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Tiny tardigrades might hold the key to the origin of life

by Rowan Hooper

Special To The Japan Times

The city of Tsuruoka in Yamagata Prefecture is famous for the three sacred mountains that are central to the practice of Shugendo. In this fascinating fusion of Buddhism and Shinto, disciples climb and descend the thousands of stone steps that lead to the summit of Mount Haguro. The ascetic practice is said to simulate the process of death and rebirth, and in the natural hot springs on Mount Yudono, pilgrims can revive their aching limbs and energize their spirits.

It was in Tsuruoka — in the parking lot of his rented apartment — that Kazuharu Arakawa found a new species of one of the most extraordinary animals on the planet. (Or off of it: This animal has been taken into orbit and survived exposure to the vacuum of space.) The animal is a tardigrade, eight-legged micro-animals also known as water bears. In essence, they are the microscopic equivalent of pilgrims practicing Shugendo’s “death and rebirth.” Tardigrades go through cycles of total stasis, whereby almost all metabolic activity shuts down, and they can stay like that for decades, maybe hundreds of years. But put them in warm water and, like the disciples heading to Mount Yudono, they come springing back to life.

Arakawa of the Institute for Advanced Biosciences at Keio University is fascinated by tardigrades — and with good reason.

“Tardigrades can come to a complete stop of metabolism with almost complete loss of water, but can still restart life upon rehydration,” he says. By removing all water, the animals can put life on hold. This form of “life pause” is called anhydrobiosis.

The animals have been studied in Japan for more than a century. With this new discovery, there are now 168 species of tardigrade in the country and, in theory, there are many more species remaining to be discovered. After all, Arakawa didn’t have to go anywhere unusual to find the new species: He just picked up some moss from his parking lot. I think it’s amazing that if you look at some ordinary moss under a microscope, you are very likely to be able to see these incredible animals.

The tardigrade Arakawa discovered (Macrobiotus shonaicus) belongs to a family that is usually carnivorous. They feed on other microscopic organisms such as rotifers and nematode worms, but also algae, so this new beast is an omnivore. But it is the ability to restart life after rehydration that makes them so fascinating, and the study of the ability is at the forefront of biological research. Tardigrades may even provide clues as to the origin of life itself.

“We have no idea how life arose from matter several billion years ago,” Arakawa says.

By studying tardigrade revival we can get an idea of how molecules assemble in the spaces between cells in the tardigrade’s body. Before cells and life evolved, organic molecules had to put themselves together without the shelter provided by the cell, so what happens in tardigrades provides a good insight into that.

“Tardigrades can endure many extreme environments,” Arakawa says, “including extreme cold, heat, space vacuum, high pressure, radiation, and chemicals.” The way they do it is fascinating. Arakawa and his colleagues recently found that tardigrades have a special protein that suppresses damage to DNA. By growing human cells in a petri dish with this protein, the team showed that the cells were able to tolerate higher doses of radiation than normal. “These new tardigrade-specific proteins may have a wide range of medical and industrial applications,” Arakawa says.

The new species he has discovered is described in the journal PLoS ONE. The team used an electron microscope to examine the tardigrade’s eggs, which helped in determining that it is a new species.

There are many species of tardigrade, but in 2016 one species had its genome sequenced. The results of that showed that the animal had “borrowed” chunks of DNA from a whole range of other lifeforms, including bacteria, fungi, plants and other animals. In fact, 17.5 percent of the tardigrade genome is not really tardigrade DNA at all, or it didn’t used to be.

It seems that when the animals dry out and rehydrate, other bits of DNA can easily slip inside. Rather than cause a problem, tardigrades have embraced the benefits this can bring.

Rowan Hooper is managing editor of New Scientist magazine. He tweets at @rowhoop and his new book, “Superhuman,” is out later this year.