A single-celled blob living in a California hot spring has smashed a heat tolerance record for an entire domain of life.
The newly described “fire amoeba” can grow at 63° Celsius (145° Fahrenheit), a temperature that’s fatal to all other known eukaryotes, researchers report September 22 in Cell. Eukaryotes include all life-forms with nuclei and organelles, such as plants, fungi and animals.
Little was known about the genetic adaptations of microbial eukaryotes that live in extreme environments. From 2023 to 2025, microbiologist Beryl Rappaport and her colleagues brought water samples from geothermal hot springs in California’s Lassen Volcanic National Park into the lab. There, the team tried to grow any amoebas they found and study their DNA.
“We set up cultures at a few temperatures, with the hottest being 57° C because that is the highest temperature previously described for an amoeba,” says Rappaport, a member of microbial ecologist Angela Oliverio’s lab at Syracuse University in New York.
After seeing an amoeba growing at 57° C that looked different from the previous amoeba record holder Echinamoeba thermarum, the team cranked up the temperature. The mystery microbe continued to replicate and thrive at a blistering 63° C, above the eukaryote record of 60° C held by some algae and fungi. This resilient amoeba could even survive up to 70° C by entering a dormant state.
The researchers named the amoeba Incendiamoeba cascadensis, which means “fire amoeba from the Cascades,” after the volcanic mountains where it was discovered.
Some other, simpler microbes like bacteria and archaea can tolerate much higher temperatures. For instance, archaeon Methanopyrus kandleri holds the heat record for all life and can grow at 122° C.
The fire amoeba appears to share some adaptations with these kinds of organisms, like chemical features on the surface of its proteins that discourage the harmful clumping that happens at high temperatures.
The amoeba can also pivot between two body shapes, which might help it deal with rapidly fluctuating temperatures in its hot spring environment. “One slower [form] seems suited to foraging and one faster [one] seems useful for escaping if temperatures become suboptimal,” Rappaport says.
Even more heat-tolerant eukaryotes may yet to be discovered. “As far as we know, there is no reason why 63° is the hard limit,” Oliverio says. “We don’t really know what the upper temperature limits are.”
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