The expedition that opens a new window on the life of the abys

At the threshold of the abyss, the darkness is total, the temperature hardly exceeds 2°C and the pressure is 400 times higher than the one we bear on the surface.

This abyssal landscape seems so harsh and so austere that one would think one was projected on a planet hostile to life. "The diversity of life forms is however surprising", says biologist Pascal Hingamp of the Mediterranean Institute of Oceanology (CNRS, IRD, Aix-Marseille University, University of Toulon). The latter speaks all the better for having just co-authored a remarkable article published in the journal Nature Communications.

The initiative for this "unprecedented window on the life of the abyss" was taken by a Spanish team led by his colleague Silvia Acinas (Institute of Marine Sciences, Barcelona) in connection with the Malaspina expedition: thanks to this laboratory boat, scientists were able to take 58 precious samples of this life of the abyss in all the tropical and sub-tropical seas of the globe. No fish or crustaceans in these samples taken at a depth of 4,000 meters... but plankton, these microscopic and mostly single-celled beings.

A long term work has then started in order to "make speak" the planktonic DNA of all these samples. The 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. On the one hand: "We did not suspect to what extent the composition of this plankton, largely made up of bacteria, could differ from one place to another, and even between two sites that are quite close," says Pascal Hingamp.

In fact, although the first analyses of DNA in the abyss had already shed some light, this is the first time that oceanologists have such an atlas that allows them to appreciate this diversity on a global scale. "This is all the more surprising since living conditions may seem relatively homogeneous at this depth, regardless of where you are," he says.

"Organic snow"

On the other hand, by reconstructing, gene by gene, the metabolisms present within these planktonic communities, scientists have been able to find clues that could explain a previously troubling observation: "Measurements of the level of respiration already suggested an activity of planktonic populations more important than the nutritive resources they receive," he recalls. These communities are supposed to feed on the "organic snow" that falls on them from the surface - in other words, the waste products of other marine life. However, scientists have confirmed that this plankton is indeed endowed with a metabolism capable of making its "honey" from local inorganic matter in order to fix the precious carbon - a fundamental atom of living processes. "And we are still far from having explored all the richness of these data," says Pascal Hingamp.