Tardigrades are microscopic organisms with a reputation for being practically indestructible. The tiny animals, colloquially known as water bears or moss piglets, can survive extreme temperatures and pressures, years without food, and direct exposure to the vacuum of space. It’s even possible that tardigrades lived on the Moon for a while, after an Israeli spacecraft spilled thousands of them on the lunar surface when it crash-landed last year.
But even these death-defying creatures might be pushed to their limits by human-driven climate change, according to a recent paper in Scientific Reports. A team led by Nadja Møbjerg, a biologist at the University of Copenhagen, found that the tardigrade species Ramazzottius varieornatus was vulnerable to temperatures that will become more common in the coming decades.
“Before our study, tardigrades were regarded as the only organism on Earth to survive a cataclysmic event,” such as “the impact of an asteroid that could cause the boiling of all water in the oceans,” said study author Ricardo Cardoso Neves, a postdoc at the University of Cogenhagen, in an email.
“But now we know this is not true,” he added. “Albeit being among the most resilient organisms that inhabit our planet, it is now clear that tardigrades are vulnerable to high temperatures.”
To figure out how much heat tardigrades could potentially endure—a value known as “thermotolerance”—the researchers collected dozens of R. varieornatus tardigrades from a roof gutter in Nivå, Denmark.
One group of tardigrades was kept in an active state in which they were normally hydrated and able to move around. Another group was prompted to enter a desiccated “tun” state, which is one of the secrets of tardigrade evolutionary success.
As tuns, the animals are dormant dehydrated husks, adapted to outlast whatever pressures are temporarily placed on them. If more favorable conditions return, the tuns wake up, rehydrate, and become active again.
Neves and his colleagues measured the maximum temperatures that the tardigrades could withstand in active and tun states, on both short and long timescales. They also examined whether an acclimation period before exposure had any effect on the animals. In each case, the goal was to pinpoint the median lethal temperature, at which at least 50 percent of the tardigrades died.
The results showed that active tardigrades that had not been acclimated were “surprisingly” vulnerable, the team said, with half dying after 24 hours of exposure to just 37.1°C. The active tardigrades that had been acclimated had a slightly better result, hitting the median lethal temperature 37.6°C.
The team noted that these temperatures are only slightly above the highest temperature ever measured in Denmark, which is 36.4°C. The finding that “is quite worrying in our opinion,” Neves said, because it suggests that tardigrade mortality may increase as intense heat waves become more common, which could have ripple effects for the ecosystems in which they live.
“Active tardigrades seem also to have the ability to acclimatize to rising temperatures,” Neves noted. “How fast temperatures are increasing and how well can tardigrades acclimatize remains to be seen, though. Therefore, it seems that active tardigrades will have (at least) a hard time handling rising temperatures due to global warming.”
Møbjerg said the severity of this potential problem would "depend on whether the heatwave evaporates all water from their habitat."
"Tardigrades are only active when surrounded by water and the species that live on land in temporary freshwater microhabitats enter the tun state when the water evaporates," she noted in an email.
To that point, the desiccated specimens were able to endure up to 82.7°C for one hour before half died, while the median lethal temperature dropped to 63.1°C for tuns over a 24-hour period. Since those temperatures exceed the hottest known temperatures in the current record, tuns have a much better shot at survival than in their active hydrated state, as expected.
The research demonstrates that even tardigrades—the poster species for resilience—will be affected by climate change, though they still have a far better shot of long-term survival than many other animals.
“We expected the tardigrades—both in their active state and desiccated state—to survive higher temperatures, which was clearly not the case,” Neves said. “We had found their Achilles heel. Tardigrades are definitely not the almost indestructible organism as advertised in so many popular science websites.”
Update: This article has been updated to include comments from authors Ricardo Cardoso Neves and Nadja Møbjerg, and to reflect that Møbjerg was the study lead.