Water may be an essential element of life, but to the scarce microbes that inhabit the Mars-like Atacama Desert in Chile, it's death. Studying the effects of once-in-a-century rainfall in the hyperarid core of the desert, a team of astrobiologists led by Cornell University found that instead of causing a bloom of growth, the unexpected abundance of water killed off three quarters to seven/eighths of the microbe species present.

As scientists seek signs of present or past life on Mars, they need to readjust their thinking as to what is a normal environment. For this reason, Chile's Atacama Desert has been the focus of attention for astrobiologists trying to understand what Martian life might be like.

Compared to the Red Planet, the Atacama is a paradise, but by terrestrial standards it's the most lifeless spot on Earth that isn't covered with boiling lava. The area is so cold and so dry that nothing can live there – not even bacteria, except in a few isolated subsurface colonies.

Climate models indicate that it only rains about once a century in the Atacama and evidence on the ground suggests that some parts only see rain once every 500 years. Because of this, what few scatters of microbes there are have had to adapt to these hyperarid conditions.

If you've ever seen a desert before and just after a heavy rain, the transformation can be dramatic. At one moment, the sands seem as dry and lifeless as the Moon, but after the showers pass, there's a sudden burst of life as plants sprout up and grow, insects hurry through their life cycles, and even fish and amphibians appear where their presence seemed impossible only the day before.

This sort of opportunistic growth seems logical in arid conditions, but the Atacama runs by its own logic, as could be seen when rains came to its core on March 25 and August 9, 2015, and again on June 7, 2017. These left behind ephemeral lagoons and death.

"When the rains came to the Atacama, we were hoping for majestic blooms and deserts springing to life," says Alberto Fairen, Cornell visiting astrobiologist. "Instead, we learned the contrary, as we found that rain in the hyperarid core of the Atacama Desert caused a massive extinction of most of the indigenous microbial species there.

"The hyperdry soils before the rains were inhabited by up to 16 different, ancient microbe species. After it rained, there were only two to four microbe species found in the lagoons. The extinction event was massive."

According to team, the problem was one of superabundance. With microbes adapted to Martian conditions, what seems to us like a blessing becomes a deadly curse as the water overwhelmed their ultra-thrifty biochemistry. What happened to the Atacama microbes also suggests that the negative results of the Viking experiments in 1976 to incubate Martian life might have killed their specimens with kindness by causing their cell membranes to burst through the provision of too much water. Something that future missions will need to keep in mind.

"Our results show for the first time that providing suddenly large amounts of water to microorganisms – exquisitely adapted to extract meager and elusive moisture from the most hyperdry environments – will kill them from osmotic shock," says Fairen.

The research was published in Scientific Reports.

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