麻豆传媒

A species of amoeba discovered in California鈥檚 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掳C. However, it only retains the ability to eat and reproduce up to temperatures of 63掳C, and it loses the ability to move around above 64掳C.

Previously, according to at Syracuse University, New York, 60掳C was considered the 鈥�4-minute mile鈥� 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.

鈥淭he 4-minute mile was long thought to be impossible,鈥� says Oliverio. 鈥淏ut once accomplished, it only took weeks to be broken again.鈥�

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鈥檚 Lassen Volcanic National Park, flowing through a 鈥渂eautiful meadow鈥� less than half a metre wide for most of its length. Temperatures in the stream range from 49掳C to 65掳C, with a pH range of 6 to 7.

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

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

鈥淚t 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,鈥� says Oliverio.

at Monash University, Australia, says the discovery means he will have to rewrite his lectures on extremophiles.

鈥淚t鈥檚 a very exciting finding that extends the temperature record for the growth of eukaryotic life by at least 3掳C 鈥� a significant leap,鈥� 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掳C to 64掳C.

Earth extremophile can grow in聽near-Mars-like conditions

A salt-loving microorganism found in places like the Dead Sea can not just survive but actively grow in conditions close to those that may be found just under the Martian surface

They found there was no growth below 40掳C or above 63掳C, with the optimal range between 55 and 57掳C. Above 64掳C, the amoeba rolls up into a cyst to protect itself from the heat. After exposure to temperatures above 70掳C, these cysts are 鈥渋nactivated鈥�.

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 Taup艒 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鈥檛 yet been collected or cultured. The search should continue for even more extreme life forms, says Oliverio.

鈥淎s far as we know, there is no reason why 63掳C is the hard limit.鈥�