DIFFERENCES IN MICROBIAL LIPID DISTRIBUTIONS BETWEEN GULF OF MEXICO COLD SEEP CRUSTS AND SEDIMENTS

The application of microbiological and organic geochemical techniques to the study of cold seeps has led to new insights into the role of syntrophic archaeal and sulfate-reducing bacterial communities as the mediators of the anaerobic oxidation of methane (AOM). Gulf of Mexico cold seeps provide an intriguing contrast to previously studied systems as they are characterized by emissions of higher-molecular-weight petroleum-type hydrocarbons as well as methane. We analysed five carbonate crusts, one hydrate-bearing sediment core and one hydrate-free control core from the Green Canyon region of the Gulf of Mexico. All sediments and crusts contain abundant biodegraded petroleum as indicated by a low abundance of n-alkanes and large unresolved complex mixtures (UCM). Archaeal and bacterial biomarkers in the crusts and hydrate-bearing core have similar distributions as those observed in cold seep sediments but are absent from the control core, suggesting that the presence of higher-molecular-weight hydrocarbons has not affected the AOM community. Specifically, 13C-depleted archaeal biomarkers (archaeol abundance ranges from 1-2 ug per g dry sediment and from 1.6 to 7.6 ug per g crust) provide direct evidence for an abundant archaeal community, whilst 13C-depleted branched fatty acids and non-isoprenoidal diethers (0.25 – 1.0 ug per g dry sediment and from 0.3 to 1.5 ug per g crust) likely derive from syntrophic sulfate reducers. Nonetheless, there are differences in archaeal and bacterial lipid distributions amongst sediments and crusts. For example, sn-2-hydroxyarchaeol is particularly abundant relative to other archaeal lipids at 4-8 cm, perhaps indicating the presence of a unique archaeal community. Also crusts generally have higher archaeal biomarker abundances, higher hydroxyarchaeol to archaeaol ratios and higher archaeal to bacterial lipid ratios than corresponding sediments. This is consistent with previous studies suggesting that while the underlying biogeochemical processes and microbial community structures are the same in diverse AOM environments, there is also overprinting variability in bacterial and archaeal populations.