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The investigation of the extent and dynamics of deep microbial ecosystems in sedimentary basins is a new and intriguing topic in today’s geoscience research (Horsfield et al., 2007). With sedimentary organic matter becoming more recalcitrant with depth, microbial communities have to endure a decrease in the available carbon and energy sources in addition to increasing temperature and pressure conditions (Parkes et al., 2000). Therefore, the search for and characterization of deep microbial ecosystems are closely connected with the investigation of potential carbon and energy sources for these microorganisms.

Hydrocarbon leakage can form a food source for deep microbial communities by liberating a wide range of biologically utilizable compounds. In a previous study (the Geomound project) computer simulations of the basin history, undertaken at GeoForschungsZentrum (GFZ) Potsdam (Naeth et al., 2005), revealed that below the carbonate mounds in the Belgica mound province specific sandstones from Cretaceous and Tertiary sequences represent important migration pathways for natural gases to the surface. Thus, hydrocarbon gases could have migrated to the mound base stimulating microbial activity at this interval. Concomitantly, microorganisms oxidizing methane in near-seafloor sediments (anaerobic oxidation of methane) might have precipitated carbonate crusts, forming potential hardgrounds for coral colonization (Henriet et al., 2002).

In this report, data on molecular microbial indicators are presented for the three sites (U1316, U1317, and U1318) drilled during Integrated Ocean Drilling Program Expedition 307, (April–May 2005) in the Porcupine Basin area offshore Ireland. Additionally, gas composition and gas isotope data are shown for the sediments from Challenger Mound Site U1317. To investigate the microbial ecosystem, phospholipids and defunctionalized hopanoids were selected. Phospholipid esters represent microbial biomass from living organisms because these bacterial cell membrane components are only stable in intact cells over geological times (White et al., 1979). Defunctionalized hopanoids are already partly degraded. Thus, they are more likely remains from past microbial populations. Furthermore, the steric configuration of hopanoids can be used to identify the occurrence of fossil hydrocarbons, because oils usually contain a series of hopanoids in the “geological” 17α(H),21β(H)-configuration. Concentrations of long-chain n-alkanes were determined to assess the supply of terrestrial organic matter into the sediments of the study area (Eglinton and Hamilton, 1967).