How a Water Bear Survives, Even When It’s Dry

“Tough” takes on new meaning when it is used to describe the tardigrade.

This microscopic animal, also known as the water bear for its tubby appearance, can survive being frozen, boiled, exposed to extreme radiation and high pressures, being starved for several years and put into the vacuum of space for several days. It can even endure having its body completely dried out, a process called desiccation, by pulling its eight legs and head into its exoskeleton and forming a tiny ball.

“They can remain like that in a dry state for years, even decades, and when you put them back in water, they revive within hours,” said Thomas Boothby, a postdoctoral researcher from University of North Carolina at Chapel Hill. “They are running around again, they are eating, they are reproducing like nothing happened.”

Anhydrobiosis in Tardigrades (Ramazzottius varieornatus) – desiccation and rehydration Video by Daiki D. Horikawa

Dr. Boothby wanted to know how tardigrades protected themselves from extreme drying, a strategy called anhydrobiosis. For years scientists had thought that the water bear relied on a sugar called trehalose to preserve its cells during desiccation. That is how brine shrimps, or sea monkeys, survive being dried out. But previous studies had shown that trehalose levels were much lower in the tardigrade than in brine shrimp when they were dried, leading Dr. Boothby to believe that there might be another factor keeping the water bears alive.

He and his colleagues found that tardigrades instead have unique genes that create proteins, which they call tardigrade-specific intrinsically disordered proteins, or TDPs, that preserve their cells during desiccation. They published their findings Thursday in the journal Molecular Cell.

The team identified the TDPs after they performed a test to see what genes were being activated during anhydrobiosis in tardigrades. They put the water bears into a humidity chamber and slowly dried them out, which resembled the type of conditions they might face in a pond or pool of water that is drying out. That process helped them identify genes for further study. They found that when they disrupted the genes that created TDPs, the tardigrades no longer survived desiccation.

Photo

Tardigrades in a dehydrated state. CreditT.C. Boothby

When the tardigrade begins to dry out, its body activates its lifesaving TDPs. These proteins protect the tardigrade in much the same way that trehalose sugar protects the brine shrimp, Dr. Boothby said. The proteins encapsulate its molecular components in a glasslike matrix within the body that preserves them during the drying process.

“The glass is coating the molecules inside of the tardigrade cells, keeping them intact,” said Dr. Boothby said. This slows down the tardigrade’s metabolism, allowing it to remain in a suspended state until it is rehydrated. When they add water, the proteins melt into the liquid, and the molecules within the tardigrade are free to carry out their functions again.

Dr. Boothby said that trehalose sugar and TDPs were an example of convergent evolution, where nature has found two different ways to protect organisms from desiccation.

“This is a well-done, convincing piece of research that provides new insights on how tardigrades survive anhydrobiosis,” said John Crowe, a professor emeritus from the University of California, Davis, in an email. He added that further research would show how specific the proteins were to tardigrades and whether they existed in other animals as well.

Ralph O. Schill, a biologist from the University of Stuttgart in Germany, agreed with the findings, but said he would have liked to have seen the experiment performed on species different from those chosen by the authors. There are more than a thousand species, and different tardigrades might react to drying out differently.

“However, tardigrades are really fascinating animals,” he said, “and the new publication is an important piece of the big puzzle of the nature of life.”

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