At the threshold of the abyss, there is total darkness, the temperature barely exceeds 2°C and the pressure is 400 times higher than that which we withstand on the surface.
This abyssal landscape seems so harsh and so austere that you might think you were on a planet hostile to life. "Yet the diversity of life forms is astonishing," says biologist Pascal Hingamp from the Institut Méditerranéen d'Océanologie (CNRS, IRD, Aix-Marseille University, Université de Toulon). He has just co-authored a remarkable article published in the journal Nature Communications.
The initiative for this "unprecedented window on life in the deep" came from a Spanish team led by Silvia Acinas (Institute of Marine Sciences, Barcelona) in conjunction with the Malaspina expedition. Thanks to this laboratory ship, scientists were able to take 58 precious samples of life in the deep from all over the world's tropical and sub-tropical seas. There were no fish or crustaceans in the samples taken at a depth of 4,000 metres... just plankton, microscopic, mostly unicellular creatures.
Long-term work then began to "make the planktonic DNA of all these samples speak". Analysis of these 58 "metagenomes" - the name given by scientists to all the DNA from millions of plankton genomes sequenced in bulk - revealed two remarkable aspects of this singular life. Firstly: "We had no idea the extent to which the composition of this plankton, which is largely made up of bacteria, could differ from one place to another, and even between two fairly close sites," says Pascal Hingamp with surprise.
While initial analyses of DNA in the abyss have already shed some light on the subject, this is the first time that oceanologists have had access to such an atlas, enabling them to assess this diversity on a global scale. "This is all the more surprising given that living conditions may seem relatively homogeneous at this depth, wherever you are", he points out.
"Organic snow
Furthermore, by reconstructing the metabolisms present within these planktonic communities, gene by gene, the scientists were able to find clues that could explain a hitherto troubling observation: "Measurements of the level of respiration already suggested that the planktonic populations were more active than the nutritional resources they received", he points out. These communities are supposed to feed on the 'organic snow' that falls on them from the surface - in other words, the waste from other forms of marine life. However, scientists have confirmed that this plankton does indeed have a metabolism capable of making its 'honey' from local inorganic matter in order to fix precious carbon - the fundamental atom in living processes. "And we are still a long way from having explored all the richness of these data", assures Pascal Hingamp.