Not a drop of water and yet there is life. Researchers have now uncovered how the microbes of the world’s driest desert supply themselves with the elixir of life. The refined bacteria get the H2O with the crowbar from the rock: they convert calcium sulfate dihydrate (gypsum) into anhydrite by means of organic acids, whereby usable water molecules are released. This chemical process of water extraction shows that microbial life is possible even in apparently water-free environments such as on Mars.
Because of the extreme conditions, the Atacama desert in Chile even serves as a model for the surface of Mars: extreme drought, salty soils and strong UV radiation make it one of the most hostile regions on earth. But here too, organisms find opportunities to exist, studies of recent years have shown. In addition to other extremophilic microbes, special representatives of the cyanobacteria live under thin rock layers in the Atacama. Protected from too hard radiation and the dry winds, they use the light that shines through them to perform photosynthesis in order to supply themselves with energy. In addition to the Atacama, the so-called Chroococcidiopsis bacteria are also known from other deserts on Earth.
What is the water source?
But how do these bizarre microbes get to the water they need in the dust-dry habitats? The researchers led by David Kisailus from the University of California at Irvine have now specifically investigated the suspicion that the Chroococcidiopsis bacteria can release chemically bound H2O from the rock. Specifically, the focus is on the gypsum component. This substance is a hydrogenated form of calcium sulfate (approx[SO4]· 2H2O) – there is water in the connection. Without the water, the mineral is called anhydrite (approx[SO4]) designated.
The scientists obtained plaster samples from the Atacama Desert for their investigations and then analyzed them in the laboratory. As they report, it was initially shown that there was an overlap in the concentrations of anhydrite and the presence of the cyanobacteria in the plaster samples collected in the Atacama. Where the microbes live, the anhydrite content is higher. “This finding already indicated that the microbes extract water from the rock in order to survive,” says Kisailus. “That’s why we then carried out experiments to further confirm this hypothesis,” said the scientist.
For this purpose, the researchers made small rock cubes, which they colonized with the Chroococcidiopsis bacteria. There were two different conditions: Some of the cubes were kept at comparatively high air humidity, which is known to serve as a water source for some microorganisms. The other part of the cubes, however, was exposed to the typical Atacama dryness. Subsequent analyzes of the rock then showed that anhydrite had formed only in the dry conditions – no gypsum had been dehydrated in the wetter conditions. “In these conditions, the cyanobacteria did not need water from the rock, because they got it from their surroundings,” explains Kisailus. “But when put under drought stress, the microbes had no choice but to extract the water from the gypsum, which triggered this transformation of the material.”
Organic acids remove the H2O from the plaster
The researchers then carried out electron microscopic and spectroscopic investigations to examine the interactions between the biological and geological parts of the system even more closely. It was shown that the microbes penetrate the rock like tiny miners to break down the precious water-containing material. According to the scientists, they release certain organic acids into the mineral structure. These lead to a mobilization of the water molecules, which the bacteria then incorporate.
“It was already suspected that microorganisms could be able to extract water from minerals – we have now clearly confirmed this hypothesis for the first time,” sums up co-author DiRuggiero from Johns Hopkins University in Baltimore. “It is an amazing survival strategy for these microorganisms that exist on the dry frontier of life. The findings can now guide us in the search for traces of life in other extreme locations, ”says the scientist with a view of Mars and Co.
Source: University of California at Irvine, technical article: PNAS, doi: 10.1073 / pnas.2001613117