The microscopic water bears that can survive desiccation, extreme cold, and even trips to the Moon have a key weakness: heat. A recent study tested the survivability of a tardigrade species at elevated temperatures over an extended period. The team found that the lethal temperature for active tardigrades is only 1.2°C hotter than the maximum recorded temperature where the samples were taken.
“We can conclude that active tardigrades are vulnerable to high temperatures, though it seems that these critters would be able to acclimatize to increasing temperatures in their natural habitat,” lead author Ricardo Neves said in a statement. Neves is a postdoctoral researcher in cell biology and physiology at the University of Copenhagen in Denmark.
When given time to adjust, the active tardigrades could withstand slightly higher temperatures for the experimental time frame. Desiccated tardigrades—inactive from being dehydrated—could withstand significantly higher temperatures for a longer time.
Tardigrades Can’t Stand the Heat
This is not the first study to test the upper limits of tardigrades’ heat tolerance, but it is the first to test the animal’s resilience for an hour or longer, the team said. The researchers gathered samples of Ramazzottius varieornatus, a tardigrade species typically found in temporary freshwater habitats, from a roof gutter in Nivå, Denmark.
The researchers exposed tardigrades to different levels of heat for 1 and 24 hours to find the lethal temperature, which they defined as the temperature at which 50% of the population died. They tested active tardigrades, desiccated tardigrades, and active tardigrades given a period of acclimation.
Active tardigrades were the most vulnerable to heat: lethal temperatures at 1 hour were 37.1°C without acclimation and 37.6°C with a short acclimation period. Desiccated tardigrades were more heat resistant than active ones, just like they are more tolerant of the cold. Half the desiccated population survived an hour at 82.7°C. For the 24-hour exposure time, however, the lethal temperature dropped significantly to just 63.1°C.
“The results indicate that hydrated or desiccated specimens of Ramazzottius varieornatus are able to tolerate high temperatures, but only for a short time,” said Lorena Rebecchi, an associate professor of zoology at the University of Modena and Reggio Emilia in Italy. “This indicates that its probability to withstand climate change is limited.”
Rebecchi, who was not involved with this research, said that the results might be applicable to other tardigrade species—there are more than 1,000. “Some species inhabiting mosses and lichens of temperate regions or Antarctica have a similar tolerance,” she said.
The hottest temperature ever recorded in Denmark was 36.4°C, only 1.2°C higher than the active, acclimated tardigrades’ 1-hour heat tolerance. On average, the maximum temperature for Denmark is around 22°C, but this value is likely to climb in the next decade.
“Tardigrades are renowned for their ability to tolerate extreme conditions,” the researchers wrote, “but their endurance towards high temperatures clearly has an upper limit—high temperatures thus seem to be their Achilles heel.”
Neves and colleagues published these results in January in Scientific Reports.
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer