Even in the harshest environments, microbes always seem to get by. They thrive everywhere, from the hydrothermal vents of the boiling seabed to the high mountains of Mount Everest. They were clusters of microbial cells attached to the hull of the International Space Station were also found.
There was no reason for microbial ecologist Noah Fierer to expect the 204 soil samples he and his colleagues collected near Shackleton Glacier in Antarctica would be any different. A spoonful of typical soil it could easily contain billions of microbes and the Antarctic soils of other regions are home to at least a few thousand per gram.
So he thought that all of his samples would harbor at least some life, even though the air around Shackleton Glacier is so cold and so arid that Fierer often left his wet cloths outside to freeze-dry.
Surprisingly, some of the colder, driest soils did not appear to be inhabited by microbes at all, he and his colleagues reported in Journal of Geophysical Research: Biogeosciences of June. As far as Fierer is aware, this it is the first time that scientists have found soils that do not appear to support any kind of microbial life.
The results suggest that extremely cold and arid conditions could place a hard limit on microbial habitability. The findings also raise questions about how negative scientific findings should be interpreted, especially in the search for life on other planets. “The challenge comes back to this kind of philosophical question, how do you prove a negative?” says Fierer.
Proving a negative result is notoriously difficult. No measurement is perfectly sensitive, which means that there is always a chance that a well-executed experiment will fail to detect something that is actually present.
It took years of experiments based on multiple independent methods before Fierer of the University of Colorado Boulder and his collaborator Nick Dragone finally felt confident enough to announce that they had found soils apparently free of microbes. And the scientists intentionally stated only who were unable to detect life in their samples, not that the land was naturally barren.
“We cannot say that the land is barren. Nobody can tell, ”says Fierer. “This is an endless quest. There is always another method or a variant of a method that you could try ”.
Microbes: even on earth, life can fail
Polar microbiologist Jeff Bowman interprets the team’s results as a false negative. “Of course, there were things there”says Bowman of the Scripps Institution of Oceanography in La Jolla, California. “This is the Earth. This is an environment massively contaminated by life “.
Even if there were some undetected microbes in the soil, Dragone said, that wouldn’t affect his team’s evidence that cold and dryness pose a serious challenge to life. “It is the combination of several very demanding environmental conditions that limits life more than a single action alone”, says Dragone. “It is a very different type of restriction than, for example, only at high temperatures.”
As scientists seek evidence of life beyond Earth, they will inevitably be forced to walk the line between evidence of absence and absence of evidence.. “What we are trying to do on Mars is a bit the opposite 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 it’s hard to sell as a scientific claim. On Mars it will be the other way around.