Microbial communities are found living in a variety of habitats both on the surface and in the sub-surface. But a finding reported in Science recently found microbes living in coals 2500 m below the seafloor. What was most unique about these microbes, was their community affinity with those normally found in forest soils. This suggests that these microbial communities, which are now 2500 m below the seafloor, have been an isolated ecosystem since these forest were growing and buried tens of millions of years ago.
Microbial life inhabits deeply buried marine sediments, but the extent of this vast ecosystem remains poorly constrained. Here we provide evidence for the existence of microbial communities in ~40° to 60°C sediment associated with lignite coal beds at ~1.5 to 2.5 km below the seafloor in the Pacific Ocean off Japan. Microbial methanogenesis was indicated by the isotopic compositions of methane and carbon dioxide, biomarkers, cultivation data, and gas compositions. Concentrations of indigenous microbial cells below 1.5 km ranged from <10 to ~104 cells cm−3. Peak concentrations occurred in lignite layers, where communities differed markedly from shallower subseafloor communities and instead resembled organotrophic communities in forest soils. This suggests that terrigenous sediments retain indigenous community members tens of millions of years after burial in the seabed.