Free Radicals in Tardigrades Explained

January 17, 2024

3 min read

Cute Little Tardigrades Are Basically Indestructible, and Scientists Just Figured Out One Reason Why

Tardigrades are microscopic animals that can survive a host of conditions that are too extreme to ever occur on Earth—and scientists want to learn their secrets

By Meghan Bartels

Microscopis image of tardigrade surounded by moss
Scanning electron microscope of tardigrade. Credit: Eye of Science/Science Source

Tiny tardigrades have three claims to fame: their charmingly pudgy appearance, delightful common names (water bear and moss piglet) and stunning resilience in the face of threats ranging from the vacuum of space to temperatures near absolute zero.

Now scientists have identified a key mechanism contributing to tardigrades’ resilience—a molecular switch of sorts that triggers a hardy dormant state of being. The researchers hope that the new work, published on January 17 in the journal PLOS ONE, will encourage further exploration of the microscopic creatures’ ability to withstand extreme conditions.

“It’s opened up a whole huge repertoire of experiments we can now pursue,” says Leslie Hicks, a chemist at the University of North Carolina at Chapel Hill and a co-author of the study.

The research began back when, on a whim, co-author Derrick Kolling, a chemist at Marshall University, put a tardigrade into a machine that detects “free radicals,” or atoms that contain unpaired electrons. And he did see such atoms being produced in the water bear. That finding isn’t surprising because an animal’s normal metabolic processes, as well as environmental stressors such as smoke and other pollutants, create free radicals inside cells.

When they build up, free radicals—most notably reactive forms of oxygen—snatch electrons from their surroundings to achieve stability in a process known as oxidation. In the process, these radicals damage cells and compounds such as DNA and proteins. But in small quantities, free radicals can act as signaling molecules, Hicks says, and her lab studies show how these atoms affect a cell’s behavior by glomming on to and popping off a variety of proteins.

When Kolling told Hicks about seeing free radicals in a tardigrade, Hicks wondered if these atoms might play a role in the animal’s hardiness. The team devised several experiments to temporarily expose little water bears to stress-inducing, free-radical-producing conditions—including high levels of salt, sugar and hydrogen peroxide.

Under these forms of stress, tardigrades curl up into a temporary protective state of dormancy called a tun. “When there’s a lot of stress, they’re masters of protecting themselves,” Kolling says. The researchers monitored whether the tardigrades entered their protective state and, if so, whether they could bounce back and resume normal activity when conditions improved.

Hicks studies signaling interactions between free radicals and cysteine, a key component of proteins, and she wondered whether the mechanism could play a role in tun formation. So the scientists exposed the tardigrades to different kinds of molecules known to block cysteine oxidation, as scientists call binding by a free radical. Under stressful conditions, with cysteine unavailable to the free radicals being produced, the tardigrades couldn’t form tuns.

Kazuharu Arakawa, a scientist at Keio University in Japan, who studies tardigrades, says that the new work aligns with previous research showing the role of cysteine oxidation in a midge known for withstanding total desiccation, or the process of drying out. The similarities suggest the mechanism may be a common trigger for tuns and other forms of hardy dormancy, a phenomenon that scientists call cryptobiosis.

Still, the researchers say there’s much more work to be done to understand how free radicals are working in tardigrades. The resilient tun state isn’t the only tactic water bears use to survive environmental stress, and the team plans to study these other strategies in close detail. The researchers also plan to study other species of tardigrades (they used only Hypsibius exemplaris). They expect to find that cysteine oxidation is widely used among the animals.

Hicks says that in the long run, she hopes the work can inform studies of aging and space travel, which both involve free radicals damaging vital cellular machinery such as DNA and proteins.

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Meghan Bartels is a science journalist based in New York City. She joined Scientific American in 2023 and is now a senior news reporter. Previously, she spent more than four years as a writer and editor at, as well as nearly a year as a science reporter at Newsweek, where she focused on space and Earth science. Her writing has also appeared in Audubon, Nautilus, Astronomy and Smithsonian, among other publications. She attended Georgetown University and earned a master’s in journalism at New York University’s Science, Health and Environmental Reporting Program.

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