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  Nearly three-quarters of terrestrial microbes hide in deep rocks

Hundreds of international researchers who are members of the Deep Carbon Observatory, including Bénédicte Ménez and Emmanuelle Gérard of IPGP, published on December 10 on the occasion of AGU 2018 the sum of their work, estimating that deep life would represent a mass of 15 to 23 billion tons of carbon, several hundred times more than that of the 7 billion human beings.


Nematode (eukaryotic) in a biofilm of microorganisms. 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)

About 70% of the Earth's microbes live in its depths, in rocks that were once considered sterile but where bacteria and other microorganisms abound. For the first time, researchers have estimated the extent of this deep or "intraterrestrial" life. After hundreds of boreholes under the continents and oceans, researchers in this 10-year international collaboration have found that these organisms live miles below the surface in the earth's crust and have apparently evolved separately from life on the surface. A Japanese ship drilled 2.5 km below the ocean floor, itself 1.2 km below the ocean surface, capturing in its cores microbes never before observed and living in a 20 million year old sediment layer.

These microbes are mainly bacteria and archaea, unicellular microorganisms. Some are in a way "zombies": they use all their energy to survive, without any activity whatsoever, in pockets isolated from the surface since ancient times, for tens of millions of years or more. Under extraordinary pressure and without nutrients, they do not reproduce and have no activity other than repairing themselves.


Other, more active microorganisms fascinate geobiologists because they operate in a system that has nothing to do with the surface where the entire food chain depends on photosynthesis, which grows plants and allows a group of organisms to feed themselves. These bacteria get their energy from the rocks that deteriorate, this is called chemosynthesis.


Candidatus Desulforudis was found alive 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, 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)

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 has been set by a single-celled organism called Geogemma barossii, found in hydrothermal springs in the deep 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 pause to reflect on the exploitation we make of our subsoil, long considered lifeless. Underground storage operations have rarely considered the potential role that these micro-organisms could play in the storage of greenhouse gases such as CO2, methane, hydrogen or radioactive waste. However, a recent study conducted by the IPGP geomicrobiology team demonstrated the very high reactivity of deep ecosystems to gas injections, influencing the fate of the injected carbon, interactions with the rock and even leading to clogging of the porosity of the reservoir.


Many questions still need to be answered about these underground microbes:

  • On their travels: how do microbes spread in the subsurface? How can identical specimens be found all over the world? Have they been dispersed following major geological events (plate tectonics, earthquakes, meteorite bombardments...) or do they move themselves?
  • On their origins: did life begin in the depths of the Earth and then migrate to the surface or is it the other way around? How have "zombie" germs survived for so long without reproducing?
  • On their energy: What are their main sources of energy and carbon, methane, hydrogen, natural radiation...?


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Contact : 

Bénédicte Ménez, IPGP geomicrobiology team

Emmanuelle Gérard, IPGP geomicrobiology team

Date de publication : 
18 December 2018