Youko Miyoshi

First cultivation and ecological investigation of a bacterium affiliated with the candidate phylum OP5 from hot springs.

ABSTRACT The phylogenetic group termed OP5 was originally discovered in the Yellowstone National Park hot spring and proposed as an uncultured phylum; the group was afterwards analyzed by applying culture-independent approaches. Recently, a novel thermophilic chemoheterotrophic filamentous bacterium was obtained from a hot spring in Japan that was enriched through various isolation procedures. Phylogenetic analyses of the isolate have revealed that it is closely related to the OP5 phylum that has mainly been constructed with the environmental clones retrieved from thermophilic and mesophilic anaerobic environments. It appears that the lineage is independent at the phylum level in the domain Bacteria. Therefore, we designed a primer set for the 16S rRNA gene to specifically target the OP5 phylum and performed quantitative field analysis by using the real-time PCR method. Thus, the 16S rRNA gene of the OP5 phylum was detected in some hot-spring samples with the relative abundance ranging from 0.2% to 1.4% of the prokaryotic organisms detected. The physiology of the above-mentioned isolate and the related environmental clones indicated that they are scavengers contributing to the sulfur cycle in nature.

Defining boundaries for the distribution of microbial communities beneath the sediment-buried, hydrothermally active seafloor

Subseafloor microbes beneath active hydrothermal vents are thought to live near the upper temperature limit for life on Earth. We drilled and cored the Iheya North hydrothermal field in the Mid-Okinawa Trough, and examined the phylogenetic compositions and the products of metabolic functions of sub-vent microbial communities. We detected microbial cells, metabolic activities and molecular signatures only in the shallow sediments down to 15.8 m below the seafloor at a moderately distant drilling site from the active hydrothermal vents (450 m). At the drilling site, the profiles of methane and sulfate concentrations and the δ13C and δD isotopic compositions of methane suggested the laterally flowing hydrothermal fluids and the in situ microbial anaerobic methane oxidation. In situ measurements during the drilling constrain the current bottom temperature of the microbially habitable zone to ~45 °C. However, in the past, higher temperatures of 106–198 °C were possible at the depth, as estimated from geochemical thermometry on hydrothermally altered clay minerals. The 16S rRNA gene phylotypes found in the deepest habitable zone are related to those of thermophiles, although sequences typical of known hyperthermophilic microbes were absent from the entire core. Overall our results shed new light on the distribution and composition of the boundary microbial community close to the high-temperature limit for habitability in the subseafloor environment of a hydrothermal field.

Sediment–Pore Water System Associated with Native Sulfur Formation at Jade Hydrothermal Field in Okinawa Trough

The subsurface sediment–pore water system in the Jade hydrothermal field was investigated along with pore water chemistry and X-ray absorption fine structure analysis for sediment samples. Sediments were collected by coring near the TBS vent (Core 1186MBL) and from near the Biwako vent (Core 1188MB), which are active hydrothermal vents with high (320 °C) and low (90 °C) temperature fluids, respectively. Core 1186MBL is characterized by occurrences of chimney fragments in the shallow part (2–9 cmbsf) and native sulfur in the deep part (6–19 cmbsf). The results of chemical analyses of the pore water suggest a seawater recharge from the seafloor into Core 1186MBL. This hydraulic characteristic of the pore water, which is commonly observed near active submarine hydrothermal vents, leads to oxidation of sulfide minerals in chimney fragments in the shallow part by the oxic recharged seawater. The resulting acidic and suboxic pore water is transported downward, and can form native sulfur in the deeper part of Core 1186MBL. Core 1188MB shows a wide distribution of native sulfur, and its pore water chemistry indicates anaerobic oxidation of methane below 8 cmbsf and a mixing of seawater and hydrothermal fluid below 10 cmbsf. The mixing of acidic and anoxic hydrothermal fluid and seawater allows the precipitation of native sulfur in Core 1188MB, and the uniform circumneutral pH condition despite the input of acidic hydrothermal fluid. The native sulfur deposits in the arc–back-arc hydrothermal fields provide important geochemical information that is useful to understand the subsurface sediment–pore water system involving hydrothermal fluids

Pore Fluid Chemistry Beneath Active Hydrothermal Fields in the Mid-Okinawa Trough: Results of Shallow Drillings by BMS During TAIGA11 Cruise

TAIGA11 cruise of R/V Hakurei-maru No.2 was conducted in June, 2011 to study subseafloor geochemical environment below active hydrothermal fields using a shallow drilling system BMS (Benthic Multi-coring System). Three active hydrothermal fields were selected as target fields; the Iheya North Knoll field (27°47′ N, 126°54′ E), the Jade field in the Izena Hole (27°16′ N, 127°05′E), and the Hakurei field in the Izena Hole (27°15′ N, 127°04′ E). In this chapter, we will report chemical composition and isotope ratios of pore fluids extracted from collected sediments. At the Hakurei field in the Izena Hole, BMS drilling attained to 610 cmbsf (cm below the seafloor) in the vicinity of a large massive sulfide mound. The obtained core showed evidence for sulfide and sulfate mineralization below 223 cmbsf. Pore fluid from the corresponding depth showed enrichment in Si, K and Ca, which could be attributed to influence of formation of alteration minerals rather than to involvement of the hydrothermal component. At the Jade field in the Izena Hole, BMS drilling attained to 529 cmbsf at about 300 m apart from the area where high temperature fluid venting congregate. The obtained core comprised grayish white hydrothermal altered mud below 370 cmbsf, although pore fluid showed seawater like composition. At the Iheya North Knoll field, BMS drilling attained to 453 cmbsf at about 200 m apart from the central mound area. The obtained core consisted almost entirely of grayish white hydrothermally altered mud. Pore fluid below 180 cmbsf showed substantial enrichment in major cations (Na, K, Ca and Mg) and Cl, which would be explained as a result of hydration during hydrothermal alteration.

Occurrence of Hydrothermal Alteration Minerals at the Jade Hydrothermal Field, in the Izena Hole, Mid-Okinawa Trough

Mineralogical and geochemical features of hydrothermal alteration minerals in the sediment cores from the Jade hydrothermal field in the Izena Hole, mid-Okinawa Trough, were studied by XRD, EPMA and TEM-EDS analyses. A core sample 1186MBL collected from the surface sediment near the sulfide chimney venting high temperature fluid up to 320 °C was characterized by occurrence of kaolinite, with sulfide minerals such as sphalerite and galena. The kaolinite would be related to be formed under acidic condition caused by oxidation and dissolution of the sulfide minerals by penetrating seawater. Core samples (1188MB, 1193MB) were collected from the surface sediment in the vicinity of clear hydrothermal fluid venting of ~100 °C, which is located in 400 m distant from the sulfide chimney. In these cores, occurrence of chlorite and smectite was identified. The chlorite in the core 1188MB had chemical composition close to Al-rich chlorite which is classified as sudoite, although chlorite found in other hydrothermal fields in the Okinawa Trough is characterized as significantly Mg-rich chlorite. Core samples of up to 4–6 m length were also collected near the low temperature fluid venting to study alteration in deep layers. One of two long core samples (BMS-J-2) was characterized by chlorite and illite assemblage below 380 cmbsf, while the other (LC-J-2) was characterized by abundant occurrence of K-feldspar below 300 cmbsf. Occurrence of euhedral crystals of K-feldspar in size up to several tens μm suggests the formation by precipitation from high temperature fluid.