The secrets of tardigrades, those cosmic Pokémon
They are clawed and mythologically beautiful. Tardigrades, microscopic invertebrates resembling Pokémon, are terrestrial creatures. But if tomorrow we discovered life elsewhere, on another planet, we might well find them curled up warm, or cold, in extraterrestrial cocoons.
Michel Cassé, French Alternative Energies and Atomic Energy Commission (CEA) – Paris-Saclay University and Simon Galas, University of Montpellier
And with good reason. Thanks to their ability to survive in the most hostile conditions and adapt to the most extreme environmental fluctuations, these organisms take the art of survival to new heights. On Earth, tardigrades can withstand anything: even when frozen, boiled, dried out, irradiated, crushed, or poisoned, they continue to live their lives. In short, we are dealing with a cosmically privileged living species. Let's not hesitate, then, to describe them as intelligent, since this quality is relative to the adaptive abilities of living beings. In any case, they are far superior to robots equipped with what is misnamed artificial intelligence.
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Life on hold
What is the secret behind this resilience? Tardigrades have several strategies which, when combined, enable them to perform a marvel: literally coming back to life after a period of suspended animation in a living crystal state, known in science as anhydrobiotic state. Their bodies can lose all their water when the environment dries out, at which point they have no metabolism. But after rehydration, they come back to life! This is thanks to specific proteins, or sugars, which replace water in their bodies ( trehalose).
That's not all. During their state of suspended animation, tardigrades can be
subjected to extremely violent attacks, such as those produced by X-ray irradiation. To counteract this, proteins protect the DNA of these microorganisms (they have also been identified in the human body as protective agents of kidney tissue).
Despite this, these beautiful sleepers can sometimes suffer damage. But they will be able to repair themselves when the time comes, thanks to a set of enzymes that act like molecular DNA surgeons.
To sum up, tardigrades can use the following strategies to survive:
- An almost total loss of water from the body, which limits attacks on DNA by water hydrolysis.
- The production of proteins capable of vitrifying the animal as it loses water. These proteins protect it from bacteria and yeast during desiccation.
- The production of DNA-protecting proteins that act as molecular umbrellas.
- The expression of enzymes that repair damage accumulated by DNA during reanimation. In order to be able to revive normally, tardigrades need to have kept their muscles in good working order. A molecular and athletic feat.
Tardigrades are also known for their ability to survive exposure to vacuum. This has been tested in the laboratory, but it was a space mission that ultimately demonstrated the tardigrades' ability to withstand the hostility of the cosmos.
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Space vacuum and radiation
It was September 2007. ESA's Foton-M3 mission and its Biopan-6 platform exposed tardigrades to the vacuum of space and ionizing solar, UV, and cosmic radiation for 10 days in low Earth orbit. During this experiment, the tardigrades were exposed to radiation levels of up to 7,000 kilojoules/square meter, a thousand times higher than at sea level. Despite these extreme conditions, the tardigrades were able to recover normal activity after their return to Earth, and their descendants are still being bred in laboratories.
Scientists are always surprised when organisms that are experimentally subjected to hostile environments that no longer exist on our planet or are inaccessible to living beings nevertheless resist them. In the latter case, the question of the origin and evolution of these extreme resistances often becomes a fundamental one. Today, tardigrades have demonstrated to scientists their incredible ability to withstand stresses that are normally inaccessible. For example, they have survived exposure to pressures of up to 7.5 GPa (Ono et al., Journal of Physics: Conference Series 2012; 377: 012053-6p.; URL:). This pressure corresponds to that prevailing at a depth of 180 km in the Earth's mantle. Typically, proteins lose their secondary structure, and therefore their function, at a pressure of 2,000 MPa, and bacteria known to be the most resistant to stress, such as Deinococcus radiodurans can withstand a pressure of only 600 MPa.
Let's return to the distant cosmos. A publication entitled "Earth-like and tardigrade survey of exoplanets" clearly indicates that tardigrades, whether alive or in an anhydrobiotic state, are likely to survive in the environmental conditions of exoplanets selected on the basis of their degree of physical and chemical similarities to Earth. Of the 1,000 known species of tardigrades, some are more resistant to stress than others.
A great journey into the cosmos
Even if the possible survival of tardigrades on these exoplanets is conceivable, they still have to get there. The question of whether tardigrades could survive a journey through space has been evaluated. This is not just a theoretical possibility: with numerous objects leaving Earth and returning, tardigrades may be able to participate in these human-organized journeys and thus create a new space for their species to evolve.
Last open question for the big trip: it has been observed that tardigrade DNA undergoes fragmentation that accumulates over time spent in anhydrobiosis. Even if these DNA alterations are repaired by tardigrades when they wake up through rehydration, the insertion of foreign DNA molecules during the DNA repair process raises the question of the possibility of acquiring foreign genome fragments and thus modifying species. This particular acquisition process is known as horizontal gene transfer (HGT) and allows certain genomes to evolve without going through sexual reproduction (vertical gene transfer).
On Earth, underground, and as in heaven, tardigrades officiate and toil; we offer our probable successors our homage and compliments.
Michel Cassé, Astrophysicist and writer, French Alternative Energies and Atomic Energy Commission (CEA) – Paris-Saclay University and Simon Galas, Professor of Genetics and Molecular Biology of Aging, CNRS – Faculty of Pharmacy, University of Montpellier
The original version of this article was published on The Conversation.