Islands of Life, Part V
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.
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.”
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.”
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.