Hitoshi Tomaru

Exploring deep microbial life in coal-bearing sediment down to ~2.5 km below the ocean floor

A deep sleep in coal beds Deep below the ocean floor, microorganisms from forest soils continue to thrive. Inagaki et al. analyzed the microbial communities in several drill cores off the coast of Japan, some sampling more than 2 km below the seafloor (see the Perspective by Huber). Although cell counts decreased with depth, deep coal beds harbored active communities of methanogenic bacteria. These communities were more similar to those found in forest soils than in other deep marine sediments. Science, this issue p. 420; see also p. 376 Coal beds more than 2 kilometers below the seafloor host methanogenic bacteria related to those found in forest soils. [Also see Perspective by Huber] Microbial life inhabits deeply buried marine sediments, but the extent of this vast ecosystem remains poorly constrained. Here we provide evidence for the existence of microbial communities in ~40° to 60°C sediment associated with lignite coal beds at ~1.5 to 2.5 km below the seafloor in the Pacific Ocean off Japan. Microbial methanogenesis was indicated by the isotopic compositions of methane and carbon dioxide, biomarkers, cultivation data, and gas compositions. Concentrations of indigenous microbial cells below 1.5 km ranged from <10 to ~104 cells cm−3. Peak concentrations occurred in lignite layers, where communities differed markedly from shallower subseafloor communities and instead resembled organotrophic communities in forest soils. This suggests that terrigenous sediments retain indigenous community members tens of millions of years after burial in the seabed.

Cultivation of methanogenic community from subseafloor sediments using a continuous-flow bioreactor

Microbial methanogenesis in subseafloor sediments is a key process in the carbon cycle on the Earth. However, the cultivation-dependent evidences have been poorly demonstrated. Here we report the cultivation of a methanogenic microbial consortium from subseafloor sediments using a continuous-flow-type bioreactor with polyurethane sponges as microbial habitats, called down-flow hanging sponge (DHS) reactor. We anaerobically incubated methane-rich core sediments collected from off Shimokita Peninsula, Japan, for 826 days in the reactor at 10 °C. Synthetic seawater supplemented with glucose, yeast extract, acetate and propionate as potential energy sources was provided into the reactor. After 289 days of operation, microbiological methane production became evident. Fluorescence in situ hybridization analysis revealed the presence of metabolically active microbial cells with various morphologies in the reactor. DNA- and RNA-based phylogenetic analyses targeting 16S rRNA indicated the successful growth of phylogenetically diverse microbial components during cultivation in the reactor. Most of the phylotypes in the reactor, once it made methane, were more closely related to culture sequences than to the subsurface environmental sequence. Potentially methanogenic phylotypes related to the genera Methanobacterium, Methanococcoides and Methanosarcina were predominantly detected concomitantly with methane production, while uncultured archaeal phylotypes were also detected. Using the methanogenic community enrichment as subsequent inocula, traditional batch-type cultivations led to the successful isolation of several anaerobic microbes including those methanogens. Our results substantiate that the DHS bioreactor is a useful system for the enrichment of numerous fastidious microbes from subseafloor sediments and will enable the physiological and ecological characterization of pure cultures of previously uncultivated subseafloor microbial life.

Relationship of pore water freshening to accretionary processes in the Cascadia margin: Fluid sources and gas hydrate abundance

[1] Drilling in the Cascadia accretionary complex enable us to evaluate the contribution of dehydration reactions and gas hydrate dissociation to pore water freshening. The observed freshening with depth and distance from the prism toe is consistent with enhanced conversion of smectite to illite, driven by increase in temperature and age of accreted sediments. Although they contain gas hydrate -as evidenced by discrete low chloride spikes- the westernmost sites drilled on Hydrate Ridge show no freshening trend

Iodine dating of pore waters associated with gas hydrates in the Nankai area, Japan

The Nankai hydrate field, Japan, is an example of gas-hydrate deposits associated with an active subduction zone. In order to determine the origin of gas hydrates in this area, 129I/I ratios together with halogen concentrations were measured in a set of pore-water samples collected from two boreholes in the Nankai hydrate field. Iodine concentrations are between 100 and 230 μM, i.e., strongly enriched compared to seawater, while Cl concentrations were found to be close to that of seawater. Except for one sample, 129I/I ratios are between 180 and 520 × 10−15, giving minimum ages between 24 and 48 Ma. Because these ages are considerably older than present host sediments (<2 Ma) and subducting marine sediments (<21 Ma) in this area, iodine (and, by association, methane in the gas hydrates) must have been derived from source formations located in the continental side of the subduction zone. The results do not support derivation of gas hydrates from present host sediments or currently subducting sediments, but could be related to release and long-time recycling of fluids from marine formations of early Tertiary age.

Age variation of pore water iodine in the eastern Nankai Trough, Japan: Evidence for different methane sources in a large gas hydrate field

The 129 I geochronology of marine pore water is useful for the understanding of the origin of methane in gas hydrates because of the close association between I and marine organic materials responsible for methane generation. We report 129 I/I ratios in pore waters from three deep cores in the eastern Nankai Trough gas hydrate field, two located on the outer ridge and one in the forearc basin. As in previous studies of gas hydrate fields, I ages of pore water are consistently older than those of the host sediments. For the first time, however, the results demonstrate that the potential I source formations vary considerably across the forearc setting: While I at the basin site reaches ages close to 50 Ma, all I ages at the two ridge sites are

Effect of massive gas hydrate formation on the water isotopic fractionation of the gas hydrate system at Hydrate Ridge, Cascadia margin, offshore Oregon

Because gas hydrate is preferentially enriched in the heavy water isotopes, the δ18O and δD values of pore waters collected from gas hydrate–bearing sediment can provide information on the abundance and mechanisms of gas hydrate formation. Pore waters sampled from deep-seated (40 to 125 mbsf) gas hydrate deposits in Hydrate Ridge during ODP Leg 204 show depletion in dissolved Cl− and enrichments in 18O and D due to gas hydrate destabilization during core recovery. The oxygen and hydrogen isotopic fractionation factors (αO = 1.0025 and αH = 1.022) estimated from an extensive data set (n = 30 samples) correspond to experimentally determined values. In contrast, pore waters from shallow samples (<25 mbsf) at the ridge summit (n = 32) are highly enriched in dissolved Cl− and depleted in 18O and D, consistent with formation of massive gas hydrate deposits at rates faster than those at which these anomalies would be removed by advection or diffusion. The water isotopic fractionation factors in the brine are significantly lower than those experimentally determined, with αO of 1.0010 (average value of 1.0012) and αH of 1.008 (average value of 1.008). We discuss several factors that may be causing this anomalous fractionation and suggest that low gas occupancy in hydrate lattice (high hydration number) may be responsible for the observed small fractionation. If this were the case, the oxygen and hydrogen fractionation may serve as an indicator of hydration number during formation of gas hydrate in natural systems.

Variance and potential niche separation of microbial communities in subseafloor sediments off Shimokita Peninsula, Japan.

Subseafloor pelagic sediments with high concentrations of organic matter form habitats for diverse microorganisms. Here, we determined depth profiles of genes for SSU rRNA, mcrA, dsrA and amoA from just beneath the seafloor to 363.3 m below the seafloor (mbsf) using core samples obtained from the forearc basin off the Shimokita Peninsula. The molecular profiles were combined with data on lithostratigraphy, depositional age, sedimentation rate and pore-water chemistry. The SSU rRNA gene tag structure and diversity changed at around the sulfate-methane transition zone (SMTZ), whereas the profiles varied further with depth below the SMTZ, probably in connection with the variation in pore-water chemistry. The depth profiles of diversity and abundance of dsrA, a key gene for sulfate reduction, suggested the possible niche separations of sulfate-reducing populations, even below the SMTZ. The diversity and abundance patterns of mcrA, a key gene for methanogenesis/anaerobic methanotrophy, suggested a stratified distribution and separation of anaerobic methanotrophy and hydrogenotrophic or methylotrophic methanogensis below the SMTZ. This study provides novel insights into the relationships between the composition and function of microbial communities and the chemical environment in the nutrient-rich continental margin subseafloor sediments, which may result in niche separation and variability in subseafloor microbial populations.

Comparison of iodine dates from mud volcanoes and gas hydrate occurrences: Relevance for the movement of fluids and methane in active margins

We compare here results of iodine dating in fluids collected from mud volcanoes and gas hydrate occurrences associated with active margins. This is a compilation of previously reported data for 7 subduction zones around the Pacific Rim with slab ages ranging from 6 Ma to 130 Ma, where we determined iodine concentrations and 129I/I ratios in more than two hundred pore water samples. Iodine ages consistently are older than the host sediments and show an age distribution independent of the slab ages associated with the subduction zones. The results suggest that iodine in gas hydrates and mud volcanoes is predominantly derived from organic matter in the upper plates of subduction zones and that iodine derived from host sediments or subducting marine sediments has only a minor presence in these fluids. Because potential source sediments typically are found at lateral distances of 20 km or more, our results also suggest that fluid movement is possible over considerable distances in fractures present in the upper plate sediments. The association between iodine and methane suggests that our results can be extrapolated to the origin and transport of methane to mud volcanoes and gas hydrates. Throughout all studied sites around the Pacific margins, ages of source sediments for iodine were found to fall into the same range, which starts at the early Eocene (∼50 Ma) and has a broad peak around 30 Ma. Our results indicate that the occurrence of iodine and methane in gas hydrates and mud volcanoes is the result of transport in aqueous fluids and long-term remobilization of carbon in the upper plates of subduction zones.

Deep-biosphere methane production stimulated by geofluids in the Nankai accretionary complex

Scientific drilling at a submarine mud volcano shows that geofluid migration stimulates methanogenesis in the deep biosphere. Microbial life inhabiting subseafloor sediments plays an important role in Earth’s carbon cycle. However, the impact of geodynamic processes on the distributions and carbon-cycling activities of subseafloor life remains poorly constrained. We explore a submarine mud volcano of the Nankai accretionary complex by drilling down to 200 m below the summit. Stable isotopic compositions of water and carbon compounds, including clumped methane isotopologues, suggest that ~90% of methane is microbially produced at 16° to 30°C and 300 to 900 m below seafloor, corresponding to the basin bottom, where fluids in the accretionary prism are supplied via megasplay faults. Radiotracer experiments showed that relatively small microbial populations in deep mud volcano sediments (102 to 103 cells cm−3) include highly active hydrogenotrophic methanogens and acetogens. Our findings indicate that subduction-associated fluid migration has stimulated microbial activity in the mud reservoir and that mud volcanoes may contribute more substantially to the methane budget than previously estimated.