There are microbes near the bottom of the third deepest hole in the world. The cells, recovered from rocks almost 5 kilometres below the surface in China, are the deepest so far found anywhere on land 鈥� and they may push beyond the known heat tolerances of life on Earth.
It is widely accepted that life exists at depth. Until now, the deepest known microbes on land were tiny nematode worms found 3.6 kilometres below the surface in a South African gold mine.
A team led by Hailiang Dong at the China University of Geosciences and Li Huang at the Chinese Academy of Sciences has now discovered bacterial cells at greater depths. They studied rocks extracted from a 5.1-kilometre-deep borehole in eastern China, made as part of the Chinese Continental Scientific Drilling (CCSD) project. The CCSD hole is the third deepest in the world, after a 9-kilometre-deep borehole in Germany and one in Russia that is 12 kilometres deep.
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Microscopic analysis confirmed the presence of cells in CCSD rock samples extracted from a depth of 4.85 kilometres. The team also recovered bacterial DNA from rocks at this depth.
Dong and Huang say that, to the best of their knowledge, these are the deepest known microbes ever found on land.
Demonstrating that the cells are living will be a challenge 鈥� we know from previous studies that microbes deep below the surface often operate on such a slow timescale that they . But there are reasons to suspect the microbes may be alive.
Most importantly, they are intact rather than existing as cell fragments, which might hint that they are carrying out basic cellular repair. 鈥淚t is possible that the microbes at 4.85 kilometres might represent a living community at that depth,鈥� Dong and Huang wrote in an email to 快猫短视频.
The researchers think microbes may be found at greater depths elsewhere in the world. They suspect that conditions in the CCSD borehole become too hot at 4.85 kilometres for life to survive at deeper levels. But in regions where the local geology means temperature rises more slowly with depth, life might survive several kilometres deeper before it hits the thermal limit.
Barbara Sherwood Lollar at the University of Toronto, Canada, says this focus on temperature alone may be too simplistic. 鈥淚 think the research community is beginning to consider that the limits are actually down to a complex interplay between a variety of parameters including temperature, pressure and the physical nature of the rocks 鈥� their porosity, for instance, and the water penetrating the system,鈥� she says.
Regardless of whether or not the microbes in the borehole have hit a thermal limit at 4.85 kilometres, confirming living microbes at that depth would be significant. Previous studies suggest that at that depth in the CCSD borehole, the temperature is about 137掳C. This is higher than 122掳C, the .
But Dong and Huang point out that theoretical calculations suggest life might be possible up to 150掳C, which would allow for the possibility of bacteria living at 137掳C.
鈥淚 don鈥檛 think any of us would be hugely surprised if there鈥檚 an organism that grows at 130掳C or at 135掳C,鈥� says Sean McMahon at the University of Edinburgh, UK.
Finding such a microbe would have implications for the widening of the search for extraterrestrial life. 鈥淚n astrobiology we define habitability as an environment that can support the growth of at least one known organism,鈥� says McMahon. 鈥淚f we discover a new organism that grows at a higher temperature, then our definition of habitability immediately changes to accommodate that new limit.鈥�
Geobiology
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