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Almost three quarters of terrestrial microbes hide in deep rock

Hundreds of international researchers who are members of the Deep Carbon Observatory, including Bénédicte Ménez and Emmanuelle Gérard from the Institut de Physique du Globe de Paris, published the sum total of their work on 10 December at the AGU 2018, estimating that deep life would represent a mass of 15 to 23 billion tonnes of carbon, several hundred times more than that of 7 billion human beings.

Almost three quarters of terrestrial microbes hide in deep rock

Publication date: 18/12/2018

Press, Research

Related themes : Origins

Nematode (eukaryote) in a biofilm of micro-organisms. This unidentified nematode (Poikilolaimus sp.) from the Kopanang gold mine in South Africa lives 1.4 km below the surface. © Gaetan Borgonie (Extreme Life Isyensya, Belgium)

Around 70% of the Earth’s microbes live deep underground, in rocks that were once considered sterile, but where bacteria and other micro-organisms abound. For the first time, researchers have estimated the extent of this deep or “intraterrestrial” life. Following hundreds of drillings beneath continents and oceans, the researchers in this ten-year international collaboration have found that these organisms live kilometres below the surface, in the Earth’s crust, and have apparently evolved separately from surface life. A Japanese ship drilled 2.5 km below the ocean floor, itself 1.2 km below the surface of the oceans, capturing in its cores never-before-seen microbes living in a layer of sediment 20 million years old.

These microbes are mainly bacteria and archaea, single-celled micro-organisms. Some are “zombies” of sorts: they use all their energy to survive, without any activity, in pockets isolated from the surface since time immemorial, tens of millions of years or more. Subjected to extraordinary pressure and deprived of nutrients, they do not reproduce and have no activity other than repairing themselves.

Other, more active micro-organisms fascinate geobiologists because they function in a system that has nothing to do with the surface, where the entire food chain depends on photosynthesis, which makes plants grow and provides food for a range of organisms. These bacteria draw their energy from the rocks that are being altered, a process known as chemosynthesis.

The absolute limits in terms of pressure, temperature and energy for life to be possible remain a mystery. To date, the observed record for heat resistance belongs to a unicellular organism called Geogemma barossii, found in hydrothermal springs deep in the oceans. It lives, grows and replicates at 121°C (21° above the boiling point of water).

Imagining these deep environments teeming with life gives us food for thought about the way we exploit our subsoil, long thought to be devoid of life. Underground storage operations have rarely considered the potential role that these micro-organisms could play in storing greenhouse gases such asCO2, methane, hydrogen or radioactive waste.

Candidatus Desulforudis was found living in a fluid- and gas-filled fault 2.8 km below the Earth's surface in the Mponeng gold mine near Johannesburg, South Africa. Surprisingly, the scientists found no other organisms in their samples, making this deep ecosystem the first to be discovered on Earth with a single species. © Greg Wanger (California Institute of Technology, USA) and Gordon Southam (The University of Queensland, Australia)

However, a recent study by the IPGP geomicrobiology team has demonstrated that deep-sea ecosystems are highly reactive to gas injections, influencing the fate of the carbon injected, interactions with the rock and even clogging the porosity of the reservoir.

There are still many mysteries to be unravelled about these subterranean microbes:

  • Their movements: how do microbes spread underground? How can identical specimens be found in the four corners of the globe? Have they been dispersed by major geological events (plate tectonics, earthquakes, meteorite bombardments, etc.) or are they on the move themselves?
  • About their origins: did life begin in the depths of the Earth and then migrate to the surface, or is it the other way round? How have “zombie” microbes survived for so long without reproducing?
  • Their energy: What are their main sources of energy and carbon, methane, hydrogen, natural radiation, etc.?



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