Viruses Thrive Even in one of Earth’s Toughest Deserts

To prosper here, their microbe hosts live in the rocks and draw water from the atmosphere.

Salt nodules (whitish rocks in the foreground) are the last outpost of life in the hyperarid core of the Atacama Desert (see hammer for scale). (Dirk Schulze-Makuch)
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In a new paper published in Environmental Microbiology, Gherman Uritskiy and co-authors from Johns Hopkins University report on their findings in one of the driest environments on Earth: the core of the Atacama Desert. You would think life there is all but impossible, given the extreme scarcity of water, high levels of ultraviolet irradiation, shortage of nutrients, and high salt concentrations. But it’s not so. We know from previous studies that life nevertheless gains a foothold by practicing an endolithic lifestyle: bacteria—mostly cyanobacteria capable of photosynthesis—live within the rock, where they are protected from radiation. And they gain life-sustaining water with the help of salts that extract what little moisture the dry atmosphere holds.

The rocks in which these hardy microbes live are halite-rich salt nodules, which are recognized as one of the most Mars-like environments you can find on Earth. Just when researchers wrapped their heads around the idea that cyanobacteria and other organisms survive in such settings, Uritskiy and colleagues found another surprise: a species of algae and active viruses living within the rocks.

The presence of an alga is hugely significant, because it shows that not only archaea and bacteria can live under these conditions, but also the third domain of life, the one to which we humans belong, the eukaryotes. And that’s not all: This alga, called Dolichomastix, was the most active member of its ecological community, more productive than the cyanobacteria. What this shows—not for the first time—is that more complex organisms have the necessary toolset to thrive in extreme environments. Other famous examples of that are tardigrades from the animal kingdom and resurrection plants.

The ecosystem would not be complete if there were no viruses. The research group discovered a diverse group of them, many previously unknown. The most abundant and active viruses belonged to a new clade of Myoviridae. Characteristic of this group is that they have a head-tail geometry and are double-stranded DNA viruses that are non-enveloped. The viruses sampled by Uritskiy’s team in the Atacama seemed to target salt-loving archaea and bacteria.

Certainly biomass and biodiversity in this forbidding environment are low compared to most places on Earth. But it’s still stunning to see how much diversity there is, and to find active viruses in one of the most Mars-like environments on Earth. Does this raise the specter of life on Mars?

It’s hard to say. Mars is still a notch more challenging place to live than the core of the Atacama Desert. However, in another paper that just appeared in the journal Life, Jacob Heinz and co-authors from the Technical University Berlin showed that microbial life, particularly fungi, can tolerate much higher perchlorate concentrations than previously thought. And we know that many environments in extreme deserts on Earth and on Mars contain salts (including perchlorates) that can absorb water from the atmosphere.

What we can say for sure is that finding such rugged organisms in Earth’s harshest deserts shows us which tricks life might use if it indeed still exists on Mars: 1) live in the rocks to shield yourself from radiation, and 2) use the salt to get your water!

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