Tardigrades: Nature’s most extreme survivors

Tardigrades are nature’s ultimate survivors. As five-time mass extinction champions, they are the terrestrial organism most likely to survive in space, even beating the hated cockroach in the Animal Planet most extreme survivor award. These slow-moving moss piglets (their phylum Tardigrada, literally means slow steppers) had already been around 270 million years before the first tyrannosaurus rex made an appearance, and, if history is any indication, will far outlive any animal currently on the planet today. Known most commonly as “water bears,” these microscopic invertebrates are smaller than a single grain of sand. Recently they have made headlines as our first, albeit accidental, earthling to colonize the moon.

But why do scientists care about them so much anyway?

Water bears, nicknamed for their resemblance to a sort of eight-legged panda, are one of the most distinct members of the animal kingdom. Despite their small size, they have a full brain and nervous system, as well as a shell-like cuticle that they must shed in order to grow. Like everything else in their life, their cells divide very slowly, leading some early scientists to believe that tardigrades were eutelic, meaning that tardigrades would have the same number of cells in their bodies no matter the stage development. While research showed this wasn’t the case, tardigrades remained an organism of significant interest for another reason: their ability to survive some of the most extreme environments on earth… and in space.

From the hottest desserts to the coldest, driest regions of Antarctica, tardigrades have proven themselves to be nearly indestructible in all environments. After noticing that UV-C radiation, a powerful form of radiation blocked from earth due to our atmosphere, had little effect on tardigrade health, scientists had a great idea: these organisms may be able to survive space. And survive they did. The scientists launched a culture of tardigrades into space and left them attached to the outside of a satellite for 10 days to confirm that space wouldn’t shake the bears at all. The tardigrades survived.

Now, with space travel technology allowing longer crewed voyages into space, scientists are looking to water bears for ideas to help astronauts survive the perils of space travel. Tardigrades can engage in cryptobiosis, a state of stalled metabolism during which water bears operate using 0.01% of their usually expended energy. This is similar to an extreme version of hibernation during which the tardigrades dehydrate, forming a small, shriveled ball called a tun. In this form, they can survive the harshest conditions imaginable, including a pure vacuum, temperatures over 300 °F and under -400 °F, X-ray and ultraviolet radiation, extreme pressures equivalent to 6,000 atmospheres, and more. Researchers around the world have one not-so-simple question… how?

There are several ongoing theories regarding tardigrade survival. One concerns trehalose, a synthesized sugar that can occupy up to 25% of the tardigrade’s biomass. Researchers think this sugar is used to displace water surrounding proteins and membranes during the tardigrades cryptobiotic state. This is an important adaptation as water forms crystals when freezing, piercing membranes and leading to cellular damage. Other researchers think the secret to their survival lies elsewhere, such as with intrinsically disordered proteins. While most proteins have a highly regulated folding pattern and shape that correlates with function, intrinsically disordered proteins are more random, taking on different shapes according to surrounding environments. Researchers at UNC credit the high number of these randomized proteins with tardigrade desiccation survival, since these proteins, like trehalose, expel water and prevent damaging crystallization. Other research has proposed a more coordinated metabolic effort in which genes controlling replication and metabolism are downregulated during cryptobiosis while those serving a more protective purpose are upregulated, followed by an increase of DNA repair proteins following revival. 

Regardless of the method, these tiny animals’ ability to survive the most extreme environmental conditions our universe has to offer will surely be of interest to researchers for decades to come. 

Peer edited by Kasey Skinner