A microbial buffer zone defined by a discontinuity in prokaryote vertical distribution in a deep groundwater system in Japan sustained by paleo-seawater on a million-year timescale

We obtained samples from a borehole (depth > 1,200 m) drilled in a sedimentary geological setting in Horonobe, Hokkaido, Japan, to assess the prokaryotes inhabiting the deep terrestrial subsurface environment. The examined borehole is located in a coastal area, where paleo-seawater with an age of millions of years is preserved below ca. 800 m depth. A diffusion zone is interposed between an upper zone with meteoric water and a connate water zone containing paleo-seawater.