Known for being one of the most resilient organisms on Earth, tardigrades are microscopic organisms that can survive extreme temperatures and diverse pressures, such as years without food or exposure to space. Nevertheless, there’s a threat that does challenge them – climate change and a warmer world.

A team led by cell biologist Ricardo Cardoso Neves discovered that one species of tardigrade, the Ramazzottius varieornatus, was more vulnerable to the higher temperatures likely to become common in the next few decades because of climate change.

Global warming has already exceeded 1 degree Celsius compared to pre-industrial levels. Under the Paris Agreement on climate change, nations have vowed to make efforts to avoid the temperature increase exceeding 2 degrees Celcius.

“Tardigrades are renowned for their ability to tolerate extreme conditions, but their endurance towards high temperatures clearly has an upper limit,” Neves and his colleagues wrote in the study. “Global warming is already having harmful effects on habitats worldwide.”

The researchers collected dozens of tardigrades from a roof gutter in Denmark in order to understand how much heat they could endure. One group was kept in an active state, normally hydrated, while another group was prompted to enter a desiccated “tun” state.

Tardigrades usually live for only a few months when fully active. When short on water, they may curl up in a ball, entering the so-called “tun” state, named because it looks like a large barrel called a tun.

Neves and the team looked at the maximum temperatures that the tardigrades could tolerate in their active and tun states, using short and long timescales. They also looked at whether an acclimation period before exposure had any consequences on the animals. The goal was to pinpoint the median lethal temperature.

The active tardigrades that hadn’t been acclimated were “surprisingly” vulnerable, according to the results, with half of them dying after 24 hours of exposure to 37.1 degrees Celsius. The active ones that had been acclimated had a better result, with a median lethal temperature at 37.6 degrees Celsius.

To the surprise of the researchers, this level of temperature is not far from the currently measured maximum temperature in Denmark, which is 36.4°C. This would indicate that the tardigrade mortality increases as the heat waves become more common, having a ripple effect on the ecosystems in which they live.

At the same time, the tardigrades in a tun state could tolerate up to 82.7 degrees Celsius for an hour before half of them died. Then, the median lethal temperature decreased to 63.1 degrees Celcius for the desiccated specimens for a 24-hour period. This means tuns would have a better chance of survival than in their active state.

“Acclimatization in natural habitats may provide active state tardigrades the ability to tolerate rising temperatures,” Neves and his colleagues told Vice. “Nevertheless, tardigrades—and especially active state specimens—are clearly sensitive to high temperatures, which seem to be an Achilles heel for their otherwise extraordinary tolerance towards extreme environmental conditions.”

Key facts about tardigrades

Tardigrades are semi-aquatic. They can survive in terrestrial as well as watery environments such as oceans, lakes to mountains, forests and sand dunes. They’re found all over the world, from frigid Antarctic glaciers to active lava fields.

They are nature’s pioneers, colonizing new, potentially harsh environments, providing food for larger creatures that follow. Most eat algae and flowering plants, piercing plant cells and sucking out their contents through their tube-shaped mouths.

Tardigrades were discovered by Johann August Ephraim Goeze in 1773. He named them Tardigrada, which means “slow stepper.” In 1776, Italian biologist Lazzaro Spallanzani discovered that they could survive extreme conditions by making transforming their bodies.