By Nicole Tolmachev

As far as we know, there are no other planets where life could possibly flourish as much as here on Earth. Most of the planets that meet the condition of a “hospitable” environment are hundreds and millions of light-years away. The answer though could lie right at our own doorstep. Other planets or their moons of our solar system are also contesters in this competition for the search of another Earth. If there only was a way to actually check whether the speculations that we have are true… Well, if space ships, robots or satellites can travel to these heavenly worlds, they could take these microscopic creatures with them, as they are so resistant to any harmful environment that they are considered immortal by some people.

These little wonders of nature are called tardigrades.

What are tardigrades?

Tardigrades (a.k.a water bears or moss piglets), are moulting animals. At 0.1 to 1.5mm long, they have four sets of legs and move around clumsily-hence their more common names. Water bears have a chubby, cylindrical body consisting of four body segments plus a head segment.

An interesting fact is that tardigrades don’t have any form of respiratory organs, like lungs. They breathe through their skin, which means that the gas exchange takes place via diffusion. But there’s one condition: There has to be a fine water film on their skin, so the gas exchange can occur. Therefore, tardigrades prefer moist habitats.

Even though they prefer moist habitats, moss piglets can be found on all continents including Antarctica and in the ocean. They are everywhere: inside of middle European rain gutters, arctic pools, jungles all over the world, in the Himalayan Mountains and even deep down in the abyssal zone of the Indian Ocean. Because tardigrades are so widespread, they are divided into the three following kinds: marine (meaning oceanic), limnic (meaning living in freshwater) and terrestrial (meaning land-living). The terrestrial and limnic kinds are often combined with the limno-terrestrial species.

Moss piglets are mostly vegetarian and eat algae cells, organic waste products, and fungal spores, but there also are carnivores among them. If a water bear is carnivorous it feeds on protozoa, rotifers, nematodes and even others of its kind. Although it’s rarer than the cases above, there are tardigrades, that live as endoparasites. All tardigrades eat through pressing their mouth cone against their food, pricking it and sucking out its contents.

Remember: moss piglets need a slim water film on their skin to be able to breathe. Even though they are really resistant to dehydration, it happens sometimes. So, what do these creatures do, once living conditions start worsening?

The Holy Trinity of survival

Tardigrades have several abilities, that make them extremely or perhaps almost unnaturally persistent against dryness, the cold, strong fluctuations of oxygen and salt levels in the water and air as well as radioactivity. In the following paragraphs, I want to explain the three main survival tactics tardigrades use.

Let us start with a defensive mechanism called anoxybiosis. It’s the tolerance against oxygen deprivation. When the oxygen levels start to drop the osmoregulation stops, which means that the body doesn’t control the nutrition and water intake of cells. That way the body can consume large amounts of water and swell as a result. Because the metabolism is anaerobic (which means without air), waste products start to accumulate in the tardigrade’s body.

Usually, water bears can survive three to five days in the anoxybiotic state, but there are some kinds, which can survive for up to six months. Depending on the duration of the anoxybiosis the process of returning to normal can last a couple of minutes or hours.

The cosmos not only lacks oxygen to breathe but is also terribly cold. Our little friends also have a solution for that- cryobiosis. It’s the ability to withstand low temperatures. In the cryobiotic state, the metabolism is almost completely shut down. Most body enzymes become inactive and are replaced by special catalysts and other cryoprotective substances. They protect sensitive body tissue and replace molecules in their bodies, that are bound by water. This ability allows water bears to survive temperatures as low as -237°C. Due to this, they can survive up to ten whole days in the vacuum of outer space.

Tardigrades possess a third survival tactic that allows them to withstand dehydration. Its name is anhydrobiosis. Since this ability almost completely shuts down the animal’s metabolism, it often falls under the term of cryobiosis. Anhydrobiosis is the most important defence mechanism for tardigrades because they can’t breathe without the slightest amount of water. During the process of barrel formation, the moss piglets retract their legs and their body’s surface area becomes smaller. Their skin also becomes impermeable to water at a certain point, slowing the dehydration process down. They also produce substances, that protect them from fungal attacks as well as trehalose. It keeps the tardigrade’s body from oxidizing uncontrollably. Water bound enzymes get replaced with more dry resistant ones during this transformation.

It takes tardigrades five to seven hours to complete it and it makes them immune to temperatures as low as -237°C and as high as +150°C. Even high pressure and the vacuum of space won’t be able to harm them in the barrel state. As soon as moss piglets have access to water again, the rehydration process will commence.

An answer to the question

Before I answer the main question of the article, I want to briefly discuss another. Throughout this post, I have defined tardigrades as living things, when it would be more accurate to call them beings in a grey zone. In school, we all got told that for something to be alive it has to move, have a metabolism, reproduce, grow and must be able to interact with its surroundings. But during the time that moss piglets spend in any of the defensive states listed above, they do not fulfil these requirements. Due to this defining life just by five simple characteristics proofs to be difficult.

Now that this is out of the way, let’s establish what these critters have to do with space travel. Their survival mechanisms are far superior to almost any other being we know. If we could figure out a way, to use them in our favour it would bring a tremendous breakthrough to all scientific fields. As I mentioned at the beginning of my article their sustainability could be used to check theories about living conditions on other planets. Of course, we would need to find ways to monitor them efficiently and build devices, that could reach these faraway worlds first. Still, in my opinion, researching tardigrades and forming solutions with their abilities is a game-changing part of future space exploration.