How extinct Antarctic microbes make it harder to find aliens

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Even in the harshest environments, germs always seem to get away with it. They thrive everywhere from searing hydrothermal vents on the seabed to the high peaks of Mt. Everest. Clusters of microbial cells have even been found clinging to the hull of the International Space Station (SN: 08/26/20).

There was no reason for microbial ecologist Noah Fierer to expect the 204 soil samples he and his colleagues collected near the Shackleton Glacier in Antarctica would be any different. A typical spoonful of soil could easily hold billions of microbes, and Antarctic soils in other regions harbor at least a few thousand per gram. So he assumed that all of his samples would harbor at least some life, even though the air around Shackleton Glacier is so cold and arid that Fierer often left his damp laundry outside to freeze dry.

Surprisingly, some of the cooler, drier soils did not appear to be inhabited by microbes at all, report he and his colleagues in the Journal of Geophysical Research: Biogeosciences. To Fierer’s knowledge, this is the first time that scientists have discovered soils that do not appear to harbor microbial life.

The results suggest that extremely cold and arid conditions could impose a strict limit on microbial habitability. The findings also raise questions about how negative scientific results should be interpreted, especially in the search for life on other planets. “The challenge comes down to this kind of [question], how do you prove a negative? Said Fierer.

Noah Fierer and his colleagues found soil samples from the Shackleton Glacier region in Antarctica that showed no evidence of life, an unexpected observation because samples from the continent typically contain thousands of microbes.Courtesy of N. Fierer

Proving a negative result is notoriously difficult. No measurement is perfectly sensitive, which means there is always a possibility that a well-executed experiment will fail to detect something that is actually there. It took years of experiments based on multiple independent methods before Fierer at the University of Colorado Boulder and his collaborator Nick Dragone finally felt confident enough to report that they had found soils that were apparently free of microbes. And the scientists intentionally said they were unable to detect life in their samples, not that the soils were naturally barren. “You cannot say that the soils are sterile. Nobody can say that, ”says Fierer. “It’s a never-ending quest. There is always another method or variation of a method that you can try.

Polar microbiologist Jeff Bowman interprets the team’s findings as a false negative. “Sure, there were things there,” says Bowman of the Scripps Institution of Oceanography in La Jolla, Calif. “It’s Earth. It’s an environment that is heavily contaminated with life.

Even if there were a few undetected microbes in the soil, Dragone said, that wouldn’t call into question his team’s evidence that cold and aridity pose a serious challenge to life. “It is the combination of several very difficult environmental conditions that limits life more than just one acting by itself,” explains Dragone. “It’s a very different kind of restriction than, say, just a high temperature. “

As scientists search for evidence of life beyond Earth (NS: 07/28/20), they will inevitably be forced to cross the line between proof of absence and absence of proof. “What we are trying to do on Mars is a bit of the reverse of what we have tried to do on Earth,” says polar microbiologist Lyle Whyte of McGill University in Montreal. On Earth, claiming that an environment is lifeless is a tough scientific sell. On Mars, it will be the other way around.


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