10:45 AM - 11:00 AM
[HTT16-11] Insights from a depth transect approach: methane-related subsurface biosphere inhabiting the sedimentary layer at Suwa Basin.
Keywords:deep biosphere, methane, Suwa Basin, Itoigawa-Shizuoka Tectonic Line, Median Tectonic Line
Introduction:
The subsurface environment is one of the last frontiers of the biosphere on the Earth where mostly uncultured prokaryotes are dominantly present. Deep biosphere is generally under energy and nutrient-limited conditions. However, subsurface ecosystems in sedimentary layers are exceptionally accessible to substrates in high concentrations. It is meaningful to understand microbial activities with energy and nutrient flux into the deep biosphere. In this context, we conducted a multi-depth subsurface investigation of the pull-apart basin associated with an organic-rich sedimentary layer ( – 300 m below ground level), hot springs ( – 1000 mbgl) on the active faults of Itoigawa-Shizuoka Tectonic Line (ISTL) and the Median Tectonic Line (MTL).
Geological setting and sampling:
Suwa basin (36.0°N, 138.1°E) is located at the intersection of the ISTL and the MTL. The movement of tectonic lines resulted in active faults and hot springs located within the basin, making complex subsurface hydrogeology. These geological features enable the connection of biogeochemical cycles through the deep subsurface to the surface. In a previous study, active methane emission into the surface along active faults is reported [1]. In addition to methane, deep aquifer from the subsurface is potentially important for ecosystems inhabiting layers above by providing energy and reducing compounds.
In this study, we focused on the biosphere in the sedimentary layer which accounts for – 300 m depth of the basin [2,3]. The deep aquifer contains organic carbon in high concentrations (13 – 20 mg-C/L) [4]. This layer is thought to host a variety of active methanotrophic subsurface ecosystems depending on a range of ionic composition and redox condition. We investigated microbial ecosystems inhabiting this layer and estimated factors constraining their structure. To further understand deep fluid dynamics, we also analyzed hot spring water derived from the deep subsurface environment ( – 1000 mbgl).
We obtained groundwater and gas samples from two depths of the sedimentary layer (10 – 100 m) using three wells. The gas samples were directly introduced from the outlet of wells into vacuumed glass vials. The Kamisuwa hot spring waters were retrieved at seven sites (max. depth: 1000 m). The microbial cells were collected by filtration (pore size: 0.3 µm), and filtrates were served to analyze major ions and trace elements. Using these samples, we performed the parallel analysis of organic/inorganic geochemistry and isotope measurements (δ13C, δD, δ15N, Δ14C), including methane-specific carbon and hydrogen isotope analysis (δ13CCH4-δDCH4). We also analyzed microbial composition by SSU rRNA gene amplicon sequencing and quantification of membrane lipids. The concentrations of dissolved cations (e.g., Na, Mg, Fe) and anions (e.g., Cl, F) were measured using ion chromatography and inductively coupled plasma mass spectrometry.
Results and Discussion:
The results of carbon and hydrogen isotopic profiles indicated that the methane in the subsurface aquifers is originated from methanogenic archaea. Its substrates were also estimated to be both methyl compounds and carbonate based on the isotopic profiles [5]. The high abundance of hydrogenotrophs in hot spring samples suggested that hydrogen gas (H2) generated in the deep subsurface layer plausibly supports methanogenesis. It is interesting to note that methanotrophic bacterial lineages were commonly dominant in the subsurface aquifer. This result illuminated the importance of methane as a carbon source for the whole subsurface biosphere within the sedimentary layer. Although groundwater samples shared the major types of metabolic features (aerobic methanotrophs and chemoheterotrophs), the abundant phylogenetic lineages in each sample are different at the class level. In this presentation, detailed results and discussion about deeply buried microbial heterogeneity and fluid chemistry of deep aquifer will be assessed from the view of deep biosphere on the active fault.
References:
[1] Urai et al., 2022. ACS Earth Space Chem., 6, 1689-1697.
[2] Motojima et al., 1952. Bull. Geol. Survey Jpn., 3, 644-649 (in Japanese)
[3] Hatano et al., 2023. Palaeogeogr. Palaeoclimatol. Palaeoecol., 111439.
[4] Miyabara 2012. Ann. Environ. Sci., Shinshu Univ., 34, 10-16 (in Japanese)
[5] Whiticar 1999, Chem. Geol., 161, 291-31
The subsurface environment is one of the last frontiers of the biosphere on the Earth where mostly uncultured prokaryotes are dominantly present. Deep biosphere is generally under energy and nutrient-limited conditions. However, subsurface ecosystems in sedimentary layers are exceptionally accessible to substrates in high concentrations. It is meaningful to understand microbial activities with energy and nutrient flux into the deep biosphere. In this context, we conducted a multi-depth subsurface investigation of the pull-apart basin associated with an organic-rich sedimentary layer ( – 300 m below ground level), hot springs ( – 1000 mbgl) on the active faults of Itoigawa-Shizuoka Tectonic Line (ISTL) and the Median Tectonic Line (MTL).
Geological setting and sampling:
Suwa basin (36.0°N, 138.1°E) is located at the intersection of the ISTL and the MTL. The movement of tectonic lines resulted in active faults and hot springs located within the basin, making complex subsurface hydrogeology. These geological features enable the connection of biogeochemical cycles through the deep subsurface to the surface. In a previous study, active methane emission into the surface along active faults is reported [1]. In addition to methane, deep aquifer from the subsurface is potentially important for ecosystems inhabiting layers above by providing energy and reducing compounds.
In this study, we focused on the biosphere in the sedimentary layer which accounts for – 300 m depth of the basin [2,3]. The deep aquifer contains organic carbon in high concentrations (13 – 20 mg-C/L) [4]. This layer is thought to host a variety of active methanotrophic subsurface ecosystems depending on a range of ionic composition and redox condition. We investigated microbial ecosystems inhabiting this layer and estimated factors constraining their structure. To further understand deep fluid dynamics, we also analyzed hot spring water derived from the deep subsurface environment ( – 1000 mbgl).
We obtained groundwater and gas samples from two depths of the sedimentary layer (10 – 100 m) using three wells. The gas samples were directly introduced from the outlet of wells into vacuumed glass vials. The Kamisuwa hot spring waters were retrieved at seven sites (max. depth: 1000 m). The microbial cells were collected by filtration (pore size: 0.3 µm), and filtrates were served to analyze major ions and trace elements. Using these samples, we performed the parallel analysis of organic/inorganic geochemistry and isotope measurements (δ13C, δD, δ15N, Δ14C), including methane-specific carbon and hydrogen isotope analysis (δ13CCH4-δDCH4). We also analyzed microbial composition by SSU rRNA gene amplicon sequencing and quantification of membrane lipids. The concentrations of dissolved cations (e.g., Na, Mg, Fe) and anions (e.g., Cl, F) were measured using ion chromatography and inductively coupled plasma mass spectrometry.
Results and Discussion:
The results of carbon and hydrogen isotopic profiles indicated that the methane in the subsurface aquifers is originated from methanogenic archaea. Its substrates were also estimated to be both methyl compounds and carbonate based on the isotopic profiles [5]. The high abundance of hydrogenotrophs in hot spring samples suggested that hydrogen gas (H2) generated in the deep subsurface layer plausibly supports methanogenesis. It is interesting to note that methanotrophic bacterial lineages were commonly dominant in the subsurface aquifer. This result illuminated the importance of methane as a carbon source for the whole subsurface biosphere within the sedimentary layer. Although groundwater samples shared the major types of metabolic features (aerobic methanotrophs and chemoheterotrophs), the abundant phylogenetic lineages in each sample are different at the class level. In this presentation, detailed results and discussion about deeply buried microbial heterogeneity and fluid chemistry of deep aquifer will be assessed from the view of deep biosphere on the active fault.
References:
[1] Urai et al., 2022. ACS Earth Space Chem., 6, 1689-1697.
[2] Motojima et al., 1952. Bull. Geol. Survey Jpn., 3, 644-649 (in Japanese)
[3] Hatano et al., 2023. Palaeogeogr. Palaeoclimatol. Palaeoecol., 111439.
[4] Miyabara 2012. Ann. Environ. Sci., Shinshu Univ., 34, 10-16 (in Japanese)
[5] Whiticar 1999, Chem. Geol., 161, 291-31