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      Expedition 337 summary

      Expedition 337 Scientists

      Proceedings of the IODP

      Integrated Ocean Drilling Program

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          Abstract

          Integrated Ocean Drilling Program (IODP) Expedition 337 was the first expedition dedicated to subseafloor microbiology that used riser drilling technology. The site examined during this expedition (Site C0020) is located off the Shimokita Peninsula, Japan, at a water depth of 1180 m in a forearc basin formed by the subduction of the Pacific plate under the Okhotsk plate. Previously conducted seismic profiles suggested the presence of deep, coal-bearing horizons at ~2 km below the seafloor. Our primary scientific objectives during Expedition 337 were thus to study the relationship between these deep coalbeds and carbon cycling, as well as to explore the distribution of subseafloor life at the greatest depths that have ever been sampled by scientific ocean drilling. A key question that guided our research strategy was “Do deeply buried hydrocarbon reservoirs, such as coalbeds, act as geobiological reactors that sustain subseafloor life by releasing nutrients and carbon substrates?” To address this question and other objectives, we drilled through a 2466 m deep sedimentary sequence with a series of coal layers at ~2 km below the seafloor. Hole C0020A is thus the deepest borehole in the history of scientific ocean drilling, surpassing the previous maximum penetration depth by 355 m and providing the opportunity to extend the maximum depth of subseafloor life detection by >800 m. Site C0020 also provides the first geological record of a dynamically changing depositional environment in the former forearc basin off the Shimokita Peninsula during the late Oligocene and Miocene. This record comprises a rich diversity of lithologic facies reflecting environments ranging from warm-temperate coastal backswamps to cool-water continental shelf. The use of riser drilling technology in very deep sediment created both unique opportunities and new challenges in the study of subseafloor life. The continual use of drilling mud during riser drilling operations required implementation of a rigorous program dedicated to sample quality assurance and quality control. We successfully added chemical tracers to drilling mud to monitor levels of drilling mud contamination of samples and quantified levels of mud-derived solutes in interstitial fluid. Therefore, our data provide a framework for differentiating signals of indigenous microbes from those of contaminants. For the first time ever in scientific ocean drilling, we conducted downhole in situ fluid analysis and sampling as part of the logging operations. Logging operations yielded data of unprecedented quality that provide a comprehensive view of sediment properties at Site C0020. With an estimated temperature gradient of 24.0°C/km, temperatures in coal-bearing horizons are near 50°C and thus well within the known temperature range of life. We conducted gas analyses using a newly installed mud-gas monitoring laboratory. Gas chemistry and isotopic compositions provide the first indication of biological activity in deep horizons associated with the coalbed. Last but not least, this expedition provided a testing ground for the use of riser drilling technology to address geobiological and biogeochemical objectives and was therefore a crucial step toward the next phase of deep scientific ocean drilling. Potential benefits of deep riser drilling for the scientific community are enormous, but its implementation will require the adaptation and further fine tuning of riser drilling technology to the needs of basic research.

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          Most cited references 64

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          Global distribution of microbial abundance and biomass in subseafloor sediment.

          The global geographic distribution of subseafloor sedimentary microbes and the cause(s) of that distribution are largely unexplored. Here, we show that total microbial cell abundance in subseafloor sediment varies between sites by ca. five orders of magnitude. This variation is strongly correlated with mean sedimentation rate and distance from land. Based on these correlations, we estimate global subseafloor sedimentary microbial abundance to be 2.9⋅10(29) cells [corresponding to 4.1 petagram (Pg) C and ∼0.6% of Earth's total living biomass]. This estimate of subseafloor sedimentary microbial abundance is roughly equal to previous estimates of total microbial abundance in seawater and total microbial abundance in soil. It is much lower than previous estimates of subseafloor sedimentary microbial abundance. In consequence, we estimate Earth's total number of microbes and total living biomass to be, respectively, 50-78% and 10-45% lower than previous estimates.
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            Distributions of microbial activities in deep subseafloor sediments.

             S D'Hondt (2004)
            Diverse microbial communities and numerous energy-yielding activities occur in deeply buried sediments of the eastern Pacific Ocean. Distributions of metabolic activities often deviate from the standard model. Rates of activities, cell concentrations, and populations of cultured bacteria vary consistently from one subseafloor environment to another. Net rates of major activities principally rely on electron acceptors and electron donors from the photosynthetic surface world. At open-ocean sites, nitrate and oxygen are supplied to the deepest sedimentary communities through the underlying basaltic aquifer. In turn, these sedimentary communities may supply dissolved electron donors and nutrients to the underlying crustal biosphere.
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              Heterotrophic Archaea dominate sedimentary subsurface ecosystems off Peru.

              Studies of deeply buried, sedimentary microbial communities and associated biogeochemical processes during Ocean Drilling Program Leg 201 showed elevated prokaryotic cell numbers in sediment layers where methane is consumed anaerobically at the expense of sulfate. Here, we show that extractable archaeal rRNA, selecting only for active community members in these ecosystems, is dominated by sequences of uncultivated Archaea affiliated with the Marine Benthic Group B and the Miscellaneous Crenarchaeotal Group, whereas known methanotrophic Archaea are not detectable. Carbon flow reconstructions based on stable isotopic compositions of whole archaeal cells, intact archaeal membrane lipids, and other sedimentary carbon pools indicate that these Archaea assimilate sedimentary organic compounds other than methane even though methanotrophy accounts for a major fraction of carbon cycled in these ecosystems. Oxidation of methane by members of Marine Benthic Group B and the Miscellaneous Crenarchaeotal Group without assimilation of methane-carbon provides a plausible explanation. Maintenance energies of these subsurface communities appear to be orders of magnitude lower than minimum values known from laboratory observations, and ecosystem-level carbon budgets suggest that community turnover times are on the order of 100-2,000 years. Our study provides clues about the metabolic functionality of two cosmopolitan groups of uncultured Archaea.
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                Author and article information

                Journal
                10.2204/iodp.proc.337.2013
                Proceedings of the IODP
                Integrated Ocean Drilling Program
                1930-1014
                30 September 2013
                Article
                10.2204/iodp.proc.337.101.2013
                d92289f1-4644-4758-8acd-c686f3d8f527

                This work is licensed under a Creative Commons Attribution 4.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

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                Self URI (journal page): http://publications.iodp.org/

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