It was thought that microbes in sediments beneath the seafloor died above 80°C, but scientists have found some that can survive up to 120°C and possibly higher temperatures
25 January 2022
Living microbes have been found in sediments 1.2 kilometres below the seafloor, where the temperature reaches 120°C. The discovery shows that life in seafloor sediments can survive higher temperatures than previously thought and is therefore present at greater depths than we realised.
“Life seems to be everywhere,” says Tina Treude at the University of California, Los Angeles. “I would speculate that wherever there’s energy that can be exploited by microorganisms, life finds a way.”
It is possible that there is life at even higher temperatures. “The only way to find out is to go back and drill deeper,” she says, though in lab experiments so far, no microbes have been found to grow above 122°C.
In 2016, Treud and her team did experiments aboard their ship on samples taken from up to 1.2 kilometres below the seafloor in the Nankai Trough off the coast of Japan. The seafloor at the drill site is 4 kilometres below the water surface, and the samples included sediments that were up to 50 million years old.
A number of experiments, including those that showed metabolic processes occurring, demonstrated that the microbes in the samples were still alive.
Although these experiments couldn’t be done at temperatures above 95°C on the ship, the fact that some of these microbes came from sediments naturally heated to 120°C shows that they do survive at this temperature, says Treude.
The researchers were also able to separate out and count cells using a centrifuge. Together, the findings show that relatively few cells survive at these temperatures, but those that do have very high metabolic rates. “It was astonishingly high,” says Treude.
This surprised the team because it is the opposite of what has been found in shallower sediments, where it is much colder. Microbes are abundant there, but their metabolisms are extremely slow, and individual microbes might live for millions of years.
At 120°C, the heat is doing a lot of damage to cells, so microbes may need high metabolisms to generate enough energy to repair this damage. It is a race to stay alive, says Treude.
It isn’t clear what these heat-loving, or thermophilic, microbes are, as the team was unable to sequence their DNA. Nor is it clear how they came to be in the sediments, given that this would have been a very cold environment for a long time after the sediments that the samples came from were first deposited.
However, a few thermophilic microbes would have been present when the sediments were deposited, and they may have somehow clung on until temperatures began to rise due to being buried under more material, says team member Felix Beulig at Aarhus University in Denmark.
“We always find a fraction of thermophiles in sediments, even Arctic sediments,” says Beulig.
As the temperatures rose, all the microbes that weren’t tolerant of heat would gradually have died off, says team member Florian Schubert at the German Research Centre for Geosciences in Potsdam. “The microbes that cannot adapt, they just die,” he says.
Patrick Forterre at the Pasteur Institute in Paris says that while there are reliable results showing microbe growth at 106°C, nobody has been able to replicate the two lab studies claiming growth at 122°C. “It’s very difficult to determine the upper temperature limit,” he says.
He is therefore sceptical of the idea of microbes living normally at 120°C, but he does think it is possible that they could somehow survive and became active again at lower temperatures.
Journal reference: Nature Communications, DOI: 10.1038/s41467-021-27802-7
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