How a hot spring amoeba earned a 63°C growth record

At 63°C (145°F), a tiny amoeba from California’s Lassen Volcanic National Park can make more amoebas. That raises the measured growth limit for organisms with a cell nucleus from 60°C to 63°C. The finding first appeared in a 2025 preprint and was published in Cell on September 22, 2026.

How can researchers tell that Incendiamoeba cascadensis actually grows at that temperature? They tracked the number of amoebas in cultures for seven days at 17 temperatures, using four flasks at each temperature. Multiple probes checked the incubation temperatures. The cultures increased in density at 63°C, the highest temperature at which the team measured growth. Researchers also imaged cells incubated at 63°C in stages of mitosis, the process that separates copied genetic material as a cell divides. The rising counts and the images support the growth claim in different ways. The preprint describes both tests.

The next two numbers answer different questions. Researchers recorded the amoeba moving at 64°C, but movement does not show that it can reproduce there. At 70°C, cells rounded into dormant cysts during a five-minute exposure; they recovered after being returned to 60°C. That shows survival of a brief hotter spell, not growth at 70°C. NASA’s account reports the three thresholds, while the preprint specifies the cooling step.

The researchers found genes associated with protecting DNA and maintaining proteins, possible clues to how the amoeba copes with heat. Those clues do not yet establish which defenses enable division at 63°C. For now, the clearest discovery is one the experiments can show directly: this cell can reproduce at a temperature where no other known organism with a nucleus had been shown to grow.