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James K Fredrickson - One of the best experts on this subject based on the ideXlab platform.
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Molecular analysis of deep subsurface Cretaceous Rock indicates abundant Fe(III)‐ and S°‐reducing bacteria in a sulfate‐rich environment
Environmental microbiology, 2006Co-Authors: William P Kovacik, Ken Takai, Melanie R Mormile, James P Mckinley, Fred J Brockman, James K Fredrickson, William E HolbenAbstract:A multi-level sampler (MLS) was emplaced in a borehole straddling anaerobic, sulfate-rich Cretaceous-era shale and sandstone Rock formations {approx}200 m below ground surface at Cerro Negro, New Mexico. Sterile quartzite sand contained in chambers in the sampler allowed in situ colonization and recovery of nucleic acids for molecular analyses. DGGE and 16S rRNA gene cloning results indicated a homogeneously distributed bacterial community across the shale/sandstone interface. ?-Proteobacteria sequences were common at all depths, and were dominated by members of the Geobacteraceae family (Pelobacter, Desulfuromonas, and Geobacter). Other members of this group are capable of dissimilatory Fe(III) and/or S0 reduction, but not sulfate reduction. RNA hybridization data also suggested that Fe(III)/S0 reducing bacteria were predominant. These findings are striking considering the lack of significant concentrations of these electron acceptors in this environment. The next most abundant bacterial group indicated was the sulfate reducers, including Desulfobacterium, Desulfocapsa and Desulfobulbus. Sequences related to fermenters, denitrifiers and acetogens were also recovered. The presence of a phylogenetically and functionally diverse microbial community in this deep subsurface environment likely reflects the complex nature of the primary energy and carbon sources, kerogen associated with the shale.
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molecular analysis of deep subsurface Cretaceous Rock indicates abundant fe iii and s reducing bacteria in a sulfate rich environment
Environmental Microbiology, 2006Co-Authors: William P Kovacik, Ken Takai, Melanie R Mormile, James P Mckinley, Fred J Brockman, James K Fredrickson, William E HolbenAbstract:A multi-level sampler (MLS) was emplaced in a borehole straddling anaerobic, sulfate-rich Cretaceous-era shale and sandstone Rock formations {approx}200 m below ground surface at Cerro Negro, New Mexico. Sterile quartzite sand contained in chambers in the sampler allowed in situ colonization and recovery of nucleic acids for molecular analyses. DGGE and 16S rRNA gene cloning results indicated a homogeneously distributed bacterial community across the shale/sandstone interface. ?-Proteobacteria sequences were common at all depths, and were dominated by members of the Geobacteraceae family (Pelobacter, Desulfuromonas, and Geobacter). Other members of this group are capable of dissimilatory Fe(III) and/or S0 reduction, but not sulfate reduction. RNA hybridization data also suggested that Fe(III)/S0 reducing bacteria were predominant. These findings are striking considering the lack of significant concentrations of these electron acceptors in this environment. The next most abundant bacterial group indicated was the sulfate reducers, including Desulfobacterium, Desulfocapsa and Desulfobulbus. Sequences related to fermenters, denitrifiers and acetogens were also recovered. The presence of a phylogenetically and functionally diverse microbial community in this deep subsurface environment likely reflects the complex nature of the primary energy and carbon sources, kerogen associated with the shale.
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shifts in archaeal communities associated with lithological and geochemical variations in subsurface Cretaceous Rock
Environmental Microbiology, 2003Co-Authors: Ken Takai, Melanie R Mormile, James P Mckinley, Fred J Brockman, William E Holben, William P Kovacik, James K FredricksonAbstract:Subsurface microbial community structure in relation to geochemical gradients and lithology was investigated using a combination of molecular phylogenetic and geochemical analyses. Discreet groundwater and substratum samples were obtained from depths ranging from 182 to 190 m beneath the surface at approximately 10-cm intervals using a multi-level sampler (MLS) that straddled Cretaceous shale and sandstone formations at a site in the southern San Juan Basin in New Mexico. DNA and RNA were extracted directly from quartzite sand substratum loaded into individual cells of the MLS and colonized in situ for six months. PCR-mediated T-RFLP analysis of archaeal rRNA genes (rDNA) in conjunction with partial sequencing analysis of archaeal rDNA libraries and quantitative RNA hybridization with oligonucleotide probes were used to probe community structure and function. Although total microbial populations remained relatively constant over the entire depth interval sampled, significant shifts in archaeal populations, predominantly methanogens, were observed. These shifts coincided with the geochemical transition from relatively high methane (26 mM), low sulfate (<3 mg l-1) concentrations in the region adjacent to the organic matter-rich shale to relatively low-methane (<0.5 mM), high-sulfate (48 mg l-1) conditions in the organic-poor sandstone beneath the shale. These results indicate that active, phylogenetically diverse archaealmore » communities were present in the subsurface Cretaceous Rock environment at this site and that major archaeal clades shifted dramatically over scales of tens of centimeters, corresponding to changes in the lithology and geochemical gradients.« less
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Shifts in archaeal communities associated with lithological and geochemical variations in subsurface Cretaceous Rock
Environmental microbiology, 2003Co-Authors: Ken Takai, Melanie R Mormile, James P Mckinley, Fred J Brockman, William E Holben, William P Kovacik, James K FredricksonAbstract:Subsurface microbial community structure in relation to geochemical gradients and lithology was investigated using a combination of molecular phylogenetic and geochemical analyses. Discreet groundwater and substratum samples were obtained from depths ranging from 182 to 190 m beneath the surface at approximately 10-cm intervals using a multi-level sampler (MLS) that straddled Cretaceous shale and sandstone formations at a site in the southern San Juan Basin in New Mexico. DNA and RNA were extracted directly from quartzite sand substratum loaded into individual cells of the MLS and colonized in situ for six months. PCR-mediated T-RFLP analysis of archaeal rRNA genes (rDNA) in conjunction with partial sequencing analysis of archaeal rDNA libraries and quantitative RNA hybridization with oligonucleotide probes were used to probe community structure and function. Although total microbial populations remained relatively constant over the entire depth interval sampled, significant shifts in archaeal populations, predominantly methanogens, were observed. These shifts coincided with the geochemical transition from relatively high methane (26 mM), low sulfate (
R.e. Criss - One of the best experts on this subject based on the ideXlab platform.
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Geochemistry of tectonically expelled fluids from the northern Coast ranges, Rumsey Hills, California, USA
Geochimica et Cosmochimica Acta, 1994Co-Authors: M.l. Davisson, T.s. Presser, R.e. CrissAbstract:Tectonic compression has created abnormally high pressure on deep basinal fluids causing their expulsion from areally exposed Upper Cretaceous Rock along the eastern margin of the California Coast ranges. The fluids emerge as near-neutral, perennial sodium chloride springs at high elevations with flow rates as high as 10 L per min. Higher spring discharges are more common around the exposure of a west-vergent fault propagation fold axis. Spring waters range from ~1000 to 27,000 mg/L TDS. The least saline water (δ18O = −7.5‰) closely represents local meteoric water that mixes with saline fluid (δ18O = +5.3‰) and forms a slope of ~3.5 on a δD vs. δ18O plot. A Na (125 to 8000 mg/L) vs. Cl (150 to 17,000 mg/L) plot shows a linear dilution trend that extends close to, but below, the values for modern seawater. Calcium (75–3000 mg/L) is considerably enriched relative to seawater and forms a nonlinear trend with chloride. In detail, the “Na deficit,” defined by the difference between the measured Na content and the Na concentration on a hypothetical seawater dilution line, is approximately balanced by the Ca excess, similarly defined by the seawater dilution line. This relationship strongly suggests that the fluid is diluted seawater that is being modified by active albitization of plagioclase at different depths. Simultaneous B and 18O enrichment of the fluids, accompanied by deuterium depletion, further suggest that the seawater modification is influenced by clay diagenesis. Bicarbonate and SiO2 concentrations show an inverse correlation with Cl, with most waters being saturated or slightly oversaturated with calcite and quartz at the discharge temperatures. Some freshwater springs with near-meteoric stable isotope values may represent mixing of young groundwater from perched aquifers, but in many cases, the freshwater springs emerge along the same structures and have the same perennial nature as the saline fluids, and expulsion of an older fresh groundwater component that is under abnormal fluid pressures cannot be ruled out. Basinal fluids elsewhere commonly show dilution trends with local meteoric water, and in the case of the Rumsey Hills, some of the dilute saline waters may indicate deep penetration of meteoric water (> 1 km) in the Pleistocene before the latest tectonic uplift. Geothermometry of the spring waters (maximum ~90°C) suggest an origin from as deep as 4.0 km. This depth is consistent with the depth of the core of a fault propagation anticline below the surface of the Rumsey Hills developed by active internal deformation of an east-tapering wedge beneath the southwestern Sacramento Valley. Active tectonic compression causes near-lithostatic fluid pressures in the shallow subsurface below the Rumsey Hills and volume strain within the core of the anticline that results in upward expulsion of the saline fluids from the indicated depths.
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Tectonically expelled seawater from the northern Coast Ranges, California
1993Co-Authors: M.l. Davisson, T.s. Presser, R.e. CrissAbstract:Tectonically expelled saline fluids derived from connate seawater emerge at high elevations from Upper Cretaceous Rock in the Rumsey Hills, eastern California Coast Ranges. The perennial spring waters are near-neutral in pH and range from [approximately]1,000 to 27,000 mg/L total dissolved solids. In detail, the Na deficit'', defined as the difference between the measured Na content and the Na concentration on a hypothetical seawater dilution line, is almost precisely balanced by the Ca excess'', similarly defined relative to a seawater dilution line. This relationship strongly suggests that the fluid is diluted seawater that is being modified by active albitization of plagioclase at different depths. Simultaneous boron and [sup 18]O enrichment of the fluids, accompanied by deuterium depletion, suggests that the seawater modification is also influenced by clay diagenesis. Bicarbonate (55 to 400 ppm) and SiO[sub 2] (2 to 25 ppm) concentrations show an inverse correlation with Cl, with most waters showing saturation or slight oversaturation with calcite and quartz at their sampling temperatures ([approximately]20 C). The highest silica and bicarbonate concentrations correspond to the most dilute spring waters, and these also emerge as perennial springs along the same structures as the saline fluids, indicating that the fresh spring waters maymore » not be modern. Fresh-water recharge possibly was deeper in the geologic past and diluted the saline fluid at depth, as typically seen in many sedimentary basins. Chemical geothermometry suggests that these saline fluids originate in the core of an anticline under near-lithostatic fluid pressures created by the active internal deformation of an east-tapering wedge below the southwestern Sacramento Valley.« less
James P Mckinley - One of the best experts on this subject based on the ideXlab platform.
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Molecular analysis of deep subsurface Cretaceous Rock indicates abundant Fe(III)‐ and S°‐reducing bacteria in a sulfate‐rich environment
Environmental microbiology, 2006Co-Authors: William P Kovacik, Ken Takai, Melanie R Mormile, James P Mckinley, Fred J Brockman, James K Fredrickson, William E HolbenAbstract:A multi-level sampler (MLS) was emplaced in a borehole straddling anaerobic, sulfate-rich Cretaceous-era shale and sandstone Rock formations {approx}200 m below ground surface at Cerro Negro, New Mexico. Sterile quartzite sand contained in chambers in the sampler allowed in situ colonization and recovery of nucleic acids for molecular analyses. DGGE and 16S rRNA gene cloning results indicated a homogeneously distributed bacterial community across the shale/sandstone interface. ?-Proteobacteria sequences were common at all depths, and were dominated by members of the Geobacteraceae family (Pelobacter, Desulfuromonas, and Geobacter). Other members of this group are capable of dissimilatory Fe(III) and/or S0 reduction, but not sulfate reduction. RNA hybridization data also suggested that Fe(III)/S0 reducing bacteria were predominant. These findings are striking considering the lack of significant concentrations of these electron acceptors in this environment. The next most abundant bacterial group indicated was the sulfate reducers, including Desulfobacterium, Desulfocapsa and Desulfobulbus. Sequences related to fermenters, denitrifiers and acetogens were also recovered. The presence of a phylogenetically and functionally diverse microbial community in this deep subsurface environment likely reflects the complex nature of the primary energy and carbon sources, kerogen associated with the shale.
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molecular analysis of deep subsurface Cretaceous Rock indicates abundant fe iii and s reducing bacteria in a sulfate rich environment
Environmental Microbiology, 2006Co-Authors: William P Kovacik, Ken Takai, Melanie R Mormile, James P Mckinley, Fred J Brockman, James K Fredrickson, William E HolbenAbstract:A multi-level sampler (MLS) was emplaced in a borehole straddling anaerobic, sulfate-rich Cretaceous-era shale and sandstone Rock formations {approx}200 m below ground surface at Cerro Negro, New Mexico. Sterile quartzite sand contained in chambers in the sampler allowed in situ colonization and recovery of nucleic acids for molecular analyses. DGGE and 16S rRNA gene cloning results indicated a homogeneously distributed bacterial community across the shale/sandstone interface. ?-Proteobacteria sequences were common at all depths, and were dominated by members of the Geobacteraceae family (Pelobacter, Desulfuromonas, and Geobacter). Other members of this group are capable of dissimilatory Fe(III) and/or S0 reduction, but not sulfate reduction. RNA hybridization data also suggested that Fe(III)/S0 reducing bacteria were predominant. These findings are striking considering the lack of significant concentrations of these electron acceptors in this environment. The next most abundant bacterial group indicated was the sulfate reducers, including Desulfobacterium, Desulfocapsa and Desulfobulbus. Sequences related to fermenters, denitrifiers and acetogens were also recovered. The presence of a phylogenetically and functionally diverse microbial community in this deep subsurface environment likely reflects the complex nature of the primary energy and carbon sources, kerogen associated with the shale.
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shifts in archaeal communities associated with lithological and geochemical variations in subsurface Cretaceous Rock
Environmental Microbiology, 2003Co-Authors: Ken Takai, Melanie R Mormile, James P Mckinley, Fred J Brockman, William E Holben, William P Kovacik, James K FredricksonAbstract:Subsurface microbial community structure in relation to geochemical gradients and lithology was investigated using a combination of molecular phylogenetic and geochemical analyses. Discreet groundwater and substratum samples were obtained from depths ranging from 182 to 190 m beneath the surface at approximately 10-cm intervals using a multi-level sampler (MLS) that straddled Cretaceous shale and sandstone formations at a site in the southern San Juan Basin in New Mexico. DNA and RNA were extracted directly from quartzite sand substratum loaded into individual cells of the MLS and colonized in situ for six months. PCR-mediated T-RFLP analysis of archaeal rRNA genes (rDNA) in conjunction with partial sequencing analysis of archaeal rDNA libraries and quantitative RNA hybridization with oligonucleotide probes were used to probe community structure and function. Although total microbial populations remained relatively constant over the entire depth interval sampled, significant shifts in archaeal populations, predominantly methanogens, were observed. These shifts coincided with the geochemical transition from relatively high methane (26 mM), low sulfate (<3 mg l-1) concentrations in the region adjacent to the organic matter-rich shale to relatively low-methane (<0.5 mM), high-sulfate (48 mg l-1) conditions in the organic-poor sandstone beneath the shale. These results indicate that active, phylogenetically diverse archaealmore » communities were present in the subsurface Cretaceous Rock environment at this site and that major archaeal clades shifted dramatically over scales of tens of centimeters, corresponding to changes in the lithology and geochemical gradients.« less
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Shifts in archaeal communities associated with lithological and geochemical variations in subsurface Cretaceous Rock
Environmental microbiology, 2003Co-Authors: Ken Takai, Melanie R Mormile, James P Mckinley, Fred J Brockman, William E Holben, William P Kovacik, James K FredricksonAbstract:Subsurface microbial community structure in relation to geochemical gradients and lithology was investigated using a combination of molecular phylogenetic and geochemical analyses. Discreet groundwater and substratum samples were obtained from depths ranging from 182 to 190 m beneath the surface at approximately 10-cm intervals using a multi-level sampler (MLS) that straddled Cretaceous shale and sandstone formations at a site in the southern San Juan Basin in New Mexico. DNA and RNA were extracted directly from quartzite sand substratum loaded into individual cells of the MLS and colonized in situ for six months. PCR-mediated T-RFLP analysis of archaeal rRNA genes (rDNA) in conjunction with partial sequencing analysis of archaeal rDNA libraries and quantitative RNA hybridization with oligonucleotide probes were used to probe community structure and function. Although total microbial populations remained relatively constant over the entire depth interval sampled, significant shifts in archaeal populations, predominantly methanogens, were observed. These shifts coincided with the geochemical transition from relatively high methane (26 mM), low sulfate (
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Confined subsurface microbial communities in Cretaceous Rock
Nature, 1997Co-Authors: Lee R. Krumholz, James P Mckinley, Glenn A. Ulrich, Joseph M. SuflitaAbstract:Deep subsurface microbial communities1 are believed to be supported by organic matter that was either deposited with the formation sediments or which migrated from the surface along groundwater flowpaths. Investigation has therefore focused on the existence of microorganisms in recently deposited or highly permeable sediments2,3. Fewer reports have focused on consolidated Rocks4–7. These findings have often been limited by inadequate tracer methodology or non-sterile sampling techniques. Here we present evidence for the presence of spatially discrete microbial communities in Cretaceous Rocks and advance a mechanism for the long-term survival of these subterranean communities. Samples were collected using aseptic methods and sensitive tracers8. Our results indicate that the main energy source for these communities is organic material trapped within shales. Microbial activity in shales appears to be greatly reduced, presumably because of their restrictive pore size9. However, organic material or its fermentation products could diffuse into adjacent, more permeable sandstones, where microbial activity was much more abundant. This process resulted in the presence of microbial communities at sandstone–shale interfaces. These microorganisms presumably ferment organic matter and carry out sulphate reduction and acetogenesis.
Faycel Ferhi - One of the best experts on this subject based on the ideXlab platform.
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Basin tectonic history and paleo‐physiography of the pelagian platform, northern Tunisia, using vitrinite reflectance data
Basin Research, 2018Co-Authors: Thibault Cavailhes, Atle Rotevatn, Ståle Monstad, Atef Ben Khala, Erich Funk, Kathryn Canner, Mirko Looser, Ali Chalabi, Anna Travé, Faycel FerhiAbstract:Constraining the thermal, burial and uplift/exhumation history of sedimentary basins is crucial in the understanding of upper crustal strain evolution and also has implications for understanding the nature and timing of hydrocarbon maturation and migration. In this study, we use Vitrinite Reflectance (VR) data to elucidate the paleo-physiography and thermal history of an inverted basin in the foreland of the Atlasic orogeny in Northern Tunisia. In doing so, it is the primary aim of this study to demonstrate how VR techniques may be applied to unravel basin subsidence/uplift history of structural domains and provide valuable insights into the kinematic evolution of sedimentary basins. VR measurements of both the onshore Pelagian Platform and the Tunisian Furrow in Northern Tunisia are used to impose constraints on the deformation history of a long-lived structural feature in the studied region, namely the Zaghouan Fault. Previous work has shown that this fault was active as an extensional structure in Lower Jurassic to Aptian times, before subsequently being inverted during the Late Cretaceous Eocene Atlas I tectonic event and Upper Miocene Atlas II tectonic event. Quantifying and constraining this latter inversion stage, and shedding light on the roles of structural inheritance and the basin thermal history, are secondary aims of this study. The results of this study show that the Atlas II WNW-ESE compressive event deformed both the Pelagian Platform and the Tunisian Furrow during Tortonian-Messinian times. Maximum burial depth for the Pelagian Platform was reached during the Middle to Upper Miocene, i.e. prior to the Atlas II folding event. VR measurements indicate that the Cretaceous to Ypresian section of the Pelagian Platform was buried to a maximum burial depth of ~3 km, using a geothermal gradient of 30°C/km. Cretaceous Rock samples VR values show that the hanging wall of the Zaghouan Fault was buried to a maximum depth of
William E Holben - One of the best experts on this subject based on the ideXlab platform.
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Molecular analysis of deep subsurface Cretaceous Rock indicates abundant Fe(III)‐ and S°‐reducing bacteria in a sulfate‐rich environment
Environmental microbiology, 2006Co-Authors: William P Kovacik, Ken Takai, Melanie R Mormile, James P Mckinley, Fred J Brockman, James K Fredrickson, William E HolbenAbstract:A multi-level sampler (MLS) was emplaced in a borehole straddling anaerobic, sulfate-rich Cretaceous-era shale and sandstone Rock formations {approx}200 m below ground surface at Cerro Negro, New Mexico. Sterile quartzite sand contained in chambers in the sampler allowed in situ colonization and recovery of nucleic acids for molecular analyses. DGGE and 16S rRNA gene cloning results indicated a homogeneously distributed bacterial community across the shale/sandstone interface. ?-Proteobacteria sequences were common at all depths, and were dominated by members of the Geobacteraceae family (Pelobacter, Desulfuromonas, and Geobacter). Other members of this group are capable of dissimilatory Fe(III) and/or S0 reduction, but not sulfate reduction. RNA hybridization data also suggested that Fe(III)/S0 reducing bacteria were predominant. These findings are striking considering the lack of significant concentrations of these electron acceptors in this environment. The next most abundant bacterial group indicated was the sulfate reducers, including Desulfobacterium, Desulfocapsa and Desulfobulbus. Sequences related to fermenters, denitrifiers and acetogens were also recovered. The presence of a phylogenetically and functionally diverse microbial community in this deep subsurface environment likely reflects the complex nature of the primary energy and carbon sources, kerogen associated with the shale.
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molecular analysis of deep subsurface Cretaceous Rock indicates abundant fe iii and s reducing bacteria in a sulfate rich environment
Environmental Microbiology, 2006Co-Authors: William P Kovacik, Ken Takai, Melanie R Mormile, James P Mckinley, Fred J Brockman, James K Fredrickson, William E HolbenAbstract:A multi-level sampler (MLS) was emplaced in a borehole straddling anaerobic, sulfate-rich Cretaceous-era shale and sandstone Rock formations {approx}200 m below ground surface at Cerro Negro, New Mexico. Sterile quartzite sand contained in chambers in the sampler allowed in situ colonization and recovery of nucleic acids for molecular analyses. DGGE and 16S rRNA gene cloning results indicated a homogeneously distributed bacterial community across the shale/sandstone interface. ?-Proteobacteria sequences were common at all depths, and were dominated by members of the Geobacteraceae family (Pelobacter, Desulfuromonas, and Geobacter). Other members of this group are capable of dissimilatory Fe(III) and/or S0 reduction, but not sulfate reduction. RNA hybridization data also suggested that Fe(III)/S0 reducing bacteria were predominant. These findings are striking considering the lack of significant concentrations of these electron acceptors in this environment. The next most abundant bacterial group indicated was the sulfate reducers, including Desulfobacterium, Desulfocapsa and Desulfobulbus. Sequences related to fermenters, denitrifiers and acetogens were also recovered. The presence of a phylogenetically and functionally diverse microbial community in this deep subsurface environment likely reflects the complex nature of the primary energy and carbon sources, kerogen associated with the shale.
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shifts in archaeal communities associated with lithological and geochemical variations in subsurface Cretaceous Rock
Environmental Microbiology, 2003Co-Authors: Ken Takai, Melanie R Mormile, James P Mckinley, Fred J Brockman, William E Holben, William P Kovacik, James K FredricksonAbstract:Subsurface microbial community structure in relation to geochemical gradients and lithology was investigated using a combination of molecular phylogenetic and geochemical analyses. Discreet groundwater and substratum samples were obtained from depths ranging from 182 to 190 m beneath the surface at approximately 10-cm intervals using a multi-level sampler (MLS) that straddled Cretaceous shale and sandstone formations at a site in the southern San Juan Basin in New Mexico. DNA and RNA were extracted directly from quartzite sand substratum loaded into individual cells of the MLS and colonized in situ for six months. PCR-mediated T-RFLP analysis of archaeal rRNA genes (rDNA) in conjunction with partial sequencing analysis of archaeal rDNA libraries and quantitative RNA hybridization with oligonucleotide probes were used to probe community structure and function. Although total microbial populations remained relatively constant over the entire depth interval sampled, significant shifts in archaeal populations, predominantly methanogens, were observed. These shifts coincided with the geochemical transition from relatively high methane (26 mM), low sulfate (<3 mg l-1) concentrations in the region adjacent to the organic matter-rich shale to relatively low-methane (<0.5 mM), high-sulfate (48 mg l-1) conditions in the organic-poor sandstone beneath the shale. These results indicate that active, phylogenetically diverse archaealmore » communities were present in the subsurface Cretaceous Rock environment at this site and that major archaeal clades shifted dramatically over scales of tens of centimeters, corresponding to changes in the lithology and geochemical gradients.« less
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Shifts in archaeal communities associated with lithological and geochemical variations in subsurface Cretaceous Rock
Environmental microbiology, 2003Co-Authors: Ken Takai, Melanie R Mormile, James P Mckinley, Fred J Brockman, William E Holben, William P Kovacik, James K FredricksonAbstract:Subsurface microbial community structure in relation to geochemical gradients and lithology was investigated using a combination of molecular phylogenetic and geochemical analyses. Discreet groundwater and substratum samples were obtained from depths ranging from 182 to 190 m beneath the surface at approximately 10-cm intervals using a multi-level sampler (MLS) that straddled Cretaceous shale and sandstone formations at a site in the southern San Juan Basin in New Mexico. DNA and RNA were extracted directly from quartzite sand substratum loaded into individual cells of the MLS and colonized in situ for six months. PCR-mediated T-RFLP analysis of archaeal rRNA genes (rDNA) in conjunction with partial sequencing analysis of archaeal rDNA libraries and quantitative RNA hybridization with oligonucleotide probes were used to probe community structure and function. Although total microbial populations remained relatively constant over the entire depth interval sampled, significant shifts in archaeal populations, predominantly methanogens, were observed. These shifts coincided with the geochemical transition from relatively high methane (26 mM), low sulfate (