Under nearly 3,000 meters of water, off the coast of the Mariana Trench, on a volcanic site, researchers from the University of Bremen discovered a mysterious slate blue mud mixed with mineral green. Its chemical composition is so alkaline that it would burn almost any organic matter that came into contact with it.
However, this does not prevent thousands of micro-organisms from living there in complete peace and quiet; extremophilic bacteria and archaea so well adapted that they have made this toxic environment a haven of peace. By studying them, this team of scientists, led by Palash Kumawat, hopes to better understand how life could emerge in such an environment. Their study about this strange discovery was published this summer, on August 13, in the journal Communications Earth & Environment.
An extreme refuge
The samples, taken during the R/V Sonne SO292/2 expedition in 2022, come from the extreme depths of one of the most inaccessible areas of the Pacific Ocean. They were extracted from Pacman Seamount, a mud volcano nestled in the northwest area of the Mariana Trench, where pressure exceeds 300 times that of Earth’s atmosphere. Plunged into permanent darkness, the area is constantly swept by violent currents and remains one of the last unexplored territories on the planetlocated days of navigation from any inhabited coast. In short, it is not good to live there, except for rare creatures.
It is from this volcano that this mud of a striking color gushes: a mixture of serpentinite and of brucitetwo minerals resulting from the transformation of the earth’s mantle in contact with sea water. Its pH (hydrogen potential) is twelve, one of the highest ever measured in nature. At this alkalinity level, the liquid is so basic that it would instantly dissolve fats and proteins from the skin, causing a chemical burn comparable to that of concentrated detergent.
However, the researchers found lipid traces there, coming from bacterial and archaeal cell membranes which proliferate there. These molecules, which remained surprisingly intact despite the causticity of the environment, are proof that several microbial communities currently live there.
For Florence Schubotz, organic geochemist and co-author of the study, this discovery is “ simply exciting, because we suspect that primitive life could have emerged precisely in this type of site “. The period to which Schubotz refers corresponds to Late Hadean and Early Archaean (between 4.2 and 3.5 billion years ago)when the first cell membranes could have formed in hydrothermal environments comparable to that of the Pacman Seamount.
How do these organisms survive?
If these microbes manage to survive where any living being would perish, it is because they are able to draw their energy from chemical reactions, and not from light, which never penetrates these depths. A way of life said chemosyntheticwhich allows them, even in the absence of organic matter, to exploit the methane (CH₄) and sulfate (H₂SO₄) dissolved in the mud to produce energy. When consuming these substances, they release large quantities of hydrogen sulfide (H₂S), a highly toxic gas.
These organisms are also members of the two major domains of life capable of tolerating such extreme conditions: bacteria and archaea. The lipids contained in their cell membranes, very resistant, are their “ first line of defense ”, allowing them to survive in an environment that, for most other organisms, would be lethal. « Until now, the presence of methane-producing microorganisms in this system was presumed, but could not be directly confirmed », specifies Florence Schubotz.
They are, moreover, extremely numerous, since, according to estimates, they represent 15% of the total biomass of our planet. All humanity is just dust compared to it, because we barely represent 0.01% of it and all other animals approximately 0.4%. Our species therefore weighs almost nothing, biologically speaking; but we are the only ones to have disrupted in barely a century the activity of these billions of organisms. Without them, Earth would probably just be a barren planet, without this underground network which circulates gases and nutrients since the dawn of time. By exploring these environments, Palash Kumawat and his team hope to obtain clues about the conditions that allowed life to emerge on early Earthwhich was nowhere near as hospitable as it is today.
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