Deliquescence in the Atacama
11 Jul 2006
(Source: Astrobiology Magazine)
If you're a microbe, looking for a place to call home in the hyperarid core of Chile's Atacama Desert, a little deliquescence goes a long way.
Yungay, Chile, was once the site of a bustling community, built up overnight in the late 1800s to extract sodium nitrate from the desert soil, for fertilizer; then abandoned, again overnight, when nitrate mining went bust back in the 1920s, after the invention of artificial fertilizer. Yungay now is home to the University of Antofagasta Desert Research Station, a ramshackle five-room building very much in the middle of nowhere. For a few weeks each year, Yungay comes alive again, as the station is occupied by researchers from around the world, who travel here not to search for minerals. They come here, to the driest desert environment on Earth, to study the limit of life.
In other deserts - the Antacrtic Dry Valleys, the Negev in Israel - scientists have found microorganisms living inside and underneath quartzite and sandstone rocks. In these microenvironments, just the right combination of moisture (scarce, but present in small quantities), translucence (to let some light through for photosynthesis, but block harmful UV radiation) and porosity (tiny pits, cracks and crevices that can trap moisture) provide an ideal habitat for Chroococcidiopsis. One of many types of photosynthetic bacteria, Chroococcidiopsis (kroh-oh-*****-sid-ee-op-sis) is a champion at surviving in dry environments.
But Yungay is drier than the Dry Valleys, drier than the Negev. And so, despite conducting an extensive search for such microorganisms over the past several years in and around Yungay, scientists have by and large come up empty-handed. They have scoured the area, exploring for miles in every direction, turning over, breaking open and grinding up rocks, looking for evidence of biological activity. And they've been both fascinated and frustrated by their inability to find it.
Until last year. That's when Jacek Wierzchos, a chemist from the Univeristy of Lleida, near Barcelona, Spain, made his first visit to Yungay. On his first day there, Weirzchos went for a walk in the salt flat just behind the research station. This salar, as it is known in Spanish, a mile or two square, is a vast field of bizarre formations - some knobby, others curved and twisting, most of them a few inches high - that poke up from the salt-encrusted ground. The protrusions, glistening white in the sun where their surfaces are exposed, a gritty dull brown where they are coated with desert sand, are rocks made of salt.
The salar, says Weirzchos, "is a dry lake. Maybe 5 million years ago, it was a lake, but it was evaporated and the salt was precipitated."
While strolling through the salar, Wierzchos noticed a thin dirty gray layer along the edge of one of these salt rocks, a few millimeters (about one-quarter of an inch) below its surface. Intrigued, he broke off a piece of the rock and brought it back to the research station. He dissolved a bit of the material in water, placed a drop on a microscope slide, and took a look. He expected it to be some kind of mineral contamination.
Instead, what he saw were living cells. There was life, thriving, inside dry salt rocks. He had discovered a previously unknown habitat for life on Earth. Microbes had been discovered living in rocks before. And they'd been found living in extremely salty - wet and salty - environments. But never inside dry salt rocks.
"In wet halite, okay," says Wierzchos, citing the Dead Sea as an example of a wet, salty environment where microbes have been found. "But this is dry halite. This is totally different stuff."
He went back, broke open another rock, and another. They were all colonized. The entire salar, which began just beyond the dirt patch out behind the research station, where scientists pitch their tents while they're working in Yungay, was a vast habitat for life.
Deliquescence is how these microbes, believed to be a species of Chroococcidiopsis, are able to survive. Deliquescence is the absorption of moisture from the atmosphere.
Here's how scientists think the microbes make their living. You know how on a muggy day the salt in your kitchen cakes up? That's because the salt has absorbed moisture form the air. The salt rocks in the Atacama do the same thing. They are made of the same type of salt that you cook with. The scientific name for it is halite. And whenever the relative humidity gets above 75 percent, halite absorbs moisture. Deliquescence.
It may seem that in the driest place on Earth, where daytime relative humidity is on the order of 1 or 2 percent, deliquescence would be a rare event. It is rare, but not that rare. At night, the temperature in Yungay often drops below freezing. (Icicles often make a dawn appearance, hanging down from the station's water storage tank, which sits atop a corrugated metal shed.)
Cold air can't hold as much moisture as warm air can. So even though there's not much moisture in the air to begin with, when the temperature plunges at night, relative humidity shoots up. A similar phenomenon is what causes morning dew. But in the Yungay salar, the moisture doesn't condense on the surface of the salt rocks; instead, the rocks suck the moisture inside, down to where the Chroococcidiopsis lies waiting for a drink.
No one knows how often this happens in Yungay. That's one of the details scientists are still studying. It's not every night. But it almost certainly is a more frequent occurrence than rainfall, which on average happens only once a decade.
During the course of the night, as the temperature continues to drop, relative humidity continues to rise. It peaks when the air is coldest, just before dawn. Then the sun comes up. And there are the microbes, tucked a few millimeters below the surface of the halite rock, having just received a tiny shot of moisture, sunbathing. All across the salar, Chroococcidiopsis cells spring to life - literally.
"They wake up, they come to life, they metabolize," says Wierzchos. They repair some DNA damage, perhaps grow a bit, and occasionally, on a really good morning, even divide in two.
It doesn't last. Within a few hours, the excitement's over. The air temperature has shot up, the moisture has evaporated. Deprived of their water, the cells go back into a dormant state.
That's the working theory. Just as researchers don't know how often the relative humidity gets above 75 percent, they also don't know the details of the Yungay Chroococcidiopsis life cycle. Maybe the moisture sticks around inside the halite for a few days. The moisture level inside the rocks does change more slowly than the humidity of the surrounding air. Maybe the microbes take a while to wake up. Maybe they remain active longer. All of that is yet to be sorted out, work in progress.
But one thing is for certain, says Wierzchos. Even "in the driest place in our planet, we have photosynthetic life. Which means that life is very resistant. It's much more resistant that we thought."
Thanks to deliquescence.
by Henry Bortman