Kentaro Nakamura

Cell proliferation at 122°C and isotopically heavy CH4 production by a hyperthermophilic methanogen under high-pressure cultivation

We have developed a technique for cultivation of chemolithoautotrophs under high hydrostatic pressures that is successfully applicable to various types of deep-sea chemolithoautotrophs, including methanogens. It is based on a glass-syringe-sealing liquid medium and gas mixture used in conjunction with a butyl rubber piston and a metallic needle stuck into butyl rubber. By using this technique, growth, survival, and methane production of a newly isolated, hyperthermophilic methanogen Methanopyrus kandleri strain 116 are characterized under high temperatures and hydrostatic pressures. Elevated hydrostatic pressures extend the temperature maximum for possible cell proliferation from 116°C at 0.4 MPa to 122°C at 20 MPa, providing the potential for growth even at 122°C under an in situ high pressure. In addition, piezophilic growth significantly affected stable carbon isotope fractionation of methanogenesis from CO2. Under conventional growth conditions, the isotope fractionation of methanogenesis by M. kandleri strain 116 was similar to values (−34‰ to−27‰) previously reported for other hydrogenotrophic methanogens. However, under high hydrostatic pressures, the isotope fractionation effect became much smaller (<−12‰), and the kinetic isotope effect at 122°C and 40 MPa was −9.4‰, which is one of the smallest effects ever reported. This observation will shed light on the sources and production mechanisms of deep-sea methane.

Origin and global tectonic significance of Early Archean cherts from the Marble Bar greenstone belt, Pilbara Craton, Western Australia

Abstract Five sections of bedded chert in mafic-ultramafic rocks of the Archean Warrawoona Group in the Marble Bar greenstone belt, Pilbara Craton, were analyzed in order to understand their depositional environment and to provide some constraints on Early Archean tectonics. The sections are divisible into two types based on their field occurrence, mineralogy and geochemistry; thicker ones (A and B) that overlie Fe-rich, low-K tholeiites and thinner ones (C1, C2, and C3) overlying komatiitic basalts. The thickest, ferruginous section (A; 45xa0m thick) is the Marble Bar Chert of the Towers Formation, and is interpreted to have been an in situ precipitate derived from a high-T hydrothermal solution emanating from a mid-oceanic ridge (MOR). The following geochemical features are similar to those of modern hydrothermal iron-rich sediments at a MOR: (i) P, V, Zn, and Y are positively correlated with Fe, (ii) a positive Eu anomaly (normalized to chondrite) decreases from 6.6 to 1.3 and the magnitude of a negative Ce anomaly decreases from 0.6 to 1.0 as ∑REE and LREE/HREE increase. The 13xa0m thick B-section in the Apex Basalt, dominated by SiO2 and containing significant amounts of Ba (up to 4330xa0ppm), originated from a low-T MOR hydrothermal solution. This section is characterized by an association with massive black/gray silica veins that were hydrothermal feeders in normal fault zones in the spreading center. Geochemical evidence from the greenstones underlying the B-section indicates that they are of MORB origin. In contrast to A and B, C1-, C2-, and C3-sections are intercalated conformably with komatiitic basalts of the Apex Basalt and are 3–6xa0m thick. The geochemical signatures of these three sections suggest that they were most likely formed by low-T, weak hydrothermal activity that may have been associated with hot-spot volcanism. They show strong enrichment of Cr and Ni, reflecting a significant contribution of komatiitic basaltic detritus during sedimentation. The presence of volcaniclastic chert in their uppermost beds indicates a decrease of hydrothermal silica precipitation due to waning hydrothermal activity. Among the three sections, the uppermost (C3) exhibits greater enrichment in Zr, Nb, Hf, and Th, and higher Th/Sc and (La/Yb)N relative to the lower C1- and C2-sections. This could mean that the depositional environment of C3 was relatively closer to a continent or to island arcs composed of granitoids and/or their volcanic equivalents. Siliceous mudstones and sandstones of the uppermost clastic rocks (T-section; Panorama Formation) have geochemical signatures analogous to those of felsic plutonic/volcanic rocks. High Th/Sc, negative Eu anomalies, and high ∑REE in some siliceous mudstones from this unit imply that differentiated granitoids (or volcanic equivalents) were already exposed (erupted), at least locally, in the Pilbara at this time. This study shows these Warrawoona Group cherts were deposited in a variety of environments ranging from a mid-oceanic spreading center to a convergent plate boundary via a hot-spot. This variation was most likely due to horizontal plate motions which accordingly support the operation of plate tectonics in the Early Archean.

The effect of magmatic activity on hydrothermal venting along the superfast-spreading East Pacific Rise

A survey of hydrothermal activity along the superfast-spreading (approximately 150 millimeters per year) East Pacific Rise shows that hydrothermal plumes overlay approximately 60 percent of the ridge crest between 13�50 and 18�40S, a plume abundance nearly twice that known from any other rige portion of comparable length. Plumes were most abundant where the axial cross section is inflated and an axial magma chamber is present. Plumes with high ratios of volatile (3He, CH4, and H2S) to nonvolatile (Mn and Fe) species marked where hydrothermal circulation has been perturbed by recent magmatic activity. The high proportion of volatile-rich plumes observed implies that such episodes are more frequent here than on slower spreading ridges.

Archaeal diversity and community development in deep-sea hydrothermal vents

Over the past 35 years, researchers have explored deep-sea hydrothermal vent environments around the globe and studied a number of archaea, their unique metabolic and physiological properties, and their vast phylogenetic diversity. Although the pace of discovery of new archaeal taxa, phylotypes and phenotypes in deep-sea hydrothermal vents has slowed recently, bioinformatics and interdisciplinary geochemistry-microbiology approaches are providing new information on the diversity and community composition of archaea living in deep-sea vents. Recent investigations have revealed that archaea could have originated and dispersed from ancestral communities endemic to hydrothermal vents into other biomes on Earth, and the community structure and productivity of chemolithotrophic archaea are controlled primarily by variations in the geochemical composition of hydrothermal fluids.

Discovery of New Hydrothermal Activity and Chemosynthetic Fauna on the Central Indian Ridge at 18°–20°S

Indian Ocean hydrothermal vents are believed to represent a novel biogeographic province, and are host to many novel genera and families of animals, potentially indigenous to Indian Ocean hydrothermal systems. In particular, since its discovery in 2001, much attention has been paid to a so-called ‘scaly-foot’ gastropod because of its unique iron-sulfide-coated dermal sclerites and the chemosynthetic symbioses in its various tissues. Despite increasing interest in the faunal assemblages at Indian Ocean hydrothermal vents, only two hydrothermal vent fields have been investigated in the Indian Ocean. Here we report two newly discovered hydrothermal vent fields, the Dodo and Solitaire fields, which are located in the Central Indian Ridge (CIR) segments 16 and 15, respectively. Chemosynthetic faunal communities at the Dodo field are emaciated in size and composition. In contrast, at the Solitaire field, we observed faunal communities that potentially contained almost all genera found at CIR hydrothermal environments to date, and even identified previously unreported taxa. Moreover, a new morphotype of ‘scaly-foot’ gastropod has been found at the Solitaire field. The newly discovered ‘scaly-foot’ gastropod has similar morphological and anatomical features to the previously reported type that inhabits the Kairei field, and both types of ‘scaly-foot’ gastropods genetically belong to the same species according to analyses of their COI gene and nuclear SSU rRNA gene sequences. However, the new morphotype completely lacks an iron-sulfide coating on the sclerites, which had been believed to be a novel feature restricted to ‘scaly-foot’ gastropods. Our new findings at the two newly discovered hydrothermal vent sites provide important insights into the biodiversity and biogeography of vent-endemic ecosystems in the Indian Ocean.

Compositional, Physiological and Metabolic Variability in Microbial Communities Associated with Geochemically Diverse, Deep-Sea Hydrothermal Vent Fluids

Deep-sea hydrothermal vent environments represent one of the most physically and chemically diverse biomes in Earth. The chemical and thermal gradients (e.g., >350°C across distances as small as several centimeters in active chimneys) provide a wide range of niches for microbial communities living there (Huber and Holden 2008; Nakagawa and Takai 2008; Reysenbach et al. 2000; Takai et al. 2006a). Psychrophiles, mesophiles, thermophiles and hyperthermophiles (organisms growing best from 4°C to above 80°C) thrive by chemolithoautotrophy or heterotrophy, utilizing abundant available inorganic and organic chemical energy, carbon and other element sources. They reside as free-living forms in the rocky and sedimentary mixing interfaces between hot, highly reductive hydrothermal fluids (high temperatures of endmember hydrothermal fluids) and ambient seawaters beneath and at the seafloor (along subseafloor hydrothermal fluid paths, and in and on chimneys and sediments) and within lower temperatures of diffuse fluids mainly resulting from subseafloor mixing between endmember hydrothermal fluids and ambient seawaters, and as facultative or obligate symbionts on and within invertebrate hosts (Takai et al. 2006a).

Theoretical constraints of physical and chemical properties of hydrothermal fluids on variations in chemolithotrophic microbial communities in seafloor hydrothermal systems

In the past few decades, chemosynthetic ecosystems at deep-sea hydrothermal vents have received attention as plausible analogues to the early ecosystems of Earth, as well as to extraterrestrial ecosystems. These ecosystems are sustained by chemical energy obtained from inorganic redox substances (e.g., H2S, CO2, H2, CH4, and O2) in hydrothermal fluids and ambient seawater. The chemical and isotope compositions of the hydrothermal fluid are, in turn, controlled by subseafloor physical and chemical processes, including fluid–rock interactions, phase separation and partitioning of fluids, and precipitation of minerals. We hypothesized that specific physicochemical principles describe the linkages among the living ecosystems, hydrothermal fluids, and geological background in deep-sea hydrothermal systems. We estimated the metabolic energy potentially available for productivity by chemolithotrophic microorganisms at various hydrothermal vent fields. We used a geochemical model based on hydrothermal fluid chemistry data compiled from 89 globally distributed hydrothermal vent sites. The model estimates were compared to the observed variability in extant microbial communities in seafloor hydrothermal environments. Our calculations clearly show that representative chemolithotrophic metabolisms (e.g., thiotrophic, hydrogenotrophic, and methanotrophic) respond differently to geological and geochemical variations in the hydrothermal systems. Nearly all of the deep-sea hydrothermal systems provide abundant energy for organisms with aerobic thiotrophic metabolisms; observed variations in the H2S concentrations among the hydrothermal fluids had little effect on the energetics of thiotrophic metabolism. Thus, these organisms form the base of the chemosynthetic microbial community in global deep-sea hydrothermal environments. In contrast, variations in H2 concentrations in hydrothermal fluids significantly impact organisms with aerobic and anaerobic hydrogenotrophic metabolisms. Particularly in H2-rich ultramafic rock-hosted hydrothermal systems, anaerobic and aerobic hydrogenotrophy is more energetically significant than thiotrophy. The CH4 concentration also has a considerable impact on organisms with aerobic and anaerobic methanotrophic metabolisms, particularly in sediment-associated hydrothermal systems. Recently clarified patterns and functions of existing microbial communities and their metabolisms are generally consistent with the results of our thermodynamic modeling of the hydrothermal mixing zones. These relationships provide important directions for future research addressing the origin and early evolution of life on Earth as well as for the search for extraterrestrial life.

Iron-based microbial ecosystem on and below the seafloor: a case study of hydrothermal fields of the southern mariana trough.

Microbial community structures in deep-sea hydrothermal vents fields are constrained by available energy yields provided by inorganic redox reactions, which are in turn controlled by chemical composition of hydrothermal fluids. In the past two decades, geochemical and microbiological studies have been conducted in deep-sea hydrothermal vents at three geographically different areas of the Southern Mariana Trough (SMT). A variety of geochemical data of hydrothermal fluids and an unparalleled microbiological dataset of various samples (i.e., sulfide structures of active vents, iron-rich mats, borehole fluids, and ambient seawater) are available for comparative analyses. Here, we summarize the geochemical and microbiological characteristics in the SMT and assess the relationship between the microbial community structures and the fluid geochemistry in the SMT by thermodynamic modeling. In the high temperature vent fluids, aerobic sulfide-oxidation has the potential to yield large amounts of bioavailable energy in the vent fluids, which is consistent with the detection of species related to sulfide-oxidizing bacteria (such as Thiomicrospira in the Gammaproteobacteria and Sulfurimonas in the Epsilonproteobacteria). Conversely, the bioavailable energy yield from aerobic iron-oxidation reactions in the low-temperature fluids collected from man-made boreholes and several natural vents were comparable to or higher than those from sulfide-oxidation. This is also consistent with the detection of species related to iron-oxidizing bacteria (Mariprofundus in the Zetaproteobacteria) in such low-temperature samples. The results of combination of microbiological, geochemical, and thermodynamic analyses in the SMT provide novel insights into the presence and significance of iron-based microbial ecosystems in deep-sea hydrothermal fields.

Classification of two-dimensional split trianguline representations of p -adic fields

The aim of this paper is to classify two dimensional split trianguline representations of

High connectivity of animal populations in deep-sea hydrothermal vent fields in the Central Indian Ridge relevant to its geological setting.

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