density-homogeneous micro-tufted mats—for example, those of the 3.472 Ga Middle Marker horizon. Thus, although correlative microscopy is the keystone to confirming the biogenicity of certain Precambrian stromatolites, it remains crucial to separately interrogate each putative trace of ancient life, ideally using three-dimensional analyses, to determine, where possible, palaeoenvironmental influences on morphologies. Widespread volcanism and hydrothermal effusion into the early oceans likely concentrated toxic elements in early biomes. Morphological diversity in fossil microbialites could, therefore, reflect either (or both of) differential exposure to ambient fluids enriched in toxic elements and/or changing ecosystem structure and tolerance to elements through evolutionary time—for example, after incorporation into enzymes. Proof of biogenicity by deducing morphogenesis (i.e., a process preserved in the fossil record) overcomes many of the shortcomings inherent to the proof of biogenicity by descriptions of morphology alone.
Using three different microscopy techniques (epifluorescence, electronic and atomic force microscopy), we showed that high-Mg calcite grains in calcifying microbial mats from the hypersaline lake "La Salada de Chiprana", Spain, contain viruses with a diameter of 50-80 nm. Energy-dispersive X-ray spectrometer analysis revealed that they contain nitrogen and phosphorus in a molar ratio of ~9, which is typical for viruses. Nucleic acid staining revealed that they contain DNA or RNA. As characteristic for hypersaline environments, the concentrations of free and attached viruses were high (>10(10) viruses per g of mat). In addition, we showed that acid treatment (dissolution of calcite) resulted in release of viruses into suspension and estimated that there were ~15 × 10(9) viruses per g of calcite. We suggest that virus-mineral interactions are one of the possible ways for the formation of nano-sized structures often described as "nanobacteria" and that viruses may play a role in initiating calcification.
Hemispheroidal domes (microbialites) produced by natural populations of filamentous cyanobacteria belonging to four distinct Phormidium species, and one probable new species of Schizothrix were collected alive from 0-25 m depth habitates in the lagoon of Tikehau atoll (Tuamotu, French Polynesia). This study aims to establish the biochemical control on in-situ carbonate precipitation processes (organomineralization processes) occuring merely in the alveolate network of non-coalescent microfibrils that characterizes the degraded parts of the microbialite domes. The comparison between amino acid and monosaccharide composition of purified cyanobacterially produced organic matter and that of intramineral (soluble and insoluble) organic matrices associated with carbonate precipitates emphasizes the importance of dicarboxylic (aspartic and glutamic) acids, released by the decay of cyanobacterial sheaths, in CaCO3 formation and demonstrates that the in situ precipitation of ultra-fine micrites is a highly selective process regarding the available external organic matter. This diagenetic process is thought to result from incipient hydrolysis of cyanobacterial S-layer proteins attached to extracellular polysaccharide fibrils composing the sheath. Taxonomic affinity of cyanobacterial populations responsible for microbialite construction is one of the major factors allowing biochemical discrimination of in-situ precipitated carbonates, indicating that specific mucilages or their degradational products are guiding forces for the calcification processes. Another possible source for the formation of carbonate-associated organic matrices is derived from metabolites (e.g. mucus) released in water by lagoonal dwelling benthic organisms.
Cette these se propose de determiner dans quelle mesure les caracteres du squelette aspiculaire des Spongiaires peuvent permettre d'etablir une systematique coherente, integrant a la fois les formes fossiles et actuelles. Deux essais de classification ont ete tentes, donnant successivement priorite a. L'un des criteres analysables du squelette: la morphologie puis la microstructure. L'analyse morphologique se rattache aux etudes paleontologiques classiques conduites sur le groupe des Spongiaire. L'analyse des structures carbonatees tient compte a la fois de leur organisation a l'echelle microstructurale, de la typologie des composants microstructuraux elementaires et de l'ultrastructure de ces composants, mise en evidence au moyen de destructions menagees, soit de la phase minerale, soit de la phase organique intrasquelettique. L'integration des especes actuelles dans les deux systemes de classification precedemment mentionnes, permet de preconiser l'utilisation des caracteres microstructuraux, lies aux processus de biomineralisation, pour la determination des taxons majeurs.
