Spatial distribution of biofilm-forming Archaea in the deep sedimentary rock environment of the Horonobe area, Hokkaido, Japan

Amano, Yuki   ; Beppu, Hikari*; Sato, Tomofumi*; Mochizuki, Akihito  ; Thomas, B. C.*; Banfield, J. F.*

The presence of microbes in deep subsurface environments may have important implications for the safety of underground repositories for high-level radioactive wastes. It has been suggested that biofilm may not exist in the deep subsurface due to low water flux, energy supply and physical space. We investigated spatial distribution of biofilm-forming Archaea, Candidatus Altiarchaeales, and the geochemical and microbiological properties of groundwater samples via study of boreholes drilled in the Horonobe Underground Research Laboratories (Horonobe URL), Horonobe area, Hokkaido, Japan. Samples were collected from four boreholes at 140, 250, 350 m below the surface in the Horonobe URL, and the time series changes of geochemistry, microbial abundance, and microbial diversity were monitored over several years. Analysis of metagenome and 16S rRNA genes by next generation sequencing indicates that the biofilms in methane- carbonate-rich groundwater were mainly dominated by Candidatus Altiarchaeales. Based on the section volume and the amount of discharged groundwater from the borehole, it is suggested that the biofilms occur within porous sedimentary rocks or fracture surfaces, rather than on the borehole walls. The biofilm accumulates heavy metals including Cr, Fe, Ni, Cu, Pb, Ce, Nd, U and Th. From the calculation of the bioaccumulation factor for the concentration of heavy metals, it is possible that the biofilms selectively accumulate heavy metals. Thus, the presence of biofilms provides a mechanism for retardation of radionuclide migration in the subsurface. This is of practical interest from the perspective of geological disposal systems for high-level radioactive waste in the deep subsurface.