Scientists increasingly have been linking global warming with plant and animal die-offs now happening at a rate comparable to some of Earth's worst mass extinction events.
Heat takes a toll in many different ways. Cold-blooded animals like fish and reptiles simply can't acclimate to warmer temperatures. In Australia, hotter sand in sea turtle nesting areas is turning all the hatchlings to females. And in recent years, flocks of dead birds have fallen out of the sky during extreme heat waves.
New research suggests that even some of nature's toughest microscopic species are vulnerable, including tiny tardigrades, invertebrates that live nearly everywhere, including on icy alpine summits and volcanoes, rainforests and the fringes of Antarctica.
A study by University of Copenhagen scientists published in the journal Scientific Reports suggests that tardigrades, which have a reputation for tolerating extreme conditions like freezing cold, intense radiation or lengthy dehydration, may also succumb to global warming. Temperatures above 100 degrees Fahrenheit are frequently lethal to a common tardigrade species found in Denmark, the scientists concluded.
The results were surprising, said lead author Ricardo Neves, a University of Copenhagen biologist.
"We expected the tardigrades—both in their active state and desiccated state—to survive higher temperatures, which was clearly not the case. We found their Achilles heel," Neves said. "Tardigrades are definitely not the almost indestructible organism as advertised in so many popular science websites."
Tardigrades, sometimes called water bears or moss piglets, are colonizers that can survive in emerging new habitats like new moss on freshly cooled lava, and have even survived trips to outer space.
Mark Blaxter, a scientist and tardigrade expert with the Wellcome Sanger Institute, a nonprofit genomics and genetics research organization in the UK, said that, tardigrades, like other organisms adapted to live in a 32 degrees Fahrenheit to 86 degrees Fahrenheit temperature range, will struggle in hotter environments.
"Even species renowned as being super-resistant to environmental insults will be impacted. It's good to be reminded that all of the ecosystem will be impacted, not just the megabiota we can easily see," said Blaxter, who was not involved in the most recent research.
In the new study, the researchers found that tardigrades are vulnerable to long-term exposure to high temperatures, with a mortality threshold just slightly warmer than the current maximum high temperature record in Denmark.
One of the main reasons tardigrades are able to persist in extreme environments is because they can deactivate their metabolism, entering what scientists call a cryptobiotic state. But though they can survive like that for up to 20 years in cold that reaches minus 4 degrees Fahrenheit, the new study indicates they are more sensitive to very warm temperatures.
"With global warming, some tardigrades will die, and there will be less biodiversity due to the loss of tardigrades and other organisms," said University of California, San Diego biologist Jim Kadonaga, who recently published research describing a protective protein that shields tardigrades from certain types of extreme conditions.
In the University of Copenhagen experiments, exposure to a temperature of 98.8 degrees Fahrenheit for 48 hours killed half of all active tardigrades that weren't acclimated. Those that had a chance to get used to warmer conditions gradually could withstand temperatures of 99.7 degrees Fahrenheit for 48 hours.
The length of exposure to heat is critical for tardigrades, Neves said.
It took exposing tardigrades to 180.8 degrees Fahrenheit for one hour to kill half of them in their suspended cryptobiotic state. But after 24 hours of exposure, half died at a much lower temperature, 145.6 degrees Fahrenheit.
The species tested by the researchers was collected from the rain gutter of a house in Denmark, a type of habitat where the temperature could approach the threshold level on a warm, sunny day. Global warming is expected to increase the intensity and length of heat waves in the decades ahead, potentially exposing tardigrades to more frequent lethal conditions.
The lab tests investigated the thermotolerance of active, feeding tardigrades (R. varieornatus) by exposing them to four different temperature levels: 86 degrees Fahrenheit, 95 degrees Fahrenheit, 98.6 degrees Fahrenheit and 104 degrees Fahrenehit. The survival rate was compared to that of a control group kept at 41 degrees Fahrenheit (the temperature at which they were collected from the rain gutter).
Inactive tardigrades were also exposed to a series of different temperature levels for different lengths of time, with the results showing how they can survive short periods of extreme heat, but die when they're exposed for more than 48 hours.
The findings bolster other recent studies that have shown how extreme heat waves can kill or impair a wide variety of organisms. In 2016, scientists with Oxford University and the University of Washington developed a "biological index" to help show the temperature thresholds for a variety of species.
"From an ecological perspective, heat waves cause population die-offs in both terrestrial and marine ecosystems, including mussels, coral reefs, desert birds and trees," the authors wrote in that study, published in the journal Integrative & Comparative Biology.
"It's important to remember that all the core bits of an ecosystem that we don't see, the bacteria, fungi, protozoa, nematodes, soil mites and yes, tardigrades, are going to be impacted by climatic change," said Blaxter.
"Tardigrades can be seen as a poster child for the life of soils and sediments," he said. "If they are thought to be invincible, then soils may be thought to be invincible and we don't need to worry much.
"However," he added, "if tardigrades with their superpowers are just as affected by the creep of climate change as are polar bears and rainforests, then maybe we should care more about, and work to understand better, the resilience of soil and sediment ecosystems: if the soils die, we die."