Researchers have discovered a new species of amoeba that can survive and reproduce at temperatures previously considered impossible for complex life. Nicknamed the fire amoeba, Incendiamoeba cascadensis was collected from a geothermal stream in Lassen Volcanic National Park, California. It can grow at temperatures up to 63°C, breaking a long-standing barrier for eukaryotes — the branch of life that includes all animals, plants, and fungi.
Before this discovery, 60°C was considered the upper limit for eukaryotic growth. Angela Oliverio of Syracuse University, New York, described that threshold as the «4-minute mile» barrier for complex life. «The 4-minute mile was long thought to be impossible,» she said. «But once accomplished, it only took weeks to be broken again.» The finding suggests that other similarly heat-tolerant organisms may await discovery.
The amoeba was found in a small geothermal stream less than half a metre wide, where temperatures range from 49°C to 65°C. At full stretch, it measures about 40 micrometres long and 10 micrometres wide — over ten times the size of an average bacterial cell. In this hot environment, it acts as a top predator, feeding on thermophile bacteria. Oliverio compared it to a forest where plants are abundant but bears are rare, even though bears live there too.
Back in the lab, the team cultured the amoeba and tested its limits across 17 temperatures from 30°C to 64°C. It showed no growth below 40°C or above 63°C, with optimal growth between 55°C and 57°C. Above 64°C, it rolls into a protective cyst. After exposure to temperatures above 70°C, those cysts are inactivated. The species can survive in waters heated up to 70°C, but it loses the ability to move above 64°C and to eat and reproduce above 63°C.
The discovery has implications for the search for life elsewhere. Simpler organisms without a nucleus can survive at even more extreme temperatures — the archaeon Methanopyrus kandleri can withstand up to 122°C. But the new finding shows that eukaryotes, the lineage that produced all complex life on Earth, can grow at higher temperatures than once believed. Bob Leung of Monash University in Australia said he will have to rewrite his lectures on extremophiles. «It’s a very exciting finding that extends the temperature record for the growth of eukaryotic life by at least 3°C — a significant leap,» he said.
When the team searched global e-DNA databases for similar genetic signatures, they found RNA matches from a geothermal mat in New Zealand’s Taupō Volcanic Zone and three matches from Yellowstone National Park in the US. Some of these may represent closely related species that have not yet been collected or cultured. Oliverio said the search should continue for even more extreme life forms. «As far as we know, there is no reason why 63°C is the hard limit,» she said.
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