The Atacama Desert is one of the places on Earth that most resembles Mars.Dima Gavrysh / AP

As one of the driest places on Earth, Chile’s Atacama Desert is one of the last places you’d expect a trip to be ruined by rain.

Dirk Schulze-Makuch happened to be so lucky. In early 2015, he was preparing for field work in the Atacama, which he expressly chose because he was hunting for life in extreme—i.e., dry—conditions. (On Earth, only the Dry Valley in Antarctica is drier than the Atacama.) Then in March, a freak rainstorm hit. “You had to prepare everything months before, and then, heck, it’s raining,” says Schulze-Makuch, an astrobiologist at the Technical University of Berlin.

But there was an upside. The March shower gave the team a unique opportunity to study what happens when the Atacama’s hyperarid regions—the driest of the dry—finally get water. After a rare rainstorm, the more humid parts of the Atacama actually grow a spectacular carpet of flowers, but the hyperarid sections stay barren to the eye. There is nothing to see but soil and rock. “It’s difficult to describe. It’s nearly a different planet. It’s kind of lifeless,” says Schulze-Makuch.

Not entirely. His team found that microbial life blooms even in the hyperarid regions after rain. And when Schulze-Makuch’s team returned in 2016, then again in 2017, they found microbial life diminished but seemingly still active.

Scientists have found DNA of microbes in the Atacama before. They have even, in some cases, coaxed microbes from the Atacama to multiply on petri dishes. But it’s unclear whether these tiny life forms are permanent Atacama dwellers or just transient microorganisms, carried by desert gusts. “The question is, well, are these creatures coming here to die, or are they able to survive waiting for water,” says Armando Azua-Bustos, a researcher at the Center of Astrobiology in Madrid who was not involved in the study. This new paper strongly suggests they can survive in the desert soil.

(Schulze-Makuch’s team focused on microbes that live in the desert’s soil, from the surface to a foot underground. There are, separately, microbes that live several feet lower in salt rocks, which absorb what little water there is in the soil. It’s unlikely the microbes Schulze-Makuch’s team studied can access this deeper water source.)

Researchers conduct field sampling in the Atacama Desert. (PNAS)

To prove that these microbes are actually alive, Schulze-Makuch’s team used a whole suite of methods. “They really threw the book at it,” says Julia Neilson, a soil scientist at the University of Arizona. Ordinary DNA sequencing, for example, can pick up dead cells. So the team specifically picked out intact and presumably live cells, sequencing their DNA separately. They also looked for evidence of active life: enzymes, a molecule called ATP known as the “currency of energy” in cells, DNA replication, and other molecules present when a cell is living and breathing. They found all of these when they looked in 2015.

When the team returned in 2016 and 2017, they found much, much less DNA and ATP inside intact cells in the soil. The microbial bloom was only temporary. But not all the microbes were dead or entirely dormant, either. The amount of ATP inside cells at two of the driest sites was very low but not zero. Even in these harsh, dry conditions, a few very hardy microbes could just eke out a living.

It’s also possible other microbes are active—but at levels too low to detect. “It raises the question about the limits of detection,” says Jackie Goordial, a researcher at the Bigelow Laboratory for Ocean Sciences who studies microbes in extreme environments. “In deep-sea marine sites or in permafrost soils there are estimates that cells replicate every 1,000 to 10,000 years. So how do you measure that?”

Scientists are often interested in extreme environments, like the Atacama or permafrost, because they’re interested in understanding the limits of life—not just on Earth, but in the whole universe. Schulze-Makuch’s work was funded in part by a grant from the European Research Council to study habitability on Mars. If you take the Atacama as an example, he notes, “there doesn’t have to be all the time moisture, just once in a while, and life can make a living. We think on Mars the situations are similar but a bit more extreme.”

On Earth, scientists still have many more questions about the microbes in the Atacama. For example, if they are still active in dry periods, how are they able to do it? Schulze-Makuch is leaving for another expedition in the Atacama in March. He has no idea if it’s going to rain.

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