Islands of Life, Part V
Wierzchos attacks an unusually durable halite formation in the Yungay salar. Credit: Henry Bortman
In his final report from the Atacama Desert in northern Chile, the driest place in the world, Field Research Editor Henry Bortman is brought up to speed on six years of extremophile research by expedition leader Jacek Wierzchos.
Yungay salar, Atacama Desert, Chile
May 12, 2011
I first met Jacek Wierzchos five years ago, at the desert research station in Yungay. As we walked out into the salar (salt flat) behind the station, he told me the story of his discovery only a year earlier of photosynthetic bacteria living inside the halite (salt) rocks that blanket the salar. He and his colleagues were then in the early stages of trying to figure out their survival strategy.
At the time, he had formulated a theory that it was the ability of halite under certain conditions to absorb moisture from the surrounding air that provided a source of water in this hyperarid environment. But his theory hadn’t yet been tested.
“After five years,” he told me on the last morning of my recent trip to Yungay, during a similar stroll through the salar, “we have much more information.”
For example, researchers now know more about the specific cyanobacterium that inhabits the halite. At first, Wierzchos told me, they thought it was a type of Chroococcidiopsis (crow-oh-cock-sid-ee-op-sis), a genus of bacteria well known for its ability to survive in very dry places. “In the Dry Valleys in Antarctica, in the Negev Desert, and in many places, Chroococcidiopsis is the main cyanobacteria which can resist very dry conditions.” And under the microscope the halite-dwelling organism looks like Chroococcidiopsis.
But genetic sequencing has shown it to be closer to the genus Halothece, discovered a few years ago living in shallow hypersaline ponds along the coast of Baja California Sur in Mexico. The same organism has been found in several different salars in the Atacama, separated by hundreds of kilometers, but nowhere else in the world. It is “probably a new genus of cyanobacteria” that exists “only here,” Wierzchos said, “in the hyperarid core of the Atacama Desert.”
The Yungay salar in late afternoon light. Credit: Henry Bortman
A better understanding has also been developed about the microenvironment inside the halite that enables the bacteria to live there. Despite harsh conditions unfavorable to life – nearly 100 percent salinity, long periods without any input of moisture and temperature extremes that can vary by as much as 50 degrees Celsius (90 degrees Fahrenheit) within a single 24-hour period – the halite also offers several qualities that make it a suitable habitat. It screens out UV light but lets through light needed for photosynthesis. And it is porous, its pore spaces being neither too large nor too small, but “just right” for microbes to inhabit them.
Most important, though, is the hygroscopic property of halite, its ability to absorb water vapor from the surrounding air when the relative humidity goes above 75 percent. Although Atacama air is very, very dry – aircraft that fly over don’t leave contrails because the water vapor disperses so quickly – at night, when the temperature drops low enough, relative humidity can sometimes rise above this threshold. When that happens, the halite rocks absorb moisture and concentrate it, a process known as deliquescence, forming brine inside the rock, which the bacteria take advantage of.
That was the original theory, published in 2008. To test it more thoroughly, researchers for the past year and a half have been “monitoring external microclimatic conditions” and have also placed “several smart sensors on the interior of the rock, in the colonization zone.” This is the information being recorded at the met station I talked about in my previous post. What they found surprised them.
“It is a paradox,” Wierzchos said. From January 2010 to the present, the “relative humidity of the air never exceeded 75 percent.” In other words, in a year and a half, there wasn’t a single deliquescence event. But during that same time period, “inside the rock we have had conditions with relative humidity [higher] than 75 percent during almost 4000 hours.” So halite appears to have the ability not only to absorb water but also to retain it, even when exterior conditions are extremely dry. Wierzchos and his colleagues are working to “elaborate an hypothesis” of “how this is possible.” It’s not a previously known phenomenon.
The halite rock formations in Salar de Navidad, shown here, are small because 20 years ago the salar was mined for salt. Credit: Henry Bortman
Other new questions have also arisen. In Salar Grande and Salar Llamara, scientists have found colonized halite like that in Yungay. But on this trip, they discovered salars that appeared similar, but where there was very little colonization. “It could be detected, but it’s not obvious, it’s not so abundant,” Wierzchos said.
So while the question five years ago was, “How does life survive inside the halite? “, a new question arises now: “Why, in some environments, in some places of the Atacama Desert, in the same substrate, in apparently the same climatic conditions, is life absent?”
Wierzchos went on to talk about other similar environments where one might expect to find microbial life similar to that in the Atacama. “There are many environments, habitats, that conditions look very good for endolithic life, and this endolithic life is not present. For example, in the Negev Desert. “We know that halite is present in the Negev Desert, not far from the Dead Sea.” But, he said, “the Negev Desert is too wet for microbial life inside halite rocks. In fact it is too wet for formation of these salt-composed pinnacles. It is a paradox that a desert could be too wet for life, but this is true.” Even the occasional rain there is enough to wash away any stable halite formations that might begin to grow, thus destroying the potential microhabitat inside these rocks.
So, said Wierzchos, he was beginning to come to the conclusion that it was not merely the specifics of the extreme environment in the Atacama that have enabled the specialized halite environment to develop, but rather that “most important is the stability of the system. We know on our planet many places with extreme environments which are colonized by microbial life. My observations indicate that environmental conditions could be apparently extreme for life, but if they are constant for long period of time, the microorganisms could find the manner to survive in these conditions. And they might make the transition from ‘extremotolerant’ to ‘extremophiles.’ The Atacama Desert, probably the oldest desert on Earth, is an excellent example of [the type of environment that leads to] this kind of life, which survived in conditions that were extremely harsh but constant for millions years.”
A gypsum outcrop (not colonized) at the eastern edge of the Yungay salar. Credit: Henry Bortman
Oh, and one more thing. Researchers have now found life not only in halite but also in gypsum. Like halite, “gypsum is semi-transparent and porous but not as salty as halite,” he said, so it looked like “it could be a perfect target, a perfect substrate for colonization.” In Bea Hills (see my third report in this series), however, where there is a lot of gypsum, researchers didn’t find any colonization.
So Wierzchos and his colleagues also checked out other locations. “and about 300, 400 kilometers to the north,” in a place that has been dubbed Kilometer 37, “we found one area with gypsum crust and in this area gypsum is colonized by many kinds of micro-organisms. We have lichens, we have algae, fungi, cyanobacteria,” Wierzchos said. “We have endolithic colonization and also we have epilithic colonization, which means that the microorganisms can live also on the surface of the gypsum crust.”
Halite “triplets” in the salar at Yungay. Credit: Henry Bortman
“Rain is not a source of water in this place. The source of water is the high enough relative humidity of the air.” Almost every night, he said, “the relative humidity in that area is higher than 80 percent” and dew condenses on the gypsum crust. “Gypsum doesn’t deliquesce.” So in Bea Hills, near Yungay, where the relative humidity is much lower, “gypsum is devoid of life.” But in locations where there is high relative humidity at night, the environment provides enough water for a virtual menagerie of microbes to inhabit the gypsum.
The work of understanding that environment, too, is ongoing.
And with all that swimming around in my head, I packed up my tent, said my goodbyes, climbed into my rental truck and, leaving the extremophiles to bask in the dry heat, drove back down to Antofagasta, to a room with air-conditioning, a much-needed shower and a bed with clean sheets.
Wierzchos’s investigation of extremophile microhabitats in the Atacama has been conducted in close collaboration with his colleagues from the National Museum of Natural Sciences in Madrid: Prof. Carmen Ascaso, leader of the EcoGeo investigation group; Asunción de los Ríos; and PhD candidates Beatriz Cámara and Sergio Valea.