Les tapis microbiens sont des communautés benthiques, calcifiées (i.e. microbialites) ou non, diverses à la fois phylogénétiquement et métaboliquement. Les tapis microbiens fossiles constituent les plus anciennes traces de vie sur Terre et leurs représentants modernes peuvent donc être utilisés pour comprendre le fonctionnement de ces écosystèmes anciens. J'ai étudié les communautés microbiennes (archées, bactéries et eucaryotes) de plusieurs microbialites (lac Alchichica, Mexique) et tapis microbiens (dans une mare du salar de Llamara, Chili) afin de caractériser finement leur structure phylogénétique et d'améliorer notre compréhension de leur fonctionnement. J'ai pour cela utilisé une approche combinant des outils moléculaires (métabarcoding, métagénomique) à des données environnementales (paramètres physico-chimiques de la colonne d'eau ou composition minérale des microbialites). Mon travail de thèse a permis d'affiner le modèle de formation des microbialites d'Alchichica, en montrant notamment que, en plus de la photosynthèse oxygénique cyanobactérienne, le potentiel à précipiter des carbonates de la photosynthèse eucaryote et, surtout, de la photosynthèse anoxygénique est important. Les communautés des tapis de Llamara étaient quant à elles caractérisées par la présence de nombreuses lignées d'archées et de bactéries très divergentes, dont certaines ont été identifiées pour la première fois dans ce travail. Nos analyses ont aussi souligné la diversité des organismes impliqués dans les cycles du soufre et de l'azote au sein de ces systèmes et permis d'identifier de potentielles interactions biotiques entre des lignées procaryotes dont l'écologie est peu connue. Enfin, nous avons mis en évidence que les paramètres environnementaux influencent fortement la composition des communautés associées à ces microbialites et à ces tapis microbiens. L'ensemble de ces résultats permet de mieux comprendre le fonctionnement de ces systèmes ainsi que les facteurs qui influencent leur structure phylogénétique et fonctionnelle. / Microbial mats are phylogenetically and functionally diverse benthic microbial communities, which can be sometimes calcified (i.e. microbialites). Fossil microbial mats constitute the oldest traces of life on Earth and their modern representatives are thus used as analogues of those primordial ecosystems to gain insights into their functioning. I have studied the microbial communities (archaea, bacteria and eukaryotes) of several microbialites (lake Alchichica, Mexico) and microbial mats (in a small pond in the salar de Llamara, Chile). The main objectives of my PhD were to finely characterize their phylogenetic structure and to improve our understanding of the functioning of these complex ecosystems. To do so, I have applied a multi-disciplinary approach combining molecular approaches (metabarcoding, metagenomics) to environmental data (physico-chemical parameters of the water column or mineral composition of the microbialites).The results presented in this thesis allowed refining our model of microbialite formation in Lake Alchichica. We showed that, in addition to cyanobacterial photosynthesis, both eukaryotic and, particularly, anoxygenic photosyntheses were potentially important to promote carbonate precipitation. Llamara mat communities were characterised by the presence of numerous novel archaeal and bacterial lineages, some of which were identified for the first time in this work. Our analyses have also highlighted the diversity of organisms involved in both sulphur and nitrogen cycles in these mats and identified potential biotic interactions between poorly known prokaryotic lineages. Finally, we showed that the composition of the microbial communities associated to these microbialites and microbial mats was strongly influenced by environmental parameters. Overall, these results represent a substantial contribution to our understanding of the ecology of these systems as well as of the factors that influence their phylogenetic and functional structures.

No description available.

No description available.

No description available.

<p>Modern microbialites provide the opportunity to explore the influences of biology on microbialite formation and understand how biosignatures can be preserved in these structures. In this study, we used the isotopic compositions (δ¹³C) of phospholipid fatty acids (PLFAs) and their structurally-defined profiles, in conjunction with calcium carbonate isotopic compositions and imaging to evaluate microbial autotrophic and heterotrophic processes associated with freshwater microbialites from Kelly Lake, British Columbia. This was done to determine what types of metabolism may have been influencing microbialite growth and whether a biosignature of this process was preserved. In addition, PLFA profiles from a microbialite-derived pure culture were analyzed under various growth conditions to assess environmental influences on microbial PLFA composition.</p> <p>Although the majority of the δ¹³C values of Kelly Lake microbialite surface carbonates fell within the range predicted for equilibrium precipitation, samples collected from 26 m were found to have enriched δ¹³C_carb values and are likely a biosignature of autotrophy at this depth. PLFA profiles and δ¹³C_PLFAvalues also supported the predominance of autotrophy, however, they indicated that heterotrophic organisms were also present. This data suggests that autotrophic metabolisms have influenced the local geochemistry in the past, at least at 26 m, and are likely substantial contributors to microbialite growth.</p> <p>Changes in temperature, pH, NaCl concentrations, and cell densities were found to induce variations in the PLFA profiles of the <em>Exiguobacterium</em> strain RW2. The degree of PLFA unsaturation changed in each of the different culture conditions, and was predominantly adjusted through alterations in the branched monoenoic PLFAs, particularly i-17:1Δ⁵. These results highlight the difficulties associated with applying PLFA profiles as evidence for shifts in a microbial community composition, since altered growth conditions can induce intra-specific PLFA changes.</p> / Master of Science (MSc)

No description available.

No description available.

No description available.