Linking microbes to geochemical processes: Characterization of microbial assemblages involved in sulfur and methane cycling in high-temperature petroleum reservoirs and marine methane seeps

Anaerobic microorganisms are taxonomically and metabolically diverse and are essential for nutrient cycling in anoxic environments. Despite their global importance, relatively little is known about the interplay between specific microbial populations and their function in many anaerobic ecosystems. Using DNA-based molecular techniques with enrichment cultures, geochemistry and stable isotope methodologies, the taxonomic and functional diversity of microbial assemblages from two anoxic extreme environments, high temperature subsurface oil reservoirs and marine methane cold seeps, were characterized. The diversity of microorganisms specifically involved in the cycling of methane and sulfur species in these environments was emphasized in this research.; Within petroleum reservoirs, the diversity of microorganisms detected by 16S rRNA gene surveys and enrichments reflected the physico-chemical conditions. The presence of diverse thermophilic assemblages closely related to known sulfidogenic, methanogenic and fermentative microorganisms recovered from other petroleum systems and hydrothermal vents, suggests that thermophilic Archaea and Bacteria are widespread in deep subsurface petroleum formations. The potential for methane and hydrogen sulfide production by in situ thermophilic assemblages was also correlated with the geochemistry of formation fluids. Collectively, these results strongly suggest that these thermophilic populations may be actively participating in carbon and sulfur cycling in situ.; The anaerobic oxidation of methane is thought to be the dominant microbial process in marine sediments overlying gas hydrates and methane seeps, but the identity and diversity of microorganisms mediating this process is poorly understood. The presence of specific archaeal and bacterial 16S rDNA sequences correlated with 13C-depleted lipid biomarkers in sediments from two distinct seep locations supported the hypothesis that two methanogen-related archaeal groups (ANME-1 and ANME-2), and sulfate-reducing Desulfosarcina spp., are involved in this process. Using fluorescent in situ hybridization assays, the distribution, morphology and interspecies associations of the candidate groups were examined. The direct coupling of fluorescent in situ hybridization with secondary ion mass spectrometry revealed extreme 13C depletion in the biomass of individual ANME-1 rods and in aggregations of ANME-2 and Desulfosarcina spp. This extreme 13C depletion indicates that both archaeal groups are consuming methane anaerobically, either in syntrophic cooperation with sulfate-reducing bacteria or possibly independently by a currently unrecognized mechanism.