ALBERT

Description: Deep subsurface sediments from the Nankai Trough, Japan Sea, ODP Leg 190, sites 1173, 1174, 1177, and near-surface sediments from Hydrate Ridge, NE-Pacific have been analysed by high performance liquid chromatography (HPLC)–electrospray ionisation (ESI)-mass spectrometry (MS). The main objective was to utilize the presence of intact phospholipids as a direct indicator of viable microorganisms. The extracts of Nankai Trough sediments were found to contain a variety of phospholipid (PL) structures, well-known to stem from microorganisms, to depths as great as 745 mbsf and in situ temperatures as high as 85 °C. In addition, high relative amounts of lysophospholipids (e.g. lysophosphatidylcholines) exceeding those of the regular phospholipids were detected. Diglyceride mass fragments of various PLs have been assigned to fatty acyl side-chains of typical chain length (C14, C16, C18, C20) and degree of unsaturation (zero, one or two double bonds). Similar results were obtained for the phospholipid distribution in extracts of organic matter-rich Hydrate Ridge sediments. To date, the enhanced occurrence of lysophospholipids cannot be explained completely but a response to increasing thermal and ecological stress seems probable.

Description: Earth's largest reactive carbon pool, marine sedimentary organic matter, becomes increasingly recalcitrant during burial, making it almost inaccessible as a substrate for microorganisms, and thereby limiting metabolic activity in the deep biosphere. Because elevated temperature acting over geological time leads to the massive thermal breakdown of the organic matter into volatiles, including petroleum, the question arises whether microorganisms can directly utilize these maturation products as a substrate. While migrated thermogenic fluids are known to sustain microbial consortia in shallow sediments, an in situ coupling of abiotic generation and microbial utilization has not been demonstrated. Here we show, using a combination of basin modelling, kinetic modelling, geomicrobiology and biogeochemistry, that microorganisms inhabit the active generation zone in the Nankai Trough, offshore Japan. Three sites from ODP Leg 190 have been evaluated, namely 1173, 1174 and 1177, drilled in nearly undeformed Quaternary and Tertiary sedimentary sequences seaward of the Nankai Trough itself. Paleotemperatures were reconstructed based on subsidence profiles, compaction modelling, present-day heat flow, downhole temperature measurements and organic maturity parameters. Today's heat flow distribution can be considered mainly conductive, and is extremely high in places, reaching 180 mW/m**2. The kinetic parameters describing total hydrocarbon generation, determined by laboratory pyrolysis experiments, were utilized by the model in order to predict the timing of generation in time and space. The model predicts that the onset of present day generation lies between 300 and 500 m below sea floor (5100-5300 m below mean sea level), depending on well location. In the case of Site 1174, 5-10% conversion has taken place by a present day temperature of ca. 85 °C. Predictions were largely validated by on-site hydrocarbon gas measurements. Viable organisms in the same depth range have been proven using 14C-radiolabelled substrates for methanogenesis, bacterial cell counts and intact phospholipids. Altogether, these results point to an overlap of abiotic thermal degradation reactions going on in the same part of the sedimentary column as where a deep biosphere exists. The organic matter preserved in Nankai Trough sediments is of the type that generates putative feedstocks for microbial activity, namely oxygenated compounds and hydrocarbons. Furthermore, the rates of thermal degradation calculated from the kinetic model closely resemble rates of respiration and electron donor consumption independently measured in other deep biosphere environments. We deduce that abiotically driven degradation reactions have provided substrates for microbial activity in deep sediments at this convergent continental margin.