High Temperatures Might Be Water Bears’ Achilles Heel

Scientists have calculated that water bears could survive the water pressure at the ocean floor, the coldest corners of space, and the aftermath of an asteroid impact. A study in 2017 in Scientific Reports suggested that the only way to wipe out the eight-legged, microscopic might would be to boil away Earth’s oceans. But now, researchers at the University of Copenhagen in Denmark have identified a more immediate threat to tardigrades: a warming climate.

In a new study published this week in Scientific Reports, the researchers used a species of tardigrades found in their local gutters called Ramazzottius varieornatus and exposed them to high temperatures for up to 24 hours. The team was trying to find the creatures lethal temperature, or the point at which half of the tardigrades in the sample died.

So they turned up the heat and found that active tardigrades perished at around 98 degrees Fahrenheit. The highest temperature recorded in Denmark so far is about 97 degrees Fahrenheit.

"We had found their Achilles' heel," Ricardo Neves, lead author on the study and biologist at the University of Copenhagen, told Newsweek’s Hannah Osborne. "Tardigrades are definitely not the almost indestructible organism.”

But tardigrades, also called water bears or moss piglets, get their legendary resilience from their "tun" state, according to the researchers’ statement. A tun tardigrade is like a “cellular fortress,” Vox’s Brian Resnick explains. The organism tucks in its legs and head and secretes glycerol, which acts as an antifreeze, and trehalose, which crystalizes around the rolled-up water bear. Their metabolism drops to 0.01 percent as the creatures bide their time for a more habitable environment. In this state, water bears have survived in outer space and then revived.

The researchers in Copenhagen divided their tardigrades into three groups: active tardigrades, tardigrades with some acclimation to the high temperature, and tun tardigrades. Acclimation only made the tardigrades resilient against one extra degree of warming, with half dying in about 99 degree heat. In the tun state, tardigrades could withstand a full day at 145 degrees.

Tardigrades only go into the tun state when they’ve been dried out, Neves tells Matt Simon at Wired. “But if there is some humidity around them, they will not form the tun state, which means that they will stay active.” So if a pool of water in a gutter reaches deadly temperatures before it dries out, the tardigrades would be stuck in their vulnerable, active state.

More than 1,000 species of tardigradesexist, so the results of this study may not apply to all of them. But R. varieornatus is considered relatively resilient, which doesn’t bode well for similar or weaker water bears, as Lorena Rebecchi, a zoologist at the University of Modena and Reggio Emilia in Italy, tells Eos’s Kimberly M. S. Cartier.

“Some species inhabiting mosses and lichens of temperate regions or Antarctica have a similar tolerance,” explains Rebecchi, who was not involved in the study. She says the results show that tardigrades “are able to tolerate high temperatures, but only for a short time. This indicates that its probability to withstand climate change is limited.”

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