Surveying the metabolic versatility of biofilm-forming Epsilonproteobacteria: a study into developing ecosystems at extreme environments

At deep-sea hydrothermal vents, mixing of reduced, super-heated, hydrothermal fluids with cold, oxygenated, seawater creates steep temperature and chemical gradients that support chemosynthetic primary production and rich communities of invertebrates. In 2006, an eruption occurred on East Pacific Rise at 9° 50’N, 104° 17’W. Direct observations of the post-eruptive diffuse flow vents clearly indicated that the earliest colonizers were microbial biofilms. A series of cruises in 2006-07 allowed us to monitor the recovery of the ecosystem. The main objectives of this dissertation are to assess the taxonomic and functional diversity of chemosynthetic bacteria following the eruption, and to correlate it to macrofaunal colonization. To this end, I investigated several microbial biofilms that developed at the bottom of the ocean during exposure to different temperature, redox and biological regimes. Furthermore, I selected pure cultures of vent bacteria representative of these biofilms and designed experiments to investigate their expression of diagnostic genes involved in carbon fixation and respiration. Finally, I used the information obtained from the pure cultures and from metatranscriptomic studies of the vent biofilms to design experiments for the detection of gene transcripts in chemosynthetic microbial biofilm communities collected from deep-sea hydrothermal vents, and to interpret the results. My data showed that the biofilm communities that were exposed to active venting were substantially different from the ones that formed at control sites, and that vent invertebrates could only be detected at the former sites. Furthermore, I found that various members of the Epsilonproteobacteria dominated the chemosynthetic biofilm communities, and that these bacteria fixed carbon dioxide in-situ via the reverse tricarboxylic acid (rTCA) cycle and that they expressed different terminal reductases in response to variable temperature and redox conditions. I demonstrated for the first time that different respiratory pathways (e.g., nitrate reduction, sulfur oxidation/reduction, microaerobic respiration) are expressed simultaneously in chemosynthetic biofilms. In turn, these results imply that the extremely dynamic conditions found at diffuse flow vents, where reduced hydrothermal fluids mix with oxic seawater, provide the biofilm bacteria with a diverse “metabolic menu” in the form of different redox couples that they can use to conserve energy.Ph. D.Includes bibliographical referencesby Charles Edward O'Brie