A species of amoeba discovered in California’s Lassen Volcanic National Park can survive and thrive in temperatures that had been considered impossibly high for complex life.

Nicknamed the fire amoeba by the researchers who collected it, Incendiamoeba cascadensis can survive in waters heated up to 70°C (158°F). However, it only retains the ability to eat and reproduce up to temperatures of 63°C (145.4°F), and it loses the ability to move around above 64°C (147.2°F).

Previously, according to at Syracuse University, New York, 60°C (140°F) was considered the “4-minute mile” barrier for eukaryotes, the branch of the tree of life that contains all complex life forms including animals and plants, and that is defined by the presence of cells in which DNA is protected within a nucleus.

The discovery shows that eukaryotes can grow at temperatures higher than once thought possible, Oliverio says, and now that the fire amoeba has been found, more similarly tolerant extremophile species will likely be uncovered elsewhere.

“The 4-minute mile was long thought to be impossible,” says Oliverio. “But once accomplished, it only took weeks to be broken again.”

Simpler organisms can survive at even more extreme temperatures. Methanopyrus kandleri, a single-celled archaeon that lacks a nucleus, can .

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Oliverio and her colleagues collected I. cascadensis from a small geothermal stream within California’s Lassen Volcanic National Park, flowing through a “beautiful meadow” less than half a metre wide for most of its length. Temperatures in the stream range from 49°C (120.2°F) to 65°C (149°F), with a pH range of 6 to 7.

At full stretch, I. cascadensis is, on average, about 40 micrometres long and 10 µm wide, says Oliverio.

“This is over 10 times the size of the average bacterial cell, and closer to the width of a human white blood cell,” she says. In such a hot environment, the fire amoeba is the top-order predator, feeding on thermophile bacteria, which are far more common.

“It is somewhat analogous to going into a forest where you will likely see lots of plants but very few bears, even though they are common and live in these environments, too,” says Oliverio.

at Monash University, Australia, says the discovery means 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 says.

The team cultured the amoeba back in the lab, where they tested the upper and lower limits of its survival by studying it at 17 different temperatures, ranging from 30°C (86°F) to 64°C.

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They found there was no growth below 40°C (104°F) or above 63°C, with the optimal range between 55°C (131°F) and 57°C (134.6°F). Above 64°C, the amoeba rolls up into a cyst to protect itself from the heat. After exposure to temperatures above 70°C (158°C), these cysts are “inactivated”.

Intriguingly, when the team searched worldwide e-DNA databases for a signature similar to I. cascadensis, they discovered some RNA matches from a geothermal mat in the Taupō Volcanic Zone in New Zealand, and three matches from Yellowstone National Park in the US.

Some of these may be distinct, closely related species to the fire amoeba that haven’t yet been collected or cultured. The search should continue for even more extreme life forms, says Oliverio.

“As far as we know, there is no reason why 63°C is the hard limit.”