[B-BG02] Geo-Bio Interactions and its Applications

[BBG02-P04] Altiarchaeota-dominated veins in siliceous mudstone with residual fossil seawater from the deep terrestrial subsurface

*Aki Yoshida¹, Mariko Kouduka¹, Kazuya Miyakawa^2,3, Hikari Beppu³, Yuki Amano^2,3, Yohey Suzuki¹ (1.The university of Tokyo, 2.Horonobe Underground Research Center, Japan Atomic Energy Agency, 3.Nuclear Fuel Cycle Engineering Laboratories, Japan Atomic Energy Agency)

Keywords:Deep biosphere, Subsurface microbiome, Drilling contamination control with microspheres, Terrestrial deep drilling in Horonobe, Candidatus_Altiarchaeum

The deep terrestrial subsurface is estimated to harbor most of prokaryotic cells on Earth. Subsurface microbial ecosystem has been accessed by drilling and groundwater sampling from deep boreholes where high rock permeability renders flushing of drill fluids with outflowing groundwater. From low permeability rock, it is technically difficult to obtain pristine groundwater nearly depleted in contamination from drill fluids suitable for microbiological studies. Although drill cores need to be analyzed, there are few studies successfully characterizing indigenous microbes in the rock interior by molecular phylogenetic analysis. Thus, microbial communities thriving in low permeability rock widely distributed in the deep terrestrial subsurface are largely unknown. In this study, microbiological analysis of a ~500-m deep drill core sample was attempted for low permeability rock associated with fossil seawater in Neogene siliceous mudstone where 500-m deep drilling was conducted from the land surface using fluorescent microspheres for contamination control near Horononbe Underground Research Laboratory (URL), northern Japan. Microscopic observation of the rock core sample stained with SYBR Green I revealed numerous veins where the intrusion of fluorescence microspheres was limited at the core rim. Fourier Transform Infrared microscopy analysis of the interior vein surface that were collected using 30-μm double-sided tape confirmed the dense colonization of microbial cells. Surfaces of veins and the core exterior were scratched and subjected to 16S rRNA gene sequence analysis. Candidatus_Altiarchaeum known for chemoautotrophy was dominant on the vein surface with the relatively minor methanogenic and methane-oxidizing archaea. In contrast, Thiobacillus spp. were dominant at the core exterior, which clarified that the archaeal populations were indigenous. Phylogenetically identical to nearly complete archaeal genomes obtained from groundwater samples from high-permeability borehole intervals from the Horonobe URL rather than those from the land surface in Horonobe area. Particularly, Candidatus_Altiarchaeum detected in this study is classified within the Alti-1 clade, which has been detected in circulating fluids rather than rocks originating from the deep terrestrial subsurface in Europe and North America. Maximum salinities reported for the Alti-1 clade to date are ~150 mM, whereas the porewater salinity was 285 mM at the adjacent depth. These results extended the habitability of Candidatus_Altiarchaeum to low-permeability sedimentary rocks with stagnant fluids. Given that such a rocky environment is widespread in the deep terrestrial subsurface, Candidatus_Altiarchaeum could be more prevalent than previously demonstrated.

This study was undertaken as part of an R&D supporting program titled “Research and development on Groundwater Flow Evaluation Technology in Bedrock (2021 FY)”, “The project for validating near-field system assessment methodology in geological disposal”, and “Development of the Technology for Integrating Radionuclide Migration Assessments (2022 and 2023 FY)” under contract to the Ministry of Economy, Trade and Industry (Grant Number: JPJ007597).