Chile’s Atacama Desert is the driest place on Earth. It receives just millimeters of rain each year, on average, and its parched conditions make it a commonly used stand-in for Mars.
Last year, scientists working in the Atacama were astounded to find multiple lagoons of liquid water there—the ephemeral runoff from an unusual rain event. But the rainfall that created these ponds didn’t result in a bloom of life, the researchers found; it was actually lethal to the majority of microbes adapted to the extreme aridity of the Atacama.
These findings have implications for future spacecraft missions that will collect samples from other planets: Incubating dry soil samples in aqueous solutions, as was done with the Viking landers on Mars in the 1970s, may have the inadvertent effect of killing microbial life.
Life in Lagoons
Armando Azua-Bustos, an astrobiologist at the Center for Astrobiology in Madrid, Spain, was born and raised in the Atacama. When he first spotted standing pools of water in the desert in 2015, he dismissed them as being likely due to human activities like mining. After all, there had been no significant rain events in the area for the past 500 years. The pools evaporated a few months later.
But in 2017, three more temporary lagoons appeared. After linking these lagoons to unprecedented rainfall that had occurred months earlier, Azua-Bustos and his colleagues turned to studying the effect of these unexpected ponds on local microbial life.
The three lagoons ranged in length and width from a few meters to several tens of meters, and they were between 10 and 30 centimeters deep. The scientists used four techniques to analyze the microbial populations within the ponds: They analyzed samples of lagoon water using microscopy, identified sequences of RNA genes, cultivated the samples in different media, and searched for biomarkers produced by microbes.
They found only two to four species of bacteria in each of the lagoons. That’s significantly fewer than the 16 different species of bacteria, archaea, and eukaryotes that inhabited the hyperdry soils before the rains, the team reported today in Scientific Reports. “About 70% of the species that were in the soil just disappeared,” said Azua-Bustos.
One thing looked clear: This microbial ecosystem just “doesn’t know how to deal with water,” he said.
Burst Like Balloons
Azua-Bustos and his team hypothesized that the microbes were killed by osmotic stress after they absorbed water. “They burst like balloons,” he said.
This die-off may have implications for how, in the future, soil samples are collected from other planets such as Mars, said Alberto Fairén, an astrobiologist at the Center for Astrobiology in Madrid, Spain, and Cornell University in Ithaca, N.Y., who was also involved in the work. It may be vitally important to protect dry soils by not contaminating them with water, explained Fairén.
Not using water would represent a change from previous protocols. “The Viking experiments involved incubation of Mars soil samples in aqueous solutions,” said Fairén. The Viking landers used these solutions to determine whether any hypothetical microbes on Mars were consuming carbon-based compounds.
The researchers are currently doing more fieldwork in the Atacama. They’re monitoring the microbial diversity in the wet sediments from the bottoms of the lagoons, which have since evaporated. “We are now sampling soils and mud to check if the ecosystems are recovering or are being substituted by new ecological equilibriums,” said Fairén.
“This is important work,” said Chris McKay, a planetary scientist at NASA Ames Research Center in Moffett Field, Calif., who was not involved in the research. “There is clear geological and geochemical evidence that the most arid region of the Atacama has experienced these sort of floods. This is the first analysis of such rare events.”