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The hydrothermal vent squat lobster Shinkaia crosnieri Baba & Williams harbors an epibiotic bacterial community, which is numerically and functionally dominated by methanotrophs affiliated with Methylococcaceae and thioautotrophs affiliated with Sulfurovum and Thiotrichaceae. In the present study, shifts in the phylogenetic composition and metabolic function of the epibiont community were investigated using S. crosnieri individuals, which were reared for one year in a tank fed with methane as the energy and carbon source. The results obtained indicated that indigenous predominant thioautotrophic populations, such as Sulfurovum and Thiotrichaceae members, became absent, possibly due to the lack of an energy source, and epibiotic communities were dominated by indigenous Methylococcaceae and betaproteobacterial methylotrophic members that adapted to the conditions present during rearing for 12 months with a supply of methane. Furthermore, the overall phylogenetic composition of the epibiotic community markedly changed from a composition dominated by chemolithotrophs to one enriched with cross-feeding heterotrophs in addition to methanotrophs and methylotrophs. Thus, the composition and function of the S. crosnieri epibiotic bacterial community were strongly affected by the balance between the energy and carbon sources supplied for chemosynthetic production as well as that between the production and consumption of organic compounds.

Biogenic manganese oxides (BioMnOx) can be applied for the effective removal and recovery of trace metals from wastewater because of their high adsorption capacity. Although a freshwater continuous-flow system for a nitrifier-based Mn-oxidizing microbial community for producing BioMnOx has been developed so far, a seawater continuous-flow bioreactor system for BioMnOx production has not been established. Here, we report BioMnOx production by a methanotroph-based microbial community by using a continuous-flow bioreactor system. The bioreactor system was operated using a deep-sea sediment sample as the inoculum with methane as the energy source for over 2 years. The BioMnOx production became evident after 370 days of reactor operation. The maximum Mn oxidation rate was 11.4 mg L -1 day -1 . An X-ray diffraction analysis showed that the accumulated BioMnOx was birnessite. 16S rRNA gene-based clone analyses indicated that methanotrophic bacterial members were relatively abundant in the system; however, none of the known Mn-oxidizing bacteria were detected. A continuous-flow bioreactor system coupled with nitrification was also run in parallel for 636 days, but no BioMnOx production was observed in this bioreactor system. The comparative experiments indicated that the methanotroph-based microbial community, rather than the nitrifier-based community, was effective for BioMnOx production under the marine environmental conditions.

Anaerobic oxidation of methane (AOM) in marine sediments is an important global methane sink, but the physiological characteristics of AOM-associated microorganisms remain poorly understood. Here we report the cultivation of an AOM microbial community from deep-sea methane-seep sediment using a continuous-flow bioreactor with polyurethane sponges, called the down-flow hanging sponge (DHS) bioreactor. We anaerobically incubated deep-sea methane-seep sediment collected from the Nankai Trough, Japan, for 2,013 days in the bioreactor at 10°C. Following incubation, an active AOM activity was confirmed by a tracer experiment using 13C-labeled methane. Phylogenetic analyses demonstrated that phylogenetically diverse Archaea and Bacteria grew in the bioreactor. After 2,013 days of incubation, the predominant archaeal components were anaerobic methanotroph (ANME)-2a, Deep-Sea Archaeal Group, and Marine Benthic Group-D, and Gammaproteobacteria was the dominant bacterial lineage. Fluorescence in situ hybridization analysis showed that ANME-1 and -2a, and most ANME-2c cells occurred without close physical interaction with potential bacterial partners. Our data demonstrate that the DHS bioreactor system is a useful system for cultivating fastidious methane-seep-associated sedimentary microorganisms.

Two aerobic methane-oxidizing bacterial strains were isolated from distinct marine environments in Japan. Strains IT-4(T) and T2-1 were Gram-stain-negative, aerobic, motile, plump short rods or oval-shaped bacteria with a single polar flagellum and type I intracytoplasmic membranes. They were obligate methanotrophs that grew only on methane or methanol. Each strain possessed the particulate methane monooxygenase (pMMO). The ribulose monophosphate pathway was operative for carbon assimilation. The strains grew best at 37 °C, and did not grow at 45 °C. NaCl was required for growth within a concentration range of 1-8 % (w/v). The major phospholipid fatty acids were C16 : 0, C16 : 1ω7c, and C16 : 1ω5t. The major isoprenoid quinone was MQ-8. The DNA G+C content was 50.9-51.7 mol%. The 16S rRNA gene sequences of the strains showed 99.4 % similarity to each other, and DNA-DNA hybridization analysis indicated that the strains were representatives of the same species. The 16S rRNA gene sequences were highly similar to some marine environmental sequences (94.0-97.7 % similarity), but did not show similarities more than 94 % with sequences of members of other related genera, such as Methylomicrobium, Methylobacter, Methylomonas and Methylosarcina. Phylogenies based on 16S rRNA gene sequences and deduced partial PmoA sequences, and the physiological and chemotaxonomic characteristics revealed that strains IT-4(T) and T2-1 represent a novel species of a new genus in the family Methylococcaceae, for which the name Methylomarinum vadi gen. nov., sp. nov. is proposed. The type strain is IT-4(T) ( = JCM 13665(T) = DSM 18976(T)).

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.

A novel methane-oxidizing bacterium, strain HTM55(T), was isolated from subsurface hot aquifer water from a Japanese gold mine. Strain HTM55(T) was a Gram-negative, aerobic, motile, coccoid bacterium with a single polar flagellum and the distinctive intracytoplasmic membrane arrangement of a type I methanotroph. Strain HTM55(T) was a moderately thermophilic, obligate methanotroph that grew on methane and methanol at 37-65 °C (optimum 55-60 °C). The isolate grew at pH 5.2-7.5 (optimum 5.8-6.3) and with 0-1 % NaCl (optimum 0-0.3 %). The ribulose monophosphate pathway was operative for carbon assimilation. The DNA G+C content was 54.4 mol% and the major fatty acids were C(16 : 0) (52.0 %) and C(18 : 1)ω7c (34.8 %). Phylogenetic analysis of the 16S rRNA gene sequence indicated that strain HTM55(T) was closely related to Methylothermus thermalis MYHT(T) (99.2 % 16S rRNA gene sequence similarity), which is within the class Gammaproteobacteria. However, DNA-DNA relatedness between strain HTM55(T) and Methylothermus thermalis MYHT(T) was ≤ 39 %. On the basis of distinct phylogenetic, chemotaxonomic and physiological characteristics, strain HTM55(T) represents a novel species of the genus Methylothermus, for which the name Methylothermus subterraneus sp. nov. is proposed. The type strain is HTM55(T) ( = JCM 13664(T) = DSM 19750(T)).

We report complete genome sequence of a mesophilic hydrogenotrophic methanogen Methanocella paludicola, the first cultured representative of the order Methanocellales once recognized as an uncultured key archaeal group for methane emission in rice fields. The genome sequence of M. paludicola consists of a single circular chromosome of 2,957,635 bp containing 3004 protein-coding sequences (CDS). Genes for most of the functions known in the methanogenic archaea were identified, e.g. a full complement of hydrogenases and methanogenesis enzymes. The mixotrophic growth of M. paludicola was clarified by the genomic characterization and re-examined by the subsequent growth experiments. Comparative genome analysis with the previously reported genome sequence of RC-I(MRE50), which was metagenomically reconstructed, demonstrated that about 70% of M. paludicola CDSs were genetically related with RC-I(MRE50) CDSs. These CDSs included the genes involved in hydrogenotrophic methane production, incomplete TCA cycle, assimilatory sulfate reduction and so on. However, the genetic components for the carbon and nitrogen fixation and antioxidant system were different between the two Methanocellales genomes. The difference is likely associated with the physiological variability between M. paludicola and RC-I(MRE50), further suggesting the genomic and physiological diversity of the Methanocellales methanogens. Comparative genome analysis among the previously determined methanogen genomes points to the genome-wide relatedness of the Methanocellales methanogens to the orders Methanosarcinales and Methanomicrobiales methanogens in terms of the genetic repertoire. Meanwhile, the unique evolutionary history of the Methanocellales methanogens is also traced in an aspect by the comparative genome analysis among the methanogens.

Uncultured archaeal anaerobic methanotrophs (ANMEs) are known to operate the anaerobic oxidation of methane process, an important sink for the greenhouse gas methane in natural environments. In this study, we designed 16S rRNA gene-specific primers for each of the phylogenetic groups of ANMEs (ANME-1, Guaymas Basin hydrothermal sediment clones group within the ANME-1, ANME-2a, ANME-2b, ANME-2c and ANME-3) based on previously reported sequences. The newly designed primers were used for the detection of the various groups of ANMEs in the sulphate-limited anaerobic environmental samples, i.e. methanogenic sludges, rice field soils, lotus field sediments and natural gas fields. The ANME 16S rRNA gene sequences were detected only in a natural gas field sample among the environments examined in this study and were of the ANME-1 and -2c groups. In addition, the quantitative real-time PCR analysis using the designed primers showed that abundances of ANME-1 and -2c were estimated to be <0.02% of the total prokaryotic 16S rRNA gene community. The newly designed ANME group-specific primers in this study may be useful to survey the distribution and quantitative determination of ANMEs.

A novel methane-producing archaeon, designated strain HASU(T), was isolated from a lotus field. Cells were Gram-negative, non-motile, irregular cocci, 2-3 mum in diameter, and occurred singly. Growth was observed at 15-40 degrees C (optimum, 37 degrees C) and pH 6.5-7.5 (optimum, pH 7.0). The G+C content of the genomic DNA was 60.9 mol%. Strain HASU(T) utilized ethanol, 1-propanol, 1-butanol, hydrogen and formate for growth and methane production. It converted ethanol to methane and acetate. Based on comparative 16S rRNA gene sequence analysis, strain HASU(T) was shown to be affiliated with the genus Methanofollis. It was related most closely to the type strain of Methanofollis liminatans (96.1 % 16S rRNA gene sequence similarity). Based on phylogenetic analysis and phenotypic characteristics, strain HASU(T) is considered to represent a novel species of the genus Methanofollis, for which the name Methanofollis ethanolicus sp. nov. is proposed. The type strain is HASU(T) (=NBRC 104120(T)=JCM 15103(T)=DSM 21041(T)).

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 degrees C at 0.4 MPa to 122 degrees C at 20 MPa, providing the potential for growth even at 122 degrees C under an in situ high pressure. In addition, piezophilic growth significantly affected stable carbon isotope fractionation of methanogenesis from CO(2). Under conventional growth conditions, the isotope fractionation of methanogenesis by M. kandleri strain 116 was similar to values (-34 per thousand to -27 per thousand) previously reported for other hydrogenotrophic methanogens. However, under high hydrostatic pressures, the isotope fractionation effect became much smaller (< -12 per thousand), and the kinetic isotope effect at 122 degrees C and 40 MPa was -9.4 per thousand, which is one of the smallest effects ever reported. This observation will shed light on the sources and production mechanisms of deep-sea methane.

"A meta-enzyme approach" is proposed as an ecological enzymatic method to explore the potential functions of microbial communities in extreme environments such as the deep marine subsurface. We evaluated a variety of extra-cellular enzyme activities of sediment slurries and isolates from a deep subseafloor sediment core. Using the new deep-sea drilling vessel "Chikyu", we obtained 365 m of core sediments that contained approximately 2% organic matter and considerable amounts of methane from offshore the Shimokita Peninsula in Japan at a water depth of 1,180 m. In the extra-sediment fraction of the slurry samples, phosphatase, esterase, and catalase activities were detected consistently throughout the core sediments down to the deepest slurry sample from 342.5 m below seafloor (mbsf). Detectable enzyme activities predicted the existence of a sizable population of viable aerobic microorganisms even in deep subseafloor habitats. The subsequent quantitative cultivation using solid media represented remarkably high numbers of aerobic, heterotrophic microbial populations (e.g., maximally 4.4x10(7) cells cm(-3) at 342.5 mbsf). Analysis of 16S rRNA gene sequences revealed that the predominant cultivated microbial components were affiliated with the genera Bacillus, Shewanella, Pseudoalteromonas, Halomonas, Pseudomonas, Paracoccus, Rhodococcus, Microbacterium, and Flexibacteracea. Many of the predominant and scarce isolates produced a variety of extra-cellular enzymes such as proteases, amylases, lipases, chitinases, phosphatases, and deoxyribonucleases. Our results indicate that microbes in the deep subseafloor environment off Shimokita are metabolically active and that the cultivable populations may have a great potential in biotechnology.

A quantitative fluorogenic PCR method for detecting methanogenic and methanotrophic orders was established using a refined primer set for the methyl coenzyme M reductase subunit A gene (mcrA). The method developed was applied to several microbial communities in which diversity and abundance of methanogens or anaerobic methanotrophs (ANMEs) was identified by 16S rRNA gene clone analysis, and strong correlations between the copy numbers of mcrA with those of archaeal 16S rRNA genes in the communities were observed. The assay can be applied to detecting and assessing the abundance of methanogens and/or ANMEs in anoxic environments that could not be detected by 16S rRNA gene sequence analyses.

A quantitative fluorogenic PCR method for group-specific methyl coenzyme M reductase subunit A genes (mcrA) from methanotrophic archaea was established and applied to the characterization of microbial communities in anoxic methane seep sediments at the accretionary prism of the Nankai Trough. All of the previously identified subgroups of anaerobic methanotroph (ANME) mcrA genes were detected in the cores up to 25 cm below the seafloor, but distributional patterns of mcrA genes were found to differ according to depth. These findings suggest a distinct distribution of phylogenetically and physiologically diverse methanotrophic archaea that mediate methane oxidation in the anoxic sediments. This quantification method will contribute to future investigations of methanotrophic microbial ecosystems in anoxic marine sediments.

The deep subseafloor biosphere is among the least-understood habitats on Earth, even though the huge microbial biomass therein plays an important role for potential long-term controls on global biogeochemical cycles. We report here the vertical and geographical distribution of microbes and their phylogenetic diversities in deeply buried marine sediments of the Pacific Ocean Margins. During the Ocean Drilling Program Legs 201 and 204, we obtained sediment cores from the Peru and Cascadia Margins that varied with respect to the presence of dissolved methane and methane hydrate. To examine differences in prokaryotic distribution patterns in sediments with or without methane hydrates, we studied >2,800 clones possessing partial sequences (400-500 bp) of the 16S rRNA gene and 348 representative clone sequences (approximately 1 kbp) from the two geographically separated subseafloor environments. Archaea of the uncultivated Deep-Sea Archaeal Group were consistently the dominant phylotype in sediments associated with methane hydrate. Sediment cores lacking methane hydrates displayed few or no Deep-Sea Archaeal Group phylotypes. Bacterial communities in the methane hydrate-bearing sediments were dominated by members of the JS1 group, Planctomycetes, and Chloroflexi. Results from cluster and principal component analyses, which include previously reported data from the West and East Pacific Margins, suggest that, for these locations in the Pacific Ocean, prokaryotic communities from methane hydrate-bearing sediment cores are distinct from those in hydrate-free cores. The recognition of which microbial groups prevail under distinctive subseafloor environments is a significant step toward determining the role these communities play in Earth's essential biogeochemical processes.

Samples from three submerged sites (MC, a core obtained in the methane seep area; MR, a reference core obtained at a distance from the methane seep; and HC, a gas-bubbling carbonate sample) at the Kuroshima Knoll in the southern Ryuku arc were analyzed to gain insight into the organisms present and the processes involved in this oxic-anoxic methane seep environment. 16S rRNA gene analyses by quantitative real-time PCR and clone library sequencing revealed that the MC core sediments contained abundant archaea (approximately 34% of the total prokaryotes), including both mesophilic methanogens related to the genus Methanolobus and ANME-2 members of the Methanosarcinales, as well as members of the delta-Proteobacteria, suggesting that both anaerobic methane oxidation and methanogenesis occurred at this site. In addition, several functional genes connected with methane metabolism were analyzed by quantitative competitive-PCR, including the genes encoding particulate methane monooxygenase (pmoA), soluble methane monooxygenase (mmoX), methanol dehydrogenese (mxaF), and methyl coenzyme M reductase (mcrA). In the MC core sediments, the most abundant gene was mcrA (2.5 x 10(6) copies/g [wet weight]), while the pmoA gene of the type I methanotrophs (5.9 x 10(6) copies/g [wet weight]) was most abundant at the surface of the MC core. These results indicate that there is a very complex environment in which methane production, anaerobic methane oxidation, and aerobic methane oxidation all occur in close proximity. The HC carbonate site was rich in gamma-Proteobacteria and had a high copy number of mxaF (7.1 x 10(6) copies/g [wet weight]) and a much lower copy number of the pmoA gene (3.2 x 10(2) copies/g [wet weight]). The mmoX gene was never detected. In contrast, the reference core contained familiar sequences of marine sedimentary archaeal and bacterial groups but not groups specific to C1 metabolism. Geochemical characterization of the amounts and isotopic composition of pore water methane and sulfate strongly supported the notion that in this zone both aerobic methane oxidation and anaerobic methane oxidation, as well as methanogenesis, occur.

A novel extremely thermophilic, methane-producing archaeon was isolated from a black smoker chimney at the Kairei field in the Central Indian Ridge. Cells of this isolate were irregular cocci with several flagella; motility was not observed. Growth was observed between 55 and 83 degrees C (optimum of 75 degrees C; 30 min doubling time) and between pH 6.0 and 8.5 (optimum of pH 6.7). The isolate was a strictly anaerobic, methanogenic autotroph capable of using hydrogen and carbon dioxide as sole energy and carbon sources. Formate was utilized as an alternative energy source. The G+C content of the genomic DNA was 33.3 mol%. Phylogenetic analysis based on 16S rRNA gene sequences indicated that the isolate was most closely related to Methanotorris igneus strain Kol 5T. The isolate, however, could be genetically differentiated from this species by DNA-DNA hybridization analysis and on the basis of its physiological properties. The name Methanotorris formicicus sp. nov. is proposed for this isolate; the type strain is Mc-S-70T (=JCM 11930T=ATCC BAA-687T).

A novel thermophilic, methane-producing archaeon was isolated from a deep-sea hydrothermal vent chimney at the Iheya Ridge, in the Okinawa Trough, Japan. The cells were highly motile, irregular cocci, with a polar bundle of flagella. Growth was observed between 40 and 75 degrees C (optimum 60-65 degrees C; 30 min doubling time) and between pH 4.5 and 8.5 (optimum pH 6.7). The isolate was a strictly anaerobic autotroph capable of using hydrogen and carbon dioxide as sole sources of energy and carbon. Formate can serve as an alternative energy source. The G+C content of the genomic DNA was 33.5 mol%. Phylogenetic analysis based on 16S rDNA sequences and DNA-DNA hybridization analysis indicated that the isolate was closely related to members of the genera Methanococcus and Methanothermococcus. This isolate, however, could be differentiated from the previously described species of these genera on the basis of its physiological and molecular properties. The name Methanothermococcus okinawensis sp. nov is proposed, with the type strain IH1T (=JCM 11175T=DSM 14208T).

Shinkaia crosnieri is a galatheid crab that predominantly dwells in deep-sea hydrothermal systems in the Okinawa Trough, Japan. In this study, the phylogenetic diversity of active methanotrophs in the epibiotic microbial community on the setae of S. crosnieri was characterized by reverse transcription-polymerase chain reaction (RT-PCR) of a functional gene (pmoA) encoding a subunit of particulate methane monooxygenase. Phylogenetic analysis of pmoA transcript sequences revealed that the active epibiotic methanotrophs on S. crosnieri setae consisted of gammaproteobacterial type Ia and Ib methanotrophs. The effect of different RNA stabilization procedures on the abundance of pmoA and 16S rRNA transcripts in the epibiotic community was estimated by quantitative RT-PCR. Our novel RNA fixation method performed immediately after sampling effectively preserved cellular RNA assemblages, particularly labile mRNA populations, including pmoA mRNA. Methane consumption in live S. crosnieri was also estimated by continuous-flow incubation under atmospheric and in situ hydrostatic pressures, and provided a clear evidence of methane oxidation activity of the epibiotic microbial community, which was not significantly affected by hydrostatic pressure. Our study revealed the significant ecological function and nutritional contribution of epibiotic methanotrophs to the predominant S. crosnieri populations in the Okinawa Trough deep-sea hydrothermal systems. In conclusion, our study gave clear facts about diversity and methane oxidation of active methanotrophs in the epibiotic community associated with invertebrates. [ABSTRACT FROM AUTHOR]

South Chamorro Seamount (SCS) is a blueschist-bearing serpentinite mud volcano in the Mariana forearc. Previous scientific drilling conducted at SCS revealed highly alkaline, sulfate-rich formation fluids resulting from slab-derived fluid upwelling combined with serpentinization both beneath and within the seamount. In the present study, a time-series of ROV dives spanning 1000 days was conducted to collect discharging alkaline fluids from the cased Ocean Drilling Program (ODP) Hole 1200C (hereafter the CORK fluid). The CORK fluids were analyzed for chemical compositions (including dissolved gas) and microbial community composition/function. Compared to the ODP porewater, the CORK fluids were generally identical in concentration of major ions, with the exception of significant sulfate depletion and enrichment in sulfide, alkalinity, and methane. Microbiological analyses of the CORK fluids revealed little biomass and functional activity, despite habitable temperature conditions. The post-drilling sulfate depletion is likely attributable to sulfate reduction coupled with oxidation of methane (and hydrogen), probably triggered by the drilling and casing operations. Multiple lines of evidence suggest that abiotic organic synthesis associated with serpentinization is the most plausible source of the abundant methane in the CORK fluid. The SCS formation fluid regime presented here may represent the first example on Earth where abiotic syntheses are conspicuous with little biotic processes, despite a condition with sufficient bioavailable energy potentials and temperatures within the habitable range. [ABSTRACT FROM AUTHOR]

Abstract: We investigated the chemical composition of hydrothermal fluids and deduced the origin of hydrothermal fluid methane in a sediment-associated Minami-Ensei (ME) hydrothermal field, the Okinawa Trough. The endmember hydrothermal fluid of the ME field contained isotopically 13C-enriched methane (−26.0 to −24.7‰), which suggested a thermogenic origin. However, the observed methane-dominating hydrocarbon compositions (C1/C2+ >103) suggested that biogenic methane, not thermogenic hydrocarbons, is predominant. The conflict between these geochemical indications can be resolved by the hypothesis that the thermal reaction of sedimentary organic matter initially generates the primary source of hydrocarbons and the secondary thermal decomposition of hydrocarbons increases the C1/C2+ ratio as high as that of biogenic methane. The low molecular hydrogen concentration (0.04mmol/kg) in the ME fluid suggests that the thermal fluid–sediment interaction occurred in the recharge stage, not the discharge stage, of hydrothermal circulation. Less biogenic methane in the ME fluid implies a laterally shrunken hydrothermal fluid circulation that reduced biogenic methane incorporation at the recharge stage. A comparison of hydrothermal fluid chemistry and geological/geographical settings among the global sediment-associated hydrothermal systems also suggests representative patterns in the geochemical origins of hydrothermal fluid methane and their relevance to hydrothermal fluid circulation as a whole. [Copyright &y& Elsevier]

We here report on specific chemical and microbial compositions observed at the Archaean hydrothermal site in the Southern Mariana backarc spreading center, which produces two remarkably different hydrothermal fluids. One was black smoker hydrothermal fluid at 340 °C containing a low concentration of methane with a relatively high carbon isotope ratio (δ 13 C of methane relative to VPDB), −7.8‰, indicating that methane originated from a magmatic source; thus, this is a fairly ordinary hydrothermal fluid for basalt-hosted hydrothermal activity. In contrast, the other fluid was clear smoker hydrothermal fluid at 117 °C containing a high concentration of methane with a very low carbon isotope ratio (−49.7‰). The host rock of the Archaean hydrothermal site is basalt, and therefore Fischer-Tropsch-type reactions resulting from serpentinization of mantle rocks are not feasible as a source for the “light” carbon in methane. Additionally, the carbon isotope ratio of carbon dioxide demonstrated that the sources of CO 2 and CH 4 were not organic materials. The remaining possibility, biogenic methane, was confirmed by cultivation of hyperthermophilic hydrogen-oxidizing methanogens from both rocks covering the vent of clear smoker hydrothermal fluids and in situ cultivation systems. Although the dominance and abundance of methanogens were very low in this ecosystem, the number was consistent with the relationship between hydrogen concentration and methanogen abundance (Takai et al., 2015). These results suggest that phase separation led to concentration of hydrogen and subsequent persistence of a hyperthermophilic subsurface lithoautotrophic microbial ecosystem in both serpentine-hosted and basalt-hosted hydrothermal systems. This type of ecosystem may occur in similar settings elsewhere on modern earth; in addition, similar communities may have existed in other types of deep-sea hydrothermal systems in the geological past. [ABSTRACT FROM AUTHOR]

Abstract: The vent fluids of the Kairei Hydrothermal Field (KHF) on the Central Indian Ridge near the Rodriguez Triple Junction are atypical. They have a very high H2 concentration, a relatively high Si concentration, and a remarkably low CH4/H2 ratio. Recently, particular attention has been paid to the KHF, because the hydrothermal fluids are suggested to support a hydrogen-based hyperthermophilic subsurface lithoautotrophic microbial ecosystem (HyperSLiME), which is considered to be a likely modern analogue for the early Earth ecosystems prior to photosynthesis. Despite the increasing interest in the fluid chemistry and associated biota, the origin of the unusual chemistry of the hydrothermal fluids is still uncertain. Here we suggest interaction of seawater with troctolite recently discovered from small hills near the KHF, provide a possible explanation for the composition of the KHF fluids. Dives with the manned submersible Shinkai 6500 recovered plagioclase dunite, troctolites, and olivine gabbros, which generally constitute the lower oceanic crust. Microscopic observations revealed that olivine crystals in the samples were partly to completely replaced by serpentine and magnetite, indicating the generation of H2 by serpentinization reactions between olivine and hydrothermal fluids. Theoretical model calculations predict that the high H2 and high Si concentrations of the hydrothermal fluids can be attributed to serpentinization of the troctolites and subsequent hydrothermal reactions with basaltic wall rocks under the KHF. The unique geological setting of the Kairei hydrothermal system, where deep crustal rocks are emplaced in the shallower part of oceanic crust, is considered to be responsible for the unusual chemistry of the Kairei hydrothermal fluids driving the occurrence of the HyperSLiME. [Copyright &y& Elsevier]