When Water Dooms Life

On Earth or on Mars, flooding can spell destruction for flourishing communities of microbes.

Ivan Alvarado / Reuters

The Atacama Desert in northern Chile is the driest place on Earth, a parched rockscape whose inner core supports zero animal or plant life. Only a few hearty species of lichen, algae, fungi, and bacteria can survive there—mostly by clinging to mineral and salt deposits that concentrate moisture for them. Still, it’s a precarious life, and these microbes often enter states of suspended animation during dry spells, waking up only when they have enough water to get by.

So when a few rainstorms swept through the Atacama recently, drenching some places for the first time in recorded history, it looked like a great opportunity for the microbes. Deserts often bloom at such times, and the periphery of the Atacama (which can support a little plant life) was no exception: It exploded with wildflowers. A similar blossoming seemed likely for the microbes in the core: They could drink their fill at last and multiply like mad.

Things didn’t quite work out that way. What should have been a blessing turned into a massacre, as the excess water overwhelmed the microbes and burst their membranes open—an unexpected twist that could have deep implications for life on Mars and other planets.

The Atacama has been arid for 150 million years, making it the oldest desert on Earth. Its utter lack of rain can be traced to a perfect storm of geographic factors. A cold current in the nearby Pacific Ocean creates a permanent temperature inversion offshore, which discourages rainclouds from forming. The desert also lies in a valley that’s wedged between the Andes Mountains on the east and the Chilean Coastal Range on the west. These mountains form a double “rain shadow” and block moisture from reaching the Atacama from either side. The desert’s driest point, the Yungay region, receives fewer than 0.04 inches (or 1 millimeter) of rain a year. Death Valley in California gets 50 times more rain annually, and even the driest stretch ever recorded there still averaged 0.2 inches a year.

That’s why the recent rainstorms in the Atacama—two in 2015 and one in 2017—were so startling. They left behind standing lagoons, some of which glowed a lurid yellow-green from the high concentration of dissolved mineral. Nothing like this had happened in Yungay since at least the days of Columbus, and possibly much earlier. No one quite knows what caused the freak storms, but climate change is a likely culprit, as the cold sea currents have been disrupted recently. This allowed a bank of rainclouds to form over the Pacific Ocean. The clouds then plowed over the Chilean Coastal Range and dumped water onto Yungay and surrounding areas.

Five months after the June 2017 storm, a group of scientists led by Armando Azua-Bustos, a microbiologist at the Universidad Autónoma de Chile, and Alberto Fairén, a planetary scientist at Cornell University, visited the Atacama to sample three lagoons. They wanted to study the microbes that had gotten swept into them and document how well they were handling this precious influx of water.

Not very well, it turned out. As detailed in a recent paper, the scientists found that the majority of microbes normally present in the soil had been wiped out—14 of 16 species in one lagoon (88 percent), and 12 of 16 in the others (75 percent)—leaving behind just a handful of survivors. On a local scale, the rains were every bit as devastating as the asteroid that wiped out the dinosaurs 66 million years ago, which killed off 70 to 80 percent of species globally.

The scientists traced this massacre back to the very thing that allows the microbes to survive in the Atacama: their ability to hoard water. Under normal conditions, this miserliness pays off. But when faced with a glut of water, they can’t turn off their molecular machinery and say when. They keep guzzling and guzzling, until they burst from internal pressure. Azua-Bustos and Fairén’s team found evidence of this in the lagoons, which had enzymes and other organic bits floating around in them—the exploded guts of dead microbes.

Water in the Atacama, then, plays a paradoxical role: It’s both the limiting factor for life as well as the cause of local extinctions. And while the death of some bacteria and algae might not seem like a big deal, these microbes are actually famous in some circles as analogues for life on Mars.

We don’t know whether Mars ever had life, but it seemed like a promising habitat for its first billion years, with vast liquid oceans and plenty of mineral nutrients—not much different than Earth. One billion years probably wasn’t enough time for multicellular life to arise, but Martian microbes were a real possibility.

Starting around 3.5 billion years ago, however, our planetary cousin went through a severe drying-out and began to lose its water. Some was sucked deep underground, and most of the rest got dissected into H2 and O through various chemical reactions. Eventually these processes turned most of Mars’s surface into one giant Atacama Desert, forbiddingly dry and dotted with mineral deposits. NASA, in fact, uses the Atacama landscape to test rovers and other equipment for Mars missions.

But there’s an important wrinkle here. The great drying-out didn’t happen instantly; it took eons. And during the transition, when Mars was fairly parched but still had some liquid water, it experienced floods that would have made Noah blanch. We can see evidence of them on the surface of Mars today: The dry riverbed channels and alluvial fans that those floods left behind are the largest in the solar system.

This tumultuous state—a hyper-dry climate, punctuated by massive washouts—would have been catastrophic for life on Mars. The slow drying-out would have choked off the vast majority of microbes, grinding them into dust. Any that managed to pull through, scientists have argued, probably would have resembled those in the Atacama today: water-hoarders clinging to oases of mineral deposits in a vast red desert.

But if Martian microbes did resemble their Atacama counterparts, then the washouts probably finished them off, swelling them with water and bursting them like balloons. After a certain point, in other words, Mars might have been too wet to sustain the life that evolved there.

It’s possible, of course, that a few lucky pockets on Mars escaped flooding entirely, allowing microbes there to survive until today. But if so, Azua-Bustos and Fairén point out, our current approach to finding these holdouts could be doomed to fail. NASA sent the famed Viking lander to Mars in 1976, for example, largely to search for life there. To this end, the lander scooped up several soil samples for analysis—and immediately doused them with water. Viking might have come up empty anyway, but given the Atacama results, it also might have killed off the very thing it was looking for.

What applies to Mars applies to other worlds as well. Over the next decade, several new space telescopes will expand the hunt for life beyond our solar system, to planets orbiting distant stars. Scientists are especially keen to find planets that have liquid water, since as far as we know, liquid water is essential to life.

But that statement might need qualification. Water can give life, certainly. As planets change, however, and life evolves in tandem, it can also snatch life away.