The Secrets of Tardigrades: The Cosmic Pokémon
They are clawed and mythically beautiful. Tardigrades, microscopic invertebrates that look like Pokémon, are creatures of Earth. But if tomorrow we were to discover life elsewhere, on another planet, we might very well find them curled up, warm or cold, in alien cocoons.
Michel Cassé, French Alternative Energies and Atomic Energy Commission (CEA) – Paris-Saclay University and Simon Galas, University of Montpellier
And for good reason. Thanks to their ability to survive in the harshest 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 carry on with their lives. In short, we are dealing with a living species that is cosmically privileged. Let us not hesitate, then, to call them intelligent, since this quality relates to the adaptive capabilities of living beings. Far superior, in any case, to robots equipped with a misnamed “artificial intelligence.”
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Life on Hold
What is the secret behind this resilience? Tardigrades have several strategies that, when combined, enable them to perform a miracle: literally coming back to life after a period of suspended animation in a state known as the anhydrobiotic state. Their bodies can thus lose all their water when the environment dries up, at which point their metabolism ceases. But after rehydration, they come back to life! This is thanks to specific proteins, or sugars, which replace the water in their bodies ( trehalose).
That’s not all. During their state of suspended animation, tardigrades can be subjected to extremely violent stressors, such as those caused by X-ray irradiation. To counteract this, proteins protect the DNA of these microorganisms (these proteins have also been identified in the human body as agents that protect kidney tissue).
Despite this, these "sleeping beauties" sometimes sustain damage. But they will be able to repair themselves when the time comes, thanks to a set of enzymes that act as molecular surgeons for DNA.
To sum up, tardigrades can employ the following strategies to ensure their survival:
- A near-total loss of body water, which limits attacks on DNA caused by water hydrolysis.
- The production of proteins capable of vitrifying the animal as it loses water. These proteins are capable of protecting it from bacteria and yeasts during desiccation.
- The production of DNA-protective proteins that act as molecular umbrellas.
- The expression of enzymes that repair DNA damage accumulated during the process of coming back to life. To be able to revive normally, tardigrades must have maintained their muscles in good working order. A molecular and athletic feat.
Tardigrades are also known for their ability to survive exposure to a vacuum. This has been tested in the laboratory, but it was a space mission that ultimately demonstrated tardigrades’ ability to withstand the harsh conditions of space.
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Space vacuum and radiation
It was in September 2007. ESA’s Foton-M3 mission and its Biopan-6 platform exposed tardigrades to the vacuum of space and to ionizing, UV, and cosmic radiation for 10 days in low Earth orbit. During this experiment, the tardigrades were exposed to radiation levels reaching up to 7,000 kilojoules per square meter—a thousand times higher than at sea level. Despite these extreme conditions, the tardigrades were able to resume normal activity after returning to Earth, and their descendants are still being raised in laboratories.
Scientists are always amazed when organisms, when experimentally exposed to hostile environments that no longer exist on our planet or are inaccessible to living things, manage to survive nonetheless. In such cases, 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 normally inaccessible to life. For example, they have survived exposure to pressures as high as 7.5 GPa (Ono et al., Journal of Physics: Conference Series 2012; 377: 012053-6; URL:). This pressure corresponds to that found at a depth of 180 km in the Earth’s mantle. Typically, proteins lose their secondary structure—and thus their function—at a pressure of 2,000 MPa, and even bacteria known to be the most stress-tolerant, such as Deinococcus radiodurans can withstand a pressure of only 600 MPa.
Let’s return to the distant cosmos. A study titled “Earth-like and Tardigrade Survey of Exoplanets” clearly indicates that tardigrades—whether in their living state or in an anhydrobiotic state—are likely to survive under the environmental conditions of exoplanets selected based on their degree of physical and chemical similarity to Earth. Of the 1,000 known species of tardigrades, some exhibit higher resistance to stress than others.
A Grand Journey Through the Cosmos
Even if it is conceivable that tardigrades could survive on these exoplanets, they would still have to get there. The question of whether tardigrades could survive a journey through space has been evaluated. This is not merely a theoretical possibility: given the many objects that leave Earth and return, tardigrades may eventually be able to participate in these human-organized voyages and thus create a new evolutionary space for their species.
One final unresolved question regarding their long journey: it has been observed that tardigrade DNA undergoes fragmentation that accumulates over the course of their time in anhydrobiosis. Even though these DNA alterations are repaired by tardigrades upon rehydration, the insertion of foreign DNA molecules during the DNA repair process raises the question of whether they can acquire fragments of foreign genomes and thus undergo genetic modification. This specific 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, beneath the Earth, and in the heavens, tardigrades go about their business; we offer our probable successors our respects 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.