11:45 AM - 12:00 PM
[BBG02-10] Rock properties associated with microbial colonization in the deep granitic subsurface
Keywords:Deep biosphere, Mizunami URL, Chlorite-sericite hydrothermal alteration, Earth's early biosphere
Granite is the parent rock of the oldest rock in existence, and has existed since 4 billion years ago. Deep subsurface granite, where organic matter and oxidants derived from photosynthesis from the surface are extremely limited, could be similar to the environment before the emergence of photosynthetic organisms. Groundwater-based researches have been extensively performed, but there are few microbial ecology researches using rocks due to the problem of contamination during excavation and the difficulty of analyzing microorganisms inside rocks. In this study, contamination was minimized by excavating deep subsurface granite from underground tunnels.
During drilling from the 400-m deep underground tunnel at the Mizunami URL, drilling water that contaminates the cracks was sterilized by UV irradiation. A rock core sample collected from 17.35-43 meters along borehole (mabh) was analyzed because both ends were cracked. The core sample was divided into the crack surface and the outer edge and then powdered. The powder sample was DNA stained with SYBR Green I and the total cell count was measured by observation under a fluorescent microscope. Mineralogical analysis was also performed by obtaining X-ray diffraction (XRD) patterns. A portion of the undivided sample was cut into 3-mm thick slices using a diamond bad saw and observed under a fluorescent microscope after staining with SYBR Green I. Photothermal infrared spectroscopy (O-PTIR) was then performed on the cell-stained areas of the slices. Observation and elemental composition analysis were performed using a scanning electron microscope with an energy-dispersive X-ray spectroscopy (SEM-EDS) and a field-emission electron probe microanalyzer (FE-EPMA).
The total cell numbers were significantly higher in the cracks (2.1±0.8×107 cells/g) than in the outer edge (5.2±0.6×106 cells/g), suggesting that the microorganisms found in the cracks are not contamination during excavation. XRD analysis revealed that the main minerals of the granite are quartz, albite, and potassium feldspar, and the basal spacing of the oriented samples revealed that chlorite and mica are present. Observation of thin sections after staining with SYBR Green I confirmed that microorganisms were densely packed in the colored minerals near the crack surfaces. However, no microorganisms were found in the colored minerals at the outer edges or the center of the thin sections. It was therefore inferred that the microorganisms densely packed in the colored minerals at the cracks were not contamination during excavation or thin section preparation. O-PTIR analysis of the cracks where DNA-stained microorganisms were observed detected amide peaks, confirming the presence of microbial cells. SEM-EDS and FE-EPMA observations and analyses showed that the colored minerals were chlorite, and sericite, a fine-grained muscovite. High-spatial resolution elemental mapping by FE-EPMA revealed that S and Ca were distributed locally in voids associated with chlorite and sericite. The state of S could be elemental sulfur (S), whereas Ca is likely originated from calcium carbonate. In addtion, scanning soft X-ray fluorescence microscopic analysis confirmed the mienral phases of chlorite and sericite and the presence of Fe(III) in chlorite.
This study revealed that chlorite formed in the cracks of deep subsurface granite is densely populated by indigenous microorganisms that are not resulted from contamination. On the early Earth, hydrothermal activity after the intrusion of granitic magma is thought to have universally formed Fe(III)-rich chlorite and also supplied hydrogen sulfide to produce elemental sulfur. Thus, it is suggested that granite has been habitable for 4 billion years on Earth.
Sampling for this study was conducted by collaborative effrots with Mizunami URL operated by Japan Atomic Energy Agency (JAEA).
During drilling from the 400-m deep underground tunnel at the Mizunami URL, drilling water that contaminates the cracks was sterilized by UV irradiation. A rock core sample collected from 17.35-43 meters along borehole (mabh) was analyzed because both ends were cracked. The core sample was divided into the crack surface and the outer edge and then powdered. The powder sample was DNA stained with SYBR Green I and the total cell count was measured by observation under a fluorescent microscope. Mineralogical analysis was also performed by obtaining X-ray diffraction (XRD) patterns. A portion of the undivided sample was cut into 3-mm thick slices using a diamond bad saw and observed under a fluorescent microscope after staining with SYBR Green I. Photothermal infrared spectroscopy (O-PTIR) was then performed on the cell-stained areas of the slices. Observation and elemental composition analysis were performed using a scanning electron microscope with an energy-dispersive X-ray spectroscopy (SEM-EDS) and a field-emission electron probe microanalyzer (FE-EPMA).
The total cell numbers were significantly higher in the cracks (2.1±0.8×107 cells/g) than in the outer edge (5.2±0.6×106 cells/g), suggesting that the microorganisms found in the cracks are not contamination during excavation. XRD analysis revealed that the main minerals of the granite are quartz, albite, and potassium feldspar, and the basal spacing of the oriented samples revealed that chlorite and mica are present. Observation of thin sections after staining with SYBR Green I confirmed that microorganisms were densely packed in the colored minerals near the crack surfaces. However, no microorganisms were found in the colored minerals at the outer edges or the center of the thin sections. It was therefore inferred that the microorganisms densely packed in the colored minerals at the cracks were not contamination during excavation or thin section preparation. O-PTIR analysis of the cracks where DNA-stained microorganisms were observed detected amide peaks, confirming the presence of microbial cells. SEM-EDS and FE-EPMA observations and analyses showed that the colored minerals were chlorite, and sericite, a fine-grained muscovite. High-spatial resolution elemental mapping by FE-EPMA revealed that S and Ca were distributed locally in voids associated with chlorite and sericite. The state of S could be elemental sulfur (S), whereas Ca is likely originated from calcium carbonate. In addtion, scanning soft X-ray fluorescence microscopic analysis confirmed the mienral phases of chlorite and sericite and the presence of Fe(III) in chlorite.
This study revealed that chlorite formed in the cracks of deep subsurface granite is densely populated by indigenous microorganisms that are not resulted from contamination. On the early Earth, hydrothermal activity after the intrusion of granitic magma is thought to have universally formed Fe(III)-rich chlorite and also supplied hydrogen sulfide to produce elemental sulfur. Thus, it is suggested that granite has been habitable for 4 billion years on Earth.
Sampling for this study was conducted by collaborative effrots with Mizunami URL operated by Japan Atomic Energy Agency (JAEA).