While traces of life are being searched for on Mars, researchers have found a region on Earth where no life seems possible at all. Until now, scientists had assumed that microbes could exist in any environment, no matter how hot, cold, salty or acidic, if they had enough time to adapt. However, an international team of researchers has discovered no evidence of microbial life when they analyzed soil samples from the Transantarctic Mountains. The mountains in the middle of the Antarctic continent could thus be the only known region on Earth that does not even allow extremophile microorganisms to survive.
No environment seems to be too hostile to microbes to keep them from thriving – they are found en masse in the boiling hot and acidic springs of Yellowstone National Park, in the black smokers on the ocean floor that can reach several hundred degrees, and in the dry valleys of Antarctica, one of the driest places on Earth. Now, a recent study published in the Journal of Geophysical Research shows that the coldest, driest, and saltiest soils of the Transantarctic Mountains seem to push microbes to the limit. However, as reported by Antarctic Sun, scientists cannot be absolutely certain that these soils are completely sterile. If life does exist, it will be below detection limits. Noah Fierer, a microbial ecologist at the University of Colorado Boulder and co-author of the study, tells Antarctic Sun, “Microbes can basically tolerate anything, and yet here we have evidence that, oh, maybe that’s not always true.”
“If we can assume these soils in Antarctica are kind of an analog for a study of Mars, I think we have to be prepared for the assumption that there just may be nothing there.”Noah Fierer, a microbial ecologist at the University of Colorado Boulder and co-author of the study.
Microorganisms are the most common form of life on Earth, they occur everywhere and today account for about 15 percent of the total biomass. Based on the results, the researchers can identify conditions under which it is impossible for even the most resilient of them to survive. This also has implications for the question of whether life exists on other planets, such as Mars. Mars is also extremely cold, dry and salty, so microorganisms may not be able to survive there at all.
The limits of life
The research team collected more than 200 soil samples at ten different sites in the Transantarctic Mountains in the Shackleton Glacier region during the Antarctic summer of 2017/2018 at different sea levels ranging from a few hundred meters to over 7,000 meters to cover as many different conditions as possible.
Back in the lab, they analyzed the samples for traces of DNA and applied slurries of the soil samples to culture medium to determine if microbial colonies were growing. Last, they examined whether the samples consumed glucose and produced energy.
Soils at higher elevations and farther inland are generally drier and saltier, according to the researchers, and they had fewer microbes. In about one-fifth of all samples, they were unable to detect any evidence of viable microbial life. “The environmental conditions that are getting more challenging as you increase in elevation in the Shackleton Glacier Valley are restricting microbial life to a point where we can’t detect them using these widely used methods,” said Nick Dragone, a microbial ecologist at the University of Colorado Boulder and lead author of the study. “And that pattern itself is really interesting.”
The researchers suspect that the combination of extreme conditions prevents microorganisms from colonizing the soils. “Each one of those individually, microbes can tolerate,” Fierer said. “They can tolerate super low temperatures. They can tolerate high salt concentrations. They can tolerate lack of liquid water. But I think these soils where we see no detectable microbes, it’s kind of a combination of all three. You’re hitting them with the hat trick of challenges.”
The next step will be to examine the samples in which the researchers found microbes to find out how they cope with the hostile conditions. Most of the microorganisms detected were fungi, suggesting that fungi may be best adapted to harsh environments.
Julia Hager, PolarJournal
Link to the study: Dragone, N. B., Diaz, M. A., Hogg, I. D., Lyons, W. B., Jackson, W. A., Wall, D. H., et al. (2021). Exploring the boundaries of microbial habitability in soil. Journal of Geophysical Research: Biogeosciences, 126, e2020JG006052. https://doi.org/10.1029/2020JG006052
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