Tardigrades: Nature’s X-Men

Tardigrades, water bears, moss piglets – these are all terms used to name tiny creatures related to modern insects with superpowers. They can survive incredible amounts of radiation, temperatures from near absolute zero to 151^oC (304^oF), prolonged desiccation, pressure six times greater than the deepest ocean trenches, and the vacuum of space.

In this article, we’ll take an in-depth look at the environments tardigrades can survive, what would happen to us under some of the conditions they tolerate with ease, and how these little bug things are resilient.

 

What are Tardigrades?

Tardigrades are animals in the Superphylum Ecdysozoa, with arthropods and nematodes. There are about 900 species of tardigrades. They generally have eight legs ending in claws and dagger-like teeth. They sound pretty scary until you realize that they are never larger than 1.5 mm in length and most obtain nutrition by sucking the juices out of moss, lichens, and algae.

A few are carnivorous, and some are even cannibalistic. They were likely around long before the dinosaurs, emerging in the fossil record about 500 million years ago. When you think about their superpowers, it’s no surprise they’ve held on this long.

 

Tardigrades can survive intense radiation.

Exposure to high radiation can cause burns, cancer, and death in humans and other animals. This is because radiation dismantles DNA and other molecules. Effects of radiation exposure start with a reduction in white blood cells (which protect your body from infection), nausea, vomiting, and headaches. Then, you would begin to lose your hair, and your nerve and digestive tract cells would become damaged.

Next, your immune system would weaken due to the loss of white blood cells. Your body would produce fewer platelets, which allow your blood to clot, and any bleeding would be difficult to stem. Death is almost inevitable over a certain threshold. Anyone who survives high radiation exposure is extremely likely to develop cancer.

Radiation is measured in units called Grays. 4 Grays of X-rays would cause death in humans within 30 days if untreated. Tardigrades have been exposed to up to 4,000 Grays of helium ion radiation, which is even more dangerous for humans than X-rays. Tardigrades can withstand this radiation due to a protein that creates a shield around their DNA. This protein, called “Damage suppressor,” or “Dsup” for short, surrounds tardigrade DNA without inhibiting its function. When human kidney cells were engineered to produce this protein, they could withstand higher radiation than normal, cutting the damage of X-ray exposure down by 40-50%.

 

Tardigrades are tolerant to extreme temperatures.

Extreme cold has dramatic effects on the human body. Our normal core body temperature is between 37 and 38^oC. Thus, anything above or below that can be harmful.

First, the body diverts blood from the extremities and towards vital internal organs. Both heart and respiratory rates increase along with blood pressure. You’ll start to shiver as your body tries to generate more heat. That blood diversion from earlier will cause your skin to turn white, and you’ll find that moving is complicated. Confusion sets in as your brain’s enzymes become less efficient. Your fingers and toes will start to turn blue, and you won’t be able to feel them anymore. Then your heart and respiratory rates will drop, and you’ll begin to hallucinate.

Amnesia is also an effect of cold: if a family member finds you and rescues you at this point, you might not recognize them. After the hallucination and memory loss, you might start undressing as your blood vessels dilate due to muscle fatigue and suddenly feel hot. You might try to burrow into a small space to keep yourself safe. After that, you’ll pass out, your organs will shut down, and you will die.

Tardigrades don’t have these problems. In the 1920s, a Benedictine friar named Gilbert Franz Rahm stuck tardigrades in seemingly dangerous situations. He put them in liquid air at -200^oC for almost two years, in liquid nitrogen at -253^oC for over 24 hours, and in liquid helium at -272 ^oC for 8 hours. All of them survived.

Absolute zero is -273.15^oC. For comparison, the lowest temperature recorded on Earth was –89.2^oC. A tardigrade has even survived being frozen for 30 years and six months. Two Antarctic tardigrades, Acutuncus antarcticus and one tardigrade egg were collected in November of 1983 and stored at -20^oC (-4^oF). In March 2014, they were thawed out.

Both tardigrades showed signs of life after thawing, but one didn’t eat enough after being revived and eventually died. The other thrived, laying 19 eggs soon after its recovery. Fourteen of those eggs produced healthy babies. The thawed egg also hatched, and that tardigrade laid 15 eggs, of which 7 produced healthy offspring.

Tardigrades (and some other creatures) go into a state of suspended animation when frozen, called cryptobiosis. In this state, the metabolism is shut down. They don’t do any development or repair. Their reproductive processes are also entirely halted.

It’s unclear what tardigrades do to survive being frozen, but some hypotheses exist. Researchers consider ice crystals the greatest threat to tardigrades when they freeze. Ice crystals can rip apart DNA and other molecules within cells. Either tardigrade cells are protected from ice crystals somehow, or they can repair themselves upon revival. It is currently hypothesized that they use chemicals called ice nucleating agents, which keep ice from forming within cells by causing ice to form outside of cells. The sugar trehalose may also play a role by ensuring that the ice crystals forming are too small to rupture cell membranes.

Heat also causes severe harm to humans and most other animals. Initially, in the opposite reaction from that cold, your body increases the blood flow to your skin to rid the body of excess heat. This heat dissipates as sweat. However, if you lose too much water to this process, you will develop a salt imbalance, and your muscles will begin to cramp.

If your core body temperature increases to between 39 and 40^oC, your muscles will slow down and feel fatigued. Between 40 and 41^oC, you will likely experience heat exhaustion. This occurs when your body’s water or salt becomes depleted. You’ll experience extreme sweating, dizziness, confusion, headaches, nausea and vomiting, a weak but rapid pulse, muscle cramps, and fatigue. Above 41^oC, your body will begin to shut down as your cells fall apart and your organs fail.

You will stop sweating, develop severe headaches, and become confused and disoriented. Your skin will become hot, dry, and red, and you will begin to have seizures. Eventually, you will pass out and die.

Once again, however, extreme heat doesn’t bother tardigrades. Rahm, that Benedictine friar, also heated his tardigrades. He baked them at 151^oC (304^oF) for 15 minutes and could still revive them. Researchers are stumped as to how they can survive such high heat.

At the temperatures Rahm exposed them to, their proteins and cell membranes should have come apart, and all life-sustaining chemical reactions should have ceased. Other animals that live in hot places create heat shock proteins, which protect other proteins, keeping them in one piece and repairing anything damaged by the heat. But tardigrades don’t seem to have such protectors in their cells.

 

You can completely dry out a tardigrade, and it’ll be fine

In addition to their radiation, cold, and heat tolerance, tardigrades can tolerate years of severe desiccation and be revived with some water in about an hour. We’re about 60% water, so desiccation is a non-starter.

The good news is that researchers at the University of North Carolina have figured out how tardigrades can handle such severe dehydration. When a tardigrade is dried out, it makes itself a little “glass” shell that cocoons its vital proteins and molecules and keeps them in a state of suspended animation. This shell keeps whatever it surrounds from falling apart when it dries out. Researchers are calling this shell “bioglass.” Bioglass is created by unique proteins called tardigrade-specific intrinsically disordered proteins (TDPs).

As their name suggests, these proteins are usually in a state of disorder, basically existing as a gel. When dehydration occurs, these proteins quickly order themselves into a rigid structure, much like the Magic Shell ice cream coating that hardens when exposed to cold.

The team that discovered this tardigrade shield, led by Dr. Thomas Boothby, has also found that intrinsically disordered proteins (IDPs) can be engineered in other creatures’ cells and perform the same function. Boothby’s team has tested IDPs in yeast, bacteria, desiccation-sensitive enzymes, and human skin cells and found them to produce bioglass upon desiccation. This has implications for agriculture (e.g., drought-resistant crops) and medicine (i.e., medications that you could keep at room temperature instead of refrigerated).

 

Tardigrades can survive extremely high pressure.

Like radiation, cold, heat, and desiccation, humans are sensitive to high pressure. Just a few feet beneath the surface, it’s difficult for us to draw breath due to water pressure pushing on our lungs. At 100 feet down, or four times the amount of pressure, we’re used to, lung tissue contracts while the body pushes blood toward the heart and brain. Any extra blood is pushed into the blood vessels of the chest to counteract the water pressure. Heart rate slows. During intense dives, divers’ heart rates have been clocked at 14 beats per minute. For reference, that’s about a third the rate of a coma patient’s heart. If you go deep enough, the pressure will cause your lungs to collapse, which spells instant death.

It’s no surprise that this isn’t an issue for tardigrades. Before Boothby and his team, researchers knew that when tardigrades were dehydrated, they curled up into a rigid, dry pill shape. Scientists called this state a “tun,” but they didn’t know about the TGPs or how this tun state allowed the tardigrades to survive desiccation.

In 1998, researchers at Kanagawa University in Hiratsuka, Japan, discovered that the tun state also allowed tardigrades to survive pressures up to 600 megapascals (MPa). For reference, in the deepest place on Earth, the Challenger Deep, at 10,994 meters down, the pressure is 100 MPa. Most animals’ DNA and proteins would be destroyed in the Challenger Deep. Even bacteria can’t survive beyond 300 MPa(3). So it seems this bioglass does more than just protect tardigrades from dehydration; it also protects them from intense pressure.

 

Water bears in space!

We now know what extreme radiation, temperatures, and pressures can do to a human body. We also know that tardigrades are pretty chill with all those things. What about the cold, unforgiving vacuum of space? First, a human would likely pass out within 15 seconds of being in space without a suit due to the lack of oxygen. If you held your breath when you got out there, the lack of external pressure would make the gas in your lungs expand, causing them to burst. During those 15 seconds of consciousness, your skin would swell up due to the vaporization of the water in your body.

Your skin is tough enough to keep you from exploding, but you would be highly uncomfortable. Your circulatory system could keep your blood pressure under control, so your blood wouldn’t boil, but the moisture on your tongue might. The radiation in space would severely burn you, but you would also be freezing. However, you would suffocate before you froze to death.

Tardigrades can handle the environs of space and seem to thrive. In 2007, researchers stuck thousands of tardigrades on a satellite and launched them into space. They varied the exposure level of three different groups of desiccated tardigrades. One group was exposed to the vacuum of space only, one was exposed to the vacuum of space and UV-A and UV-B radiation, and the last group was exposed to the vacuum and the full range of UV radiation in space. The fourth group of hydrated tardigrades was also placed on the satellite. When the satellite returned 12 days later, the desiccated tardigrades were rehydrated.

Most of the animals exposed only to the vacuum of space were successfully revived and went on to reproduce. Some of the tardigrades exposed to radiation also survived, but not as many. The hydrated tardigrades? They behaved as normal: while in space, they underwent molting, and females laid eggs, which produced healthy babies.

 

Summary

Tardigrades are incredible. They can survive severe radiation, temperature extremes, desiccation, intense pressure, and a trip to outer space, all of which would kill you and me. They have some remarkable adaptations that make this possible, which is why I think of them as nature’s X-men.

 

References

1. Crew, Bec (2017 Mar 17) “We Can Now Harness the Tardigrade’s Strangest Superpower – and Give It to Other Organisms.” Science Alert. Retrieved from http://www.sciencealert.com/we-can-now-harness-the-tardigrade-s-strangest-superpower-and-give-it-to-other-organisms.
2. Coghlan, Andy (2017 Mar 16) “Tardigrades turn into glass to survive complete dehydration.” New Scientist. Retrieved from https://www.newscientist.com/article/2124893-tardigrades-turn-into-glass-to-survive-complete-dehydration.
3. Fox-Skelly, Jasmin (2015 Mar 13) “Tardigrades return from the dead.” BBC Earth. Retrieved from http://www.bbc.com/earth/story/20150313-the-toughest-animals-on-earth.
4. Crew, Bec (2016 Jan 18) “A Frozen Tardigrade Has Been Brought Back to Life After 30 Years.” Science Alert. Retrieved from http://www.sciencealert.com/a-frozen-tardigrade-has-been-brought-back-to-life-after-30-years.
5. “Ecdysozoa.” Retrieved 2017 Apr 20 from the Integrated Taxonomic Information System (ITIS): https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=914158#null.
6. Nordqvist, Christian (2014 Sept 26) “What are the effects of radiation on humans? What is radiation poisoning?” Medical News Today. Retrieved from http://www.medicalnewstoday.com/articles/219615.php.
7. Foley, Katherine Ellen (2016 Sept 21) “Scientists figured out how water bears can survive a radioactive apocalypse.” Quartz. Retrieved from https://qz.com/787509/how-water-bears-can-survive-massive-amounts-of-radiation/.
8. Coghlan, Andy (2016 Sept 20) “World’s hardiest animal has evolved radiation shield for its DNA.” New Scientist. Retrieved from https://www.newscientist.com/article/2106468-worlds-hardiest-animal-has-evolved-radiation-shield-for-its-dna/.
9. Ripley, Katherine (2017) “What Happens to Your Body When You Freeze to Death.” Ranker. Retrieved from http://www.ranker.com/list/what-happens-to-your-body-when-you-freeze-to-death/katherine-ripley.
10. Choi, Charles Q. (2013 Jan 3) “Atoms Reach Record Temperature, Colder than Absolute Zero.” LiveScience. Retrieved from http://www.livescience.com/25959-atoms-colder-than-absolute-zero.html.
11. BBC (2013 Jul 18) “What happens to the body in extreme heat?” BBC News. Retrieved from http://www.bbc.com/news/health-23358290.
12. Lam, Linda (2016 Jun 20) “Four Things Extreme Heat Does To Your Body.” The Weather Channel. Retrieved from https://weather.com/health/news/four-things-extreme-heat-does-to-your-body.
13. U.S. Geological Survey (2016 Dec 2) “The water in you.” The USGS Water Science School. Retrieved from https://water.usgs.gov/edu/propertyyou.html.
14. Venosa, Ali (2015 Aug 13) “Breaking Point: How Much Water Pressure Can The Human Body Take?” Medical Daily. Retrieved from http://www.medicaldaily.com/breaking-point-how-much-water-pressure-can-human-body-take-347570.