Living Life at the Extremes: How the Water Bear Survives Without Water

Tardigrades, or water bears, are known for their robust tolerance of extremes. Bring them to outer space, expose them to extreme temperatures or radiation, and they survive. These microscopic organisms can even live when sucked nearly dry of water, despite their common name.

How exactly do water bears survive dehydration? In 1922, scientist H. Baumann found that when a tardigrade dries out, it curls up and retracts its head and legs into its body. He called this state “Tönnchenform,” now known as “tun.”

Fast-forward 90 years to 2012. A group of scientists led by Takekazu Kunieda of University of Tokyo discovered proteins known to exist only in water bears, which they called cytosolic abundant heat soluble proteins (CAHS). A few years later, Thomas Boothby at University of North Carolina-Chapel Hill found that genes encoding CAHS proteins are more active only when the tardigrades are dried out. When the function of these genes is disrupted, the tardigrades do not survive dehydration.

Boothby is now a Whitman Center Early Career Investigator at the Marine Biological Laboratory (MBL) investigating how desiccation tolerance evolved in tardigrades. Tardigrades are divided into two classes: the eutardigrades and the heterotardigrades. Eutardigrades live on land and in freshwater and are able to survive desiccation. Heterotardigrades, on the other hand, are marine organisms that do not survive dehydration.

By comparing these two classes of tardigrades, Boothby hopes to identify when desiccation tolerance arose during evolution. “We basically know nothing about the genome or the transcriptome of [marine tardigrades]. We’re going to sequence their genomes and transcriptomes to see what genes they have and what genes they have turned on. That will help us trace back and figure out where in the tardigrades lineage these [CAHS proteins] evolved,” Boothby says.

Light microscopy image of a marine tardigrade Thomas Boothby found at Stony Beach in Woods Hole, Mass. This may represent a new tardigrade species! Credit: Thomas Boothby

Boothby is collecting heterotardigrades from the beaches around Woods Hole for these studies. Heterotardigrades, though abundant, are tricky to work with. “Nobody really studies marine tardigrades because they don’t know how to keep them in the lab. Anytime you want to study them you have to go to the ocean and sort through sand,” Boothby says. While at MBL, Boothby will also be investigating what conditions are required to culture marine tardigrades in the lab.

CAHS proteins are unique because they don’t have a persistent structure, which is required for most proteins to function. While the average protein will unfold and aggregate when dried, Boothby thinks that these unstructured CAHS proteins help protect other tardigrade proteins from losing their structure. As CAHS protein concentrations increase during desiccation, they start to form gels. When CAHS proteins are in the mix, desiccation-sensitive proteins cannot unfold because they are held in place by the gel.

This ability for CAHS proteins to protect desiccation-sensitive proteins could be harnessed for pharmaceutical uses, he says. Many biological products, such as vaccines, blood, and proteins, require refrigeration, which poses a logistical and economic burden in remote or developing parts of the world. “Our idea is to take pharmaceuticals, mix them with these CAHS proteins, and dry that formulation out so you can keep the pharmaceutical in a dry, protected state at room temperature,” Boothby says.

CAHS proteins could also provide crops with drought resistance. “Engineering plants that can tolerate greater extremes in their environment would help a lot of people,” says Boothby.

While CAHS proteins do not explain the tardigrade’s robust tolerance to different conditions, it gives us a glimpse into how unique features evolved to survive at the extremes.