Living mysteries: Why teeny-weeny tardigrades are tough as nails

Link to the article

https://www.snexplores.org/article/why-tardigrades-tough-space-dehydration

Can you name an animal with 8 legs? Spiders and octopi usually come to mind, but what about tardigrades, also known as "water bears"? These very small creatures were discovered 250 years ago and have continued to amaze scientists since then.

Light microscope image (left) and electron microscope image (right), from the article

The Science News Explores article describes many of their interesting features that demonstrate just how hardy the "tardies" are. They're really amazing.

Adult tardigrades are about 0.5 mm (0.020 in) long, so they are barely visible with the naked eye. Depending on font size, they may be the size of a period. They grow naturally in moss and lichen. German zoologist Johann Goeze discovered them in a pond in 1773 and gave them the dubious nickname of "little water bear". Four years later, Italian biologist Lazzaro Spallanzani wrote up his studies on them and labeled them Tardigrada, which means "slow stepper". Some people have also called them "moss piglets", but whatever the name, they are incredible.

As the article describes, tardigrades can somehow resist extreme environmental conditions. Spallanzani let them dry up, and they survived even though he thought they were shriveled up and dead. They had simply gone into a form of suspended animation like spores from plants or bacteria. That's called their tun state. The word comes from the German or Old English word for barrel, which is just what they look like. Current research is beginning to show that tardigrades have DNA segments that code for proteins that form gels or glass-like material in their hibernating stage, to protect them from damage with little water inside.

photo from the article

The article goes on to say that not only can they survive drying up, but you can't seem to kill them with X-rays, UV radiation, or gamma rays, or temperatures down to –273 °C (–459 °F). But what about higher temps? A biologist at the University of Copenhagen tested that on active and hibernating tardigrades. One was to take active tardigrades and put 20 them in small tubes with 1.5 mL (1/3 teaspoon) of water and turn up the heat to 4 temperatures. After they cooled down to room temperature for an hour, they were kept in the refrigerator with some food to prevent them from losing energy searching for it in an empty tube.

Experiment 1

The second experiment let the active tardigrades get used to 30 °C for 2 hours, then 35 °C for 2 hours, before splitting them into a tube of 35 or 40 °C for a day.

One third to half of them didn't survive at 37 °C, and none survived at 40 °C.

Experiment 2

The tardigrades that had been given time to acclimate survived 25% better at 37°C than in experiment 1, but not much.

A third experiment dried tardigrades on paper, then put them into tubes and heated them up to 40, 50, 60, 65 or 70 °C (104, 122, 140, 149, or 158 °F) for 24 hours or in another batch they were exposed to higher temps of 70, 80, 82, 85, or 90°C (158, 176, 180, 185, or 194 °F), cooled to room temperature for an hour, then rehydrated with water for another hour, then put them in the refrigerator.

Experiment 3

The tun tardigrades were still 91-98% alive after a day at 40-60 °C of dry conditions, and more than a third made it to 65 °C. The tun tardigrades that had only an hour of high heat exposure were 89-94% alive at 70-80 °C. Almost 80% made it to 82 °C, but that was the limit of their survival. Here's a link to the actual scientific paper.

Acclimating is a more natural condition, so who knows how well they will adapt to climate change? And if you think you might have them in your drinking water, boiling certainly seems to kill them even if they are hibernating.

The strangest part of tardigrades' survival is being so resistant to radiation. They aren't exposed to it in their natural environments, so why did they develop this superpower? Nobody is sure, but some are wondering whether their DNA can be used to improve a human's ability to survive in such dangerous conditions. That would be great if you are a soldier, a worker in a nuclear plant, or an astronaut.

Japanese researchers have tested that with human kidney cells in the lab. They found a special piece of DNA unique to a tardigrade. The strain of tardigrade is called YOKOZUNA-1. Yokozuna is the highest ranking in sumo wrestling, so it's no wonder they named this tardigrade for such toughness. It was originally found on moss on a bridge in Sapporo, where I now live!

This piece of DNA is called Dsup, meaning "damage suppressor". Its function is to make a protein to cover the DNA and shield it from UV radiation damage. Tardigrades also have DNA that do more efficient repairs of damage to their genetic material, so that explains their resistance with a twofold strategy. In 2020, scientists put the Dsup into tobacco plants, and in 2021 other scientists put it into human kidney cells, and in both cases, the cells were better protected against UV radiation.

As mentioned earlier, even supercold temperatures don't kill tardigrades. They were tested in 2007 for their survival in outer space by Ingemar Jönsson of Kristianstad University in Sweden when he sent them up in the European Space Agency's FOTON-M3 spacecraft. There was much more radiation in orbit than on Earth, so they didn't that survive well.

FOTON-M3 (left) and environmental exposure chamber (right) 

Israel sent its Beresheet lunar lander to the Sea of Serenity on the Moon in 2019, where the last U.S. Apollo mission (17) had landed. Unknown to many, it carried live tardigrades. An unfortunate loss of contact just before landing resulted in a crash. But people wondered whether the tardigrades survived thus contaminating the Moon's surface for future investigations of life signs there. Initially, the scientists thought it was possible they survived and could be returned and revived on Earth. In 2021, Alejandra Traspas, a doctoral student at Queen Mary University of London, wondered if the tardigrades could survive the physical impact alone. She put some into their tun state and fired them inside a hollow bullet into a sand target! They survived with the bullet speed of 3000 km/hr (1,800 mph) but not higher. Since Beresheet was expected to hit with greater force, they concluded the tardigrades would not have survived.

Why study tardigrades, aside from dreams of protecting astronauts, radiation workers, and soldiers? Some discoveries may help find solutions to Alzheimer's disease, vaccine stability, and crop stress.

For a great simplistic animated explanation on tardigrades, watch this YouTube video.