Lipid biomolecules reveal patterns of microbial metabolism in extreme environments

This dissertation used state-of-the-art organic geochemical techniques to explore the lipid biosignatures of microorganisms inhabiting at cold and hot vents. The association of organisms and their carbon metabolisms were assessed and the current knowledge on the microbially mediated oxidation of reduced compounds under aerobic and anaerobic conditions, especially at methane-rich environments, was expanded. By using the taxonomic information encoded in intact polar lipid (IPL) molecules, this work confirmed the presence of aerobic chemotrophic bacteria living symbiotically with mussels recovered from seep and vent environments. This was the first application of IPLs in symbiont ecology, providing a semi-quantitative estimation of the importance of methanotrophic and thiotrophic symbionts within the gills of Bathymodiolus mussels. In addition, the stable carbon isotopic composition of a suite of lipid biomarkers (e.g., fatty acids, sterols, bacteriohopanepolyols) derived from symbionts and hosts highlighted the importance of a chemosynthetic lifestyle in these extreme settings. The anaerobic oxidation of methane (AOM) performed by naturally enriched anaerobic methanotrophic (ANME)archaea belonging to the ANME 1 subgroup and sulfate-reducing bacteria (SRB) belonging to the HotSeep 1 cluster from the Guaymas Basin (Gulf of California) sediments was studied using a novel dual stable isotope probing (SIP) method. Dual-SIP was applied to bulk microbial lipids (total lipid extract, TLE) and involves the assimilation of deuterated water (D2O) and 13CDIC (DIC = dissolved inorganic carbon), which allows the simultaneous assessment of auto and heterotrophic carbon fixation. This work was the first to demonstrate autotrophic lipid production for both ANME and SRB, in contrast to previous AOM studies which suggested a significant contribution of direct methane assimilation during microbial growth. These results have a direct implication in the interpretation of natural lipid stable carbon isotopic compositions in AOM systems. To further access the 13C assimilation into individual archaeal IPLs an extensive preparative HPLC separation of lipids was performed. This work demonstrated an increase of up to 35 times in sensitivity on IPLs (IPL-SIP) rather than bulk lipids (TLE-SIP). Moreover, the label assimilation of individual di and tetraether IPLs uncovered a possible role of a diether phospholipid as precursor in archaeal tetraether biosynthesis. Finally, it was hypothesized that the tetraether membrane, especially the formation of glycosidic (Gly) glycerol dibiphytanyl glycerol tetraether (GDGT), is used to reduce proton-permeability, thereby protecting the integrity of cells at extreme conditions from the Guaymas Basin. Along with this hypothesis, the ubiquity of 2Gly-GDGTs in marine sediments worldwide may likely represent an expression of cell-wall adaptation to cope with energy stress.