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Life can survive inside a furnace heated to more than 400掳C. At least, that鈥檚 the extraordinary claim being made by one group of researchers. However, others say such an unexpected conclusion will need to be supported by stronger evidence.

Conventional wisdom is that life struggles to survive when the temperature rises. The thermal limit for animal life is . Some forms of bacteria can survive life at temperatures above 100掳C, with .

at Ghent University in Belgium and her colleagues knew that one limiting factor is the water within living cells. They wondered whether bacterial spores, , could withstand higher temperatures.

The researchers placed spores of the bacterium Bacillus amyloliquefaciens in glass test tubes. After three days of incubation in a desiccator with 0 per cent relative humidity, the test tubes were transferred to a furnace and heated, over 30 to 60 minutes, to peak temperatures between 200掳C and 500掳C. Then the researchers cooled the spores and assessed whether any could still grow and form cultures.

They found that spores heated to temperatures up to and including 420掳C could germinate and grow. However, spores heated to 430掳C or higher could not.

鈥淎s far as we know, spores of B. amyloliquefaciens are the most high-temperature resistant spores studied so far,鈥� says Beladjal.

DNA destruction

The finding is all the more remarkable, the researchers say, because . They think the spores survive because they carry enzymes that can repair damaged DNA. These enzymes seem to tolerate the heat, and then patch up the broken DNA after the spores are cooled.

The study is not the first to conclude that bacterial spores can withstand temperatures in excess of 400掳C. A 2015 study reported that some . However, that study only exposed spores to high temperatures for a few milliseconds.

鈥淚 think the property described is a general phenomenon for sporulating bacteria in the Bacillus genus,鈥� says at the University of Maryland in College Park, a co-author on the 2015 study. 鈥淣onetheless, it is amazing that the bacteria can survive temperatures above 400掳C. This kind of temperature is likely to occur in nature only during wildfire and volcanic activity.鈥�

Not only do the results challenge biological conventions, says Beladjal, they have practical implications. She says some sterilisation techniques that rely on heat might need to be reassessed to ensure they are fully effective.

Heated criticism

However, other researchers struggle to accept Beladjal鈥檚 claims. 鈥淚 can鈥檛 help being very sceptical about this work,鈥� says at the University of Pretoria, South Africa.

Earlier studies found bacterial spores can survive 鈥� but the higher the temperature, the shorter the exposure time the spores can withstand. Extrapolating from those results would suggest spores can survive exposure to 190掳C for a few seconds at most, Cowan says. As such, it鈥檚 difficult to see how the spores in the new study survived 420掳C for more than 30 minutes.

at the West Chester University of Pennsylvania and at the Institute of Aerospace Medicine in Cologne, Germany, are equally sceptical. Both say the study lacks detail on important points.

For instance, says Vreeland, it鈥檚 possible that extensive DNA damage can be patched up inside a spore. 鈥淏ut there is no indication how long it took the spores to make a repair,鈥� he says. 鈥淭hat is critical, as a spore has only limited resources without germinating. So damage can only be repaired slowly.鈥�

Cowan compares the paper to a 1983 study, which reported that bacteria from a deep-sea vent called a 鈥渂lack smoker鈥� . The finding was never replicated. 鈥淏ut the real value was that it triggered a storm鈥� causing an in-depth analysis of the factors dictating and limiting high-temperature survival,鈥� he says. 鈥淢aybe this paper will do the same.鈥�