Prokaryotic cells of the deep sub-seafloor biosphere identified as living bacteria

Digging deep for bacteria Sediments beneath the seafloor do not sound a promising habitat, yet it has been estimated from microscopic cell counts that sub-seafloor sediments account for over half of all eukaryotic cells on Earth. That figure does not differentiate live cells from dead, but a new study based on ribosomal RNA detection does. The data suggest that many of the cells are alive, even in 16-million-year-old sediments 400 metres beneath the seabed. All the detectable living cells are bacteria, and they seem to be thriving as they have a turnover rate comparable with those seen in surface sediments. Chemical analyses of the pore waters from hundreds of deep ocean sediment cores have over decades provided evidence for ongoing processes that require biological catalysis by prokaryotes 1 , 2 , 3 . This sub-seafloor activity of microorganisms may influence the surface Earth by changing the chemistry of the ocean and by triggering the emission of methane, with consequences for the marine carbon cycle and even the global climate 4 , 5 , 6 . Despite the fact that only about 1% of the total marine primary production of organic carbon is available for deep-sea microorganisms 7 , 8 , sub-seafloor sediments harbour over half of all prokaryotic cells on Earth 7 . This estimation has been calculated from numerous microscopic cell counts in sediment cores of the Ocean Drilling Program 1 , 9 . Because these counts cannot differentiate between dead and alive cells, the population size of living microorganisms is unknown 10 , 11 . Here, using ribosomal RNA as a target for the technique known as catalysed reporter deposition-fluorescence in situ hybridization (CARD-FISH), we provide direct quantification of live cells as defined by the presence of ribosomes. We show that a large fraction of the sub-seafloor prokaryotes is alive, even in very old (16 million yr) and deep (> 400 m) sediments. All detectable living cells belong to the Bacteria and have turnover times of 0.25–22 yr, comparable to surface sediments.