Cold Seep Environments Research Articles

Sediments hosting gas hydrate: oases for metazoan meiofauna

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 337:27-37 (2007) - doi:10.3354/meps337027 Sediments hosting gas hydrate: oases for metazoan meiofauna Stefan Sommer*, Erik Gutzmann, Olaf Pfannkuche Leibniz-Institut für Meereswissenschaften, IFM-GEOMAR, Wischhofstraße 1–3, 24148 Kiel, Germany *Email: ssommer@ifm-geomar.de ABSTRACT: The effect of methane seepage from sediments harbouring shallow gas hydrates on standing stocks and the distribution pattern of meiobenthic organisms, in particular Nematoda and Rotifera, was studied at about 800 m water depth at Hydrate Ridge, Cascadia subduction zone, off Oregon. The presence of shallow gas hydrates, buried only a few 10s of centimetres below the sediment surface, was indicated by extensive bacterial mats of chemosynthetic Beggiatoa sp. and clam fields of the bivalve mollusk Calyptogena spp. Mean abundances of meiobenthic organisms integrated over the upper 10 cm of the sediment were highest (1294 ind. 10 cm–2) at clam fields, closely followed by control sediments least affected by gas hydrates (1199 ind. 10 cm–2) and lowest in sediments covered with bacterial mats (762 ind. 10 cm–2). Average meiobenthic biomass was highest at the clam field site (262.2 µg C 10 cm–2), 210.4 µg C 10 cm–2 at the control site and very low in sediments covered with bacterial mats (61.4 µg C 10 cm–2). The dominant taxa of meiobenthic organisms at the investigated sites were nematodes and, unexpectedly, Rotifera that are almost unknown from the deep marine habitat. In terms of abundance, rotifera dominated the meiobenthic community in gas-hydrate-influenced sediments, while control sediments and deeper basins adjoined to Hydrate Ridge were dominated by nematodes. Nematodes were concentrated in the sediment surface at all sites, whereas rotifers were almost evenly distributed at all depths, with a slight preference for deeper sediment horizons. The horizontal as well as vertical distribution of nematodes and rotifers is likely to be determined by competition or predation, and by the high adaptive capability of rotifers to highly sulphidic and anoxic conditions. Estimates of meiobenthic carbon turnover in relation to the bulk organic carbon supply indicate that, in contrast to other meiobenthic communities in cold seep environments, the meiobenthos in the studied gas-hydrate-containing sediments do not benefit from the excess availability of organic carbon via the chemoautotrophic food web. This may be because, for most meiobenthic organisms (other than rotifers), tolerance mechanisms are overwhelmed by the deleterious environmental conditions of reduced oxygen availability and extremely high sulphide fluxes. KEY WORDS: Meiofauna · Nematoda · Rotifera · Gas hydrate · Carbon turnover · Hydrate Ridge · Cascadia subduction zone Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 337. Online publication date: May 14, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Sr/Ca and Mg/Ca ratios in Niger Delta sediments: Implications for authigenic carbonate genesis in cold seep environments

We report on a reconnaissance analysis of the geochemical composition of authigenic carbonates and sediment samples collected from various seepage sites on the Niger deep-sea fan. Our aim has been to investigate whether evidence for the presence of authigenic carbonates and gas hydrates within sediments is discernible from solid-phase sediment geochemistry. We show that sedimentary Sr/Ca and Mg/Ca ratios can be used to infer the presence of authigenic aragonite (Sr-rich) and Mg-rich carbonate phases (high-Mg calcite, dolomite) in cold seep settings. Using Sr/Ca and Mg/Ca ratios, the proportion (wt.%) of authigenic carbonates in Niger Fan sediments can be calculated from a mixing model between sediment fractions of terrigenous material, biogenic calcite, aragonite and high-Mg calcite. This approach was applied to high-resolution geochemical profiles along sediment cores recovered from various cold seep settings (mud volcano, diapirs, pockmarks, gas-hydrate bearing sediments). Our data reveal that authigenic carbonates occur as discrete phases in sediments from gas-hydrate-bearing areas, suggesting that such carbonate-rich sediment layers may represent paleo-indicators for ancient methane seepage in marine sediments, possibly associated to gas-hydrate reservoirs.

Discovery of "hydrothermal" chemosynthetic community in a cold seep environment, Formosa Ridge: preliminary results from NT07-05 Cruise, off southwestern Taiwan

Discovery of "hydrothermal" chemosynthetic community in a cold seep environment, Formosa Ridge: preliminary results from NT07-05 Cruise, off southwestern Taiwan

δ 44/42 Ca in gas hydrates, porewaters and authigenic carbonates from Niger Delta sediments

We report Ca isotope ratios for gas hydrates, pore waters and authigenic carbonate concretions recovered by coring at cold seep locations from the Niger Delta deep province. Our aim in this study has been to understand the mechanisms controlling Ca-isotope fractionation during biogeochemical processes at cold seeps. Carbonate precipitation in cold seep environments is closely related to the anaerobic oxidation of methane (AOM), which is mediated by a consortium of micro-organisms and releases bicarbonate (HCO3) and sulfide (HS) into surrounding pore waters [1]. We found that carbonate precipitation causes Ca enrichment in the surrounding porewaters, as expected from previous work demonstrating that calcite prefers isotopically light Ca [2]. Our measured δCa values for pore-waters are consistent with values estimated previously by Teichert et al. [3]. In our studied sediment cores, the highest δCa ratios in pore-waters were measured at the AOM sediment depth (dashed line in Fig.1), indicating on-going carbonate formation at these horizons. Porewater and carbonate δCa measurements may therefore offere potential to identify the location and intensity of AOM processes now and in the past. We will also discuss the mineralogical control of Ca isotope fractionation at cold seeps.

Microbial diversity of cold-seep sediments in Sagami Bay, Japan, as determined by 16S rRNA gene and lipid analyses

Microbial communities in Calyptogena sediment and microbial mats of Sagami Bay, Japan, were characterized using 16S rRNA gene sequencing and lipid biomarker analysis. Characterization of 16S rRNA gene isolated from these samples suggested a predominance of bacterial phylotypes related to Gammaproteobacteria (57-64%) and Deltaproteobacteria (27-29%). The Epsilonproteobacteria commonly found in cold seeps and hydrothermal vents were only detected in the microbial mat sample. Significantly different archaeal phylotypes were found in Calyptogena sediment and microbial mats; the former contained only Crenarchaeota clones (100% of the total archaeal clones) and the latter exclusively Euryarchaeota clones, including the anaerobic oxidation of methane archaeal groups ANME-2a and ANME-2c. Many of these lineages are as yet uncultured and undescribed groups of bacteria and archaea. Phospholipid fatty acid analysis suggested the presence of sulphate-reducing and sulphur-oxidizing bacteria. Results of intact glyceryl dialkyl glyceryl tetraether lipid analysis indicated the presence of nonthermophilic marine planktonic archaea. These results suggest that the microbial community in the Sagami Bay seep site is distinct from previously characterized cold-seep environments.

Cold-seep microbial communities are more abundant at deeper depths in the Japan Trench land slope.

We have analyzed the microbial communities in the cold-seep sediment samples obtained from different depths (5800∼7500 m) of the Japan Trench land slope. The results indicated that the typical cold-seep microbial communities of bacteria and archaea were basically similar in different depth environments and consisted of the delta-Proteobacteria (including sulfate reducing bacterial group) as well as methanogenic archaea, which played an important role in sulfur circulation in the seep environment. More abundant microbes were also identified in deeper cold-seep environments. These observations suggested that the cold-seep activity at the deepest depths of the Japan Trench might be more dynamic than in the shallower land slope. This is the first suggestion describing the relationship between microbial mass and cold-seep activity.

Fluid and chemical cycling at Bush Hill: Implications for gas‐ and hydrate‐rich environments

The results of a deployment of aqueous flux meters at the Bush Hill hydrate mound show that persistent hydrologic instability is a primary feature of the globally abundant gas‐ and hydrate‐rich cold seep environment. Seven flux meters were deployed for 14 weeks in the area of the Bush Hill hydrate mound. Instruments were deployed on microbial mats, bivalves, adjacent to the surface hydrate mound, and a site without visible fauna. Flow rates were observed to range from downflow of 0.01 mm/day to upflow of >15 mm/day. Temporal variability and a major hydrological event were observed to occur on all instruments. There is evidence that this event was related to a gas expulsion episode. The two instruments which exhibited downflow during the event were near surface hydrates and bubbling vents. Nearby instruments recorded a rapid increase in flow rates at the time of the event with a subsequent decrease in rates to the previous background values. Two instruments showed significant output of seawater‐like fluids, while the others output typical methane seep‐type pore fluids. These results at a passive margin site, along with our previously published convergent margin results at gas‐rich northeast Pacific cold seeps (Hydrate Ridge, Eel River margin), illustrate the hydrologic complexity of these environments. We now propose that these and our earlier results are characteristic of seafloor environments which have abundant free gas and hydrates. These mechanisms have major consequences for the near‐surface geochemical and microbial environment and for the way we interpret measurements made in these areas.

Chemical, biological and hydrological controls on the 14C content of cold seep carbonate crusts: numerical modeling and implications for convection at cold seeps

Understanding the hydrology of cold seep environments is crucial to perform accurate estimates of fluid and chemical fluxes at sedimentary wedges. Shallow convection processes may affect fluid flux estimates and could favor the destabilization of gas hydrate accumulations, increasing the sediment-ocean methane flux. Evidence for the occurrence of convection at cold seeps, however, is still limited. We use the concentration of 14C (D 14C) in carbonate crusts formed at cold seeps of the eastern Mediterranean Sea as a tracer for convective recirculation of seawater-derived fluids. A numerical model is applied to investigate the controls on 14C incorporation in cold seep carbonates. Our simulations show that increased amounts of CH 4 in the expelled fluids result in elevated crust D 14C, while high Ca 2+ and HCO 3 − concentrations produce the opposite effect. Convection is the only transport process that can significantly increase crust D 14C. Advection, bioirrigation, eddy diffusion and bioturbation instead, have little effect on, or produce a decrease of, crust D 14C. In addition, the presence of old or modern carbon (MC) in host sediments prior to cementation and the 14C-decay associated to the time needed to form the crust contribute in defining the D 14C of carbonate crusts. We then use the model to reproduce the 14C content of the eastern Mediterranean Sea crusts to constrain the chemical and hydrological conditions that led to their formation. Some crusts contain relatively low amounts of 14C (−945.0<D 14C ‰<−930.2) which, assuming no ageing after crust formation, can be reproduced without considering convection. Other crusts from two sites (the Amsterdam and Napoli mud volcanoes), instead, have a very high 14C-content (−899.0<D 14C ‰<−838.4) which can only be reproduced by the model if convection mixes deep fluids with seawater. Order-of-magnitude calculations using the Rayleigh criterion for convection suggest that the slow seepage (about 10 cm year −1) of low salinity (20‰) fluids at the Amsterdam sites could trigger haline convection there. On the Napoli mud volcano, where high-density brines are expelled, density-driven convection cannot take place and other processes, possibly involving the rapid movement of free gas in the sediment, could be important.

Life at cold seeps: a synthesis of biogeochemical and ecological data from Kazan mud volcano, eastern Mediterranean Sea

Recent field observations have identified the widespread occurrence of fluid seepage through the eastern Mediterranean Sea floor in association with mud volcanism or along deep faults. Gas hydrates and methane seeps are frequently found in cold seep areas and were anticipated targets of the MEDINAUT/MEDINETH initiatives. The study presented herein has utilized a multi-disciplinary approach incorporating observations and sampling of visually selected sites by the manned submersible Nautile and by ship-based sediment coring and geophysical surveys. The study focuses on the biogeochemical and ecological processes and conditions related to methane seepage, especially the anaerobic oxidation of methane (AOM), associated with ascending fluids on Kazan mud volcano in the eastern Mediterranean. Sampling of adjacent box cores for studies on the microbiology, biomarkers, pore water and solid phase geochemistry allowed us to integrate different biogeochemical data within a spatially highly heterogeneous system. Geophysical results clearly indicate the spatial heterogeneity of mud volcano environments. Results from pore water geochemistry and modeling efforts indicate that the rate of AOM is ∼6 mol m −2 year −1, which is lower than at active seep sites associated with conditions of focused flow, but greater than diffusion-dominated sites. Furthermore, under the non-focused flow conditions at Kazan mud volcano advective flow velocities are of the order of a few centimeters per year and gas hydrate formation is predicted to occur at a sediment depth of about 2 m and below. The methane flux through these sediments supports a large and diverse community of micro- and macrobiota, as demonstrated by carbon isotopic measurements on bulk organic matter, authigenic carbonates, specific biomarker compounds, and macrofaunal tissues. Because the AOM community appears to be able to completely oxidize methane at the rate it is seeping through the sediments, ultimate sinks for methane in this environment are authigenic carbonates and biomass. Significant differences in organic geochemical data between this site and those of other cold seep environments, even within the eastern Mediterranean mud volcanoes, indicate that the microbiological communities carrying out AOM varies depending on specific conditions such as methane flux and salinity.

Geological, geochemical, and microbial processes at the hydrate-bearing Håkon Mosby mud volcano: a review

Submarine mud volcanoes are point sources of fluid expulsion and oases of active geological, geochemical, and microbial processes in the deep ocean. The results of multidisciplinary investigations at the Håkon Mosby mud volcano (HMMV) in the Norwegian Sea at ∼1250 m water depth are reviewed in this paper. Seafloor morphology, lithotypes, geochemical, and biological processes at the HMMV are concentrically zoned. The zonation is controlled by ejection of sediment, water, and gases (mainly methane) accompanied by high heat flow in the mud volcano crater. A structural gas hydrate accumulation is associated with the HMMV. Gas hydrate is abundant only in a relatively narrow zone where optimal conditions (low temperature and high gas concentration) exist for gas hydrate crystallization. Authigenic minerals, bacterial mats, and chemosynthetic communities typical of cold seep environments are present. Rapid anaerobic sulfate-dependant oxidation of methane is thought to be mediated by a consortium of methanogens and sulfate-reducing bacteria. Although methane is oxidized in sediments at high rates, a significant portion of this gas may escape into the ocean, mainly as diffuse flux. In the water column, methane is rapidly dissolved and oxidized. This observation supports the hypothesis that gas flux from deep-water mud volcanoes contributes to the oceanic carbon pool, but not to the atmosphere. The HMMV represents an important natural laboratory and provides insight to processes at the interface of methane-rich sediments and cold bottom water in the deep ocean. Future studies may best focus on accurate direct measurements of gas flux and quantification of the biogeochemical cycling of major chemical elements in shallow sediment.

Activity, Distribution, and Diversity of Sulfate Reducers and Other Bacteria in Sediments above Gas Hydrate (Cascadia Margin, Oregon)

Cold seep environments such as sediments above outcropping hydrate at Hydrate Ridge (Cascadia margin off Oregon) are characterized by methane venting, high sulfide fluxes caused by the anaerobic oxidation of methane, and the presence of chemosynthetic communities. Recent investigations showed that another characteristic feature of cold seeps is the occurrence of methanotrophic archaea, which can be identified by specific biomarker lipids and 16S rDNA analysis. This investigation deals with the diversity and distribution of sulfate-reducing bacteria, some of which are directly involved in the anaerobic oxidation of methane as syntrophic partners of the methanotrophic archaea. The composition and activity of the microbial communities at methane vented and nonvented sediments are compared by quantitative methods including total cell counts, fluorescence in situ hybridization (FISH), bacterial production, enzyme activity, and sulfate reduction rates. Bacteria involved in the degradation of particulate organic carbon (POC) are as active and diverse as at other productive margin sites of similar water depths. The availability of methane supports a two orders of magnitude higher microbial biomass (up to 9.6 2 10 10 cells cm m 3 ) and sulfate reduction rates (up to 8 w mol cm m 3 d m 1 ) in hydrate-bearing sediments, as well as a high bacterial diversity, especially in the group of i -proteobacteria including members of the branches Desulfosarcina/Desulfococcus , Desulforhopalus , Desulfobulbus , and Desulfocapsa . Most of the diversity of sulfate-reducing bacteria in hydrate-bearing sediments comprises seep-endemic clades, which share only low similarities with previously cultured bacteria.

Community structure of vestimentiferan-generated habitat islands from Gulf of Mexico cold seeps

Biologically generated structures create habitat and influence the distribution and abundance of species in many marine systems. In the rather monotonous and nutrient-poor environment of the deep-sea, cold seep environments and their associated chemosynthetic communities offer islands of primary production and habitat to a generally sparsely distributed macrofauna. In this study, we investigate the structure of macrofaunal assemblages associated with vestimentiferan aggregations on the upper Louisiana slope of the Gulf of Mexico and the relationships between assemblage composition and the size and complexity of the vestimentiferan-generated habitat. Using custom-designed and custom-built devices, we collected seven whole vestimentiferan aggregations along with their associated fauna during the summers of 1997 and 1998. Sixty-five species were found associated with the four vestimentiferan aggregations collected in 1998, more than doubling the number of species previously reported for seeps in this region. Individual aggregations contained between 23 and 44 different non-vestimentiferan species. General trends of increasing species richness with increasing habitat size and increasing faunal density with increasing habitat complexity were identified, but substantial variability suggested other factors also control the composition of faunal associates. Faunal abundances decreased with increasing aggregation age. Seep endemics dominated the communities of younger aggregations, but non-endemic species dominated communities of older aggregations. Relative dominance of the heterotrophic community by primary consumers decreased, while predatory secondary and higher-order consumers increased with increasing aggregation age. These trends are discussed in terms of successional changes in aggregation structure, habitat heterogeneity and environmental conditions.

化学合成がうみだすもうひとつの深海生態系 化学合成共生システム‐ベントスと共生細菌との密接な関係

Endosymbiotic associations between invertebrates and chemosynthetic bacteria are common in deep-sea hydrothermal vent and cold seep environments. Since characterization of the endosymbionts has been difficult due to their resistance to cultivation once removed from their hosts, the chemotrophic endosymbioses were initially confirmed by electron microscopic observation, stable isotope analysis, lipid analysis, and enzymatic characterization. They were classified into two types: one using sulfide, oxygen, and carbon dioxide as an electron donor, acceptor, and carbon source, respectively; another using methane as an energy and carbon source. Molecular phylogenetic analysis based on 16S ribosomal RNA gene sequences have revealed that most host species contain a single phylotype of endosymbionts belonging to a limited domain of the γ-subdivision of Proteobacteria despite the wide taxonomic range of hosts. Most symbionts are characterized as thioautotrophs or methanotrophs. Symbiont transmission mechanisms have been reported in several host taxa. Vertical transmission has been shown in vesicomyid clams and deep-sea mussels. Evidence is accumulating to suggest environmental transmission in vestimentiferans and lucinid clams. In the case of solemyid clams, several lines of evidence seem to conflict. A hypothesis is suggested that bacterial chemotrophic endosymbionts influence host distributions in deep-sea chemosynthetic ecosystems. The distribution of methanotrophic symbioses between deep-sea mussels and their symbionts correlates with high methane concentrations in deep-sea chemosynthetic ecosystems. A thyasirid clam living in the deepest yet known chemosynthetic environment has an unusual intracellular dual symbiosis, which is likely essential for the survival of host bivalves at great depth in reduced environments. These results indicate that bacterial chemotrophic endosymbionts influence host distributions in deep-sea chemosynthetic ecosystems.

Psychromonas kaikoae sp. nov., a novel from the deepest piezophilic bacterium cold-seep sediments in the Japan Trench.

Two strains of obligately piezophilic bacteria were isolated from sediment collected from the deepest cold-seep environment with chemosynthesis-based animal communities within the Japan Trench, at a depth of 7434 m. The isolated strains, JT7301 and JT7304T, were closely affiliated with members of the genus Psychromonas on the basis of 16S rDNA sequence analysis. Hybridization values for DNA-DNA relatedness between these strains and the Psychromonas antarctica reference strain were significantly lower than that accepted as the phylogenetic definition of a species. The optimal temperature and pressure for growth of the isolates were 10 degrees C and 50 MPa and they produced both eicosapentaenoic acid (C20:5omega3) and docosahexaenoic acid (C22:6) in the membrane layer. Based on the taxonomic differences observed, the isolated strains appear to represent a novel obligately piezophilic Psychromonas species. The name Psychromonas kaikoae sp. nov. (type strain JT7304T = JCM 11054T = ATCC BAA-363T) is proposed. This is the first proposed obligately piezophilic species of the genus Psychromonas.

Psychromonas kaikoae sp. nov., a novel piezophilic bacterium from the deepest cold-seep sediments in the Japan Trench

Two strains of obligately piezophilic bacteria were isolated from sediment collected from the deepest cold-seep environment with chemosynthesis-based animal communities within the Japan Trench, at a depth of 7434 m. The isolated strains, JT7301 and JT7304 T , were closely affiliated with members of the genus Psychromonas on the basis of 16S rDNA sequence analysis. Hybridization values for DNA-DNA relatedness between these strains and the Psychromonas antarctica reference strain were significantly lower than that accepted as the phylogenetic definition of a species. The optimal temperature and pressure for growth of the isolates were 10 °C and 50 MPa and they produced both eicosapentaenoic acid (C20:5ω3) and docosahexaenoic acid (C22:6) in the membrane layer. Based on the taxonomic differences observed, the isolated strains appear to represent a novel obligately piezophilic Psychromonas species. The name Psychromonas kaikoae sp. nov. (type strain JT7304 T = JCM 11054 T = ATCC BAA-363 T ) is proposed. This is the first proposed obligately piezophilic species of the genus Psychromonas.

Monterey Bay cold-seep biota: Assemblages, abundance, and ultrastructure of living foraminifera

Although there is a growing body of evidence indicating benthic foraminifera inhabit hydrocarbon and cold seep environments, biochemical and ultrastructural data on seep foraminiferal communities are not available. Therefore, sediments collected from cold seeps in Monterey Bay, CA (900–1000 m), were examined for the presence of live benthic foraminifera. Results from three independent methods (ATP assay, ultrastructural analysis, rose Bengal staining) indicate that certain species inhabit the Clam Flat and Clam Field seeps. Abundances in our seep samples were lower than in comparable non-seep sites, although not atypical for these bathyal depths. Of 38 species represented at these two seep sites by cytoplasm-containing specimens, only Spiroplectammina biformis was restricted to the seep environment. However, because S. biformis is also known from non-seep sites in other areas, it should not be considered as endemic to seeps. Ultrastructural studies show abundant peroxisomes in seep specimens, which may allow inhabitation of such environments. One specimen of Uvigerina peregrina had prokaryotes nestled in test pores, suggesting that bacteria may play a role in the survival of foraminifera in this seep environment.

Archaea mediating anaerobic methane oxidation in deep-sea sediments at cold seeps of the eastern Aleutian subduction zone

Cold seeps in the Aleutian deep-sea trench support prolific benthic communities and generate carbonate precipitates which are dependent on carbon dioxide delivered from anaerobic methane oxidation. This process is active in the anaerobic sediments at the sulfate reduction-methane production boundary and is probably performed by archaea working in syntrophic co-operation with sulfate-reducing bacteria. Diagnostic lipid biomarkers of archaeal origin include irregular isoprenoids such as 2,6,11,15-tetramethylhexadecane (crocetane) and 2,6,10,15,19-pentamethylicosane (PMI) as well as the glycerol ether lipid archaeol (2,3-di-O-phytanyl-sn-glycerol). These biomarkers are prominent lipid constituents in the anaerobic sediments as well as in the carbonate precipitates. Carbon isotopic compositions of the biomarkers are strongly depleted in 13C with values of δ13C as low as −130.3‰ PDB. The process of anaerobic methane oxidation is also reflected in the carbon isotope composition of organic matter with δ13C-values of −39.2 and −41.8‰ and of the carbonate precipitates with values of −45.4 and −48.7‰. This suggests that methane-oxidizing archaea have accumulated within the microbial community, which is active at the cold seep sites. The dominance of crocetane in sediments at one station indicates that, probably due to decreased methane venting, archaea might no longer be growing, whereas high amounts of crocetenes found at other more active stations may indicate recent fluid venting and active archaea. Comparison with other biomarker studies suggests that various archaeal assemblages might be involved in the anaerobic consumption of methane. The assemblages are apparently dependent on specific conditions found at each cold seep environment. Selective conditions probably include water depth, temperature, degree of anoxia, and supply of free methane.

Microbial diversity of deep-sea extremophiles--Piezophiles, Hyperthermophiles, and subsurface microorganisms

Knowledge of our Planet's biosphere has increased tremendously during the last 10 to 20 years. In the field of Microbiology in particular, scientists have discovered novel "extremophiles", microorganisms capable of living in extreme environments such as highly acidic or alkaline conditions, at high salt concentration, with no oxygen, extreme temperatures (as low as -20 degrees C and as high as 300 degrees C), at high concentrations of heavy metals and in high pressure environments such as the deep-sea. It is apparent that microorganisms can exist in any extreme environment of the Earth, yet already scientists have started to look for life on other planets; the so-called "Exobiology" project. But as yet we have little knowledge of the deep-sea and subsurface biosphere of our own planet. We believe that we should elucidate the Biodiversity of Earth more thoroughly before exploring life on other planets, and these attempts would provide deeper insight into clarifying the existence of extraterrestrial life. We focused on two deep-sea extremophiles in this article; one is "Piezophiles", and another is "Hyperthermophiles". Piezophiles are typical microorganisms adapted to high-pressure and cold temperature environments, and located in deep-sea bottom. Otherwise, hyperthermophiles are living in high temperature environment, and located at around the hydrothermal vent systems in deep-sea. They are not typical deep-sea microorganisms, but they can grow well at high-pressure condition, just like piezophiles. Deming and Baross mentioned that most of the hyperthermophilic archaea isolated from deep-sea hydrothermal vents are able to grow under conditions of high temperature and pressure, and in most cases their optimal pressure for growth was greater than the environmental pressure they were isolated from. It is possible that originally their native environment may have been deeper than the sea floor and that there had to be a deeper biosphere. This implication suggests that the deep-sea hydrothermal vents are the windows to a deep subsurface biosphere. A vast array of chemoautotrophic deep-sea animal communities have been found to exist in cold seep environments, and most of these animals are common with those found in hydrothermal vent environments. Thus, it is possible to consider that the cold seeps are also one of slit windows to a deep subsurface biosphere. We conclude that the deep-sea extremophiles are very closely related into the unseen majority in subsurface biosphere, and the subsurface biosphere probably concerns to consider the "exobiology".

Phylogenetic characterization of endosymbionts in three hydrothermal vent mussels: influence on host distributions

The bacterial endosymbionts of 3 hydrothermal vent mussels from Japanese waters were characterized by transmission electron microscopic (TEM) observation and phylogenetic analy- ses of 16S ribosomal RNA gene sequences. Endosymbionts of Bathymodiolus septemdierum were related to sulfur-oxidizing bacteria (thioautotrophs), while endosymbionts of B. platifrons and B. japonicus were related to methane-oxidizing bacteria (methanotrophs). This is the first report of deep-sea mussels containing only methanotrophs (lacking thioautotrophs) from hydrothermal vents. Comparison of methane and hydrogen sulfide concentrations in end-member fluids from deep-sea hydrothermal vents indicated that methane concentrations were much higher in habitats containing Bathymodiolus spp. which harbored only methanotrophs than in other habitats of hydrothermal vent mussels. The known distribution of other mussels containing only methanotrophs has thus far been limited to cold-seep environments with high methane concentrations from the interstitial water. These results suggest that the distribution of methanotrophic symbioses between deep-sea mussels and methanotrophs is strongly influenced by the methane or hydrocarbon concentrations provided from hydrothermal vent and cold-seep activities (or that methane concentration is a possible limiting factor that restricts the distribution of methanotrophy-dependent symbioses in the deep sea).

Microbial Diversity in Sediments Collected from the Deepest Cold-Seep Area, the Japan Trench.

: The Japan Trench land slope at a depth of 6,400 m is the deepest cold-seep environment with Calyptogena communities. Sediment samples from inside and beside the Calyptogena communities were collected, and the microbial diversity in the sediment samples was studied by molecular phylogenetic techniques. From DNA extracted directly from the sediment samples, 16S rDNAs were amplified by the polymerase chain reaction method. The sequences of the amplified 16S rDNAs selected by restriction fragment length polymorphism analysis were determined and compared with sequences in DNA databases. The results showed that 33 different bacterial 16S rDNA sequences from the two samples analyzed fell into similar phylogenetic categories, the alpha-, gamma-, delta-, and varepsilon-subdivisions of Proteobacteria, Cytophaga, and gram-positive bacteria; some of the 16S rDNA sequences were common to both samples. delta- and varepsilon-Proteobacteria-related sequences were abundant in both sediments. These sequences are mostly related to sulfate-reducing or sulfur-reducing bacteria and epibionts, respectively. Eight different archaeal 16S rDNA sequences were cloned from the sediments. The majority of the archaeal 16S rDNA sequences clustered in Crenarchaeota and showed high similarities to marine group I archaeal rDNA. A Methanococcoides burtonii-related sequence obtained from the sediment clustered in the Euryarchaeota indicating that M. burtonii-related strains in the area of Calyptogena communities may contribute to production of methane in this environment. From these results, we propose a possible model of sulfur circulation within the microbial community and that of Calyptogena clams in the cold-seep environment.