In One of the Driest Places on Earth, the First Rain in Centuries Brought Death Not Life

The Atacama Desert, primarily located in northern Chile, is the driest nonpolar region on Earth. In its center, lies a hyperarid core in which no precipitation had been recorded for 500 years, until 2015, when a sprinkling of rain fell on the area.

You might expect that this rain would have caused a sudden blooming in the amount of life in the region—which is limited to a handful of microbe species. However, the water had quite the opposite effect, wiping out most of the microbes that lived there, according to a paper published in the journal Scientific Reports.

"When the rains came to the Atacama, we were hoping for majestic blooms and deserts springing to life," Alberto Fairen, co-author of the study from Cornell University, said in a statement. "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," he said. "After it rained, there were only two to four microbe species found in the lagoons. The extinction event was massive."

According to the team, the sudden and massive input of water in regions that have remained hyperarid for millions of years is harmful for most of the surface microbial species—which are exquisitely adapted to survive in the inhospitable conditions. The sudden overabundance of water disrupts the normal functioning of the microbes via a process known as "osmotic shock."

A changing climate over the Pacific is thought to have been responsible for bringing the first rain in centuries to the Atacama's hyperarid core. These surprise precipitation events took place on March 25 and August 9, 2015, and again on June 7, 2017.

The new findings could have significant implications for our understanding of how microbial life evolved on Mars (if it ever existed at all, of course). The Atacama has long been of interest to astrobiologists due to its similarities with the Martian surface.

Mars has experienced a complex history of global climate changes, including a first period between 4.5 billion and 3.5 billion years ago, when the planet may have sustained large amounts of water on the surface. Over time, the climate became increasingly dry until the surface became the vast desert that we recognize today.

The Atacama Desert is one of the driest places on Earth. Rains in the past few years wiped out most of the microbes that lived there. iStock

However, this transition was interrupted by enormous discharges of water that flooded regions on the surface on several occasions after 3.5 billion years ago.

As a result, "hypothetical local ecosystems existing in some places on Mars, and adapted to the increasingly dryness of the Mars surface and subsurface after 3.5 billion years ago, would have been later episodically exposed to even stronger osmotic stresses than those we have reported here for the Atacama microorganisms," the authors wrote in the study.

Consequently, "the recurrence of liquid water on the surface of Mars after the earliest times might have contributed to decimate local or regional ecosystems, instead of being an opportunity for life to bloom again in the flooded areas," they said.

In addition, the new findings may shed new light on controversial samples collected by the Viking space probes in 1976. One experiment conducted by the NASA landers found evidence indicating the presence of life, while another on the same sample, found no traces of organic material.

"The negative results obtained with the life-detecting instruments onboard the 1976 Viking landers may find the simplest explanation in the fact that, in both experiments, samples were incubated with various watery solutions," the authors wrote.

"Any potential Martian cells would have not been exposed to such elevated values of water activity for at least millions of years, so their sampling and inclusion in the Viking experiments would have caused first their osmotic burst, and then the subsequent destruction of the organic molecules," the researchers said.