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Abstract:
Over the past 20 years, scientific drilling into sediments and basaltic crust all over the world ocean has revealed the omnipresence of microscopic life deep beneath the seafloor. Diverse communities of prokaryotic cells have been discovered in sediments and rock reaching a subsurface depth of 1 km. Most of these microorganisms have no cultured or known relatives in the surface world and are still only characterized by the genetic code of their DNA. Recent studies (1–4) have shed light on the ways in which they differ from microorganisms in the surface world and on the energy sources that support life in this buried ecosystem.
About 20 years ago, R. John Parkes and Barry Cragg started to systematically enumerate microorganisms in deep cores (5). Much later, rigorous contamination tests performed on the drill ship (6)showed that the cells detected were indeed indigenous to the deep subsurface. The cell counts were used for a bold extrapolation to the global ocean floor. The astonishing conclusion was that this “unseen majority” of microorganisms accounts for 55 to 85% of Earth's prokaryotic biomass and about 30% of the total living biomass (7).
The first drilling expedition focused entirely on deep biosphere exploration was launched in 2002 by the Ocean Drilling Program (ODP, Leg 201) (1). The target was the eastern tropical Pacific, with sites ranging from the continental shelf to ocean depths of 5000 m. By drilling through the seafloor an—at open-ocean site—down to the basaltic crust, sediments with ages up to 35 million years old could be sampled (8).
