Deep-dwelling bacteria could rewrite our understanding of where alien life could take hold

Finding bacteria in extreme environments, like at thermal ocean vents or in the sulfur pools at Yellowstone, isn’t all that novel. In 1991, remote-control robots brought out samples of pitch-black fungi that had been growing inside the ruins of the Chernobyl reactor. Scientists sat up and paid attention, but then sort of collectively yawned. (The same species of fungi grows in nuclear reactor pools, too, but it also does fine in everyone’s dishwasher.) Those fungi aren’t the only ones that have made friends with radioactive places, though. There’s a bacteria that owns the radioactive bottom of a 2-mile-deep South African gold mine, and astrobiologist Dimitra Atri believes its case study should make a major change in how we look for extraterrestrial life.

The bacteria in question is dubbed Candidatus Desulforudis audaxviator (audax viator, “bold traveler,” from Jules Verne). Where gold is found, there are often deposits of radioactive uranium ore, and the Mponeng mine is no exception; there’s uranium down there, and thorium too. But Ca. D. audaxviator (can we call you “audax” for short? Of course we can) is pretty copacetic with all that radiation.

Like those fungi at Chernobyl, audax seems to do just fine with the elevated radiation levels. It’s happy to co-opt the weak gamma radiation for its own ends. And it would prefer to do so in solitude. When researchers sequenced the noxious anaerobic soup at the bottom of the mine shaft in 2008, they found that audax is the only thing living where it lives. It’s the first known example of a single-species ecosystem.

How can this possibly work? Radiation from the uranium and thorium breaks down the rock, which dissolves into sulfate ions. As an obligate anaerobe, audax eats sulfate and excretes hydrogen sulfide, and then makes use of the sulfur from its own excretions. This hardy little bacteria has actually been living down in the anaerobic blackness so long that the ability to handle oxygen gas is fading from the species’ genetic memory.

For reference, sulfate is the conjugate base of sulfuric acid. Chemists can correct me here, but I’m pretty sure that since it’s in water, that means these bacteria are living in a buffered solution of sulfuric acid, happily munching away at rocks and crapping out toxic, explosive hydrogen sulfide into a radioactive mine shaft. Clearly they are difficult to perturb, even in that heavily irradiated blackness.

In light of these observations, the case Atri puts forth goes like this: Since audax does just fine in the pitch black with only the in situ radioactivity to sustain it, on planets where there isn’t a robust enough source of photons from a parent star, radioactivity and bombardment with galactic cosmic rays could fill in the energy-input gap.

Consequently, Atri says, we should change the heuristics we use to look for life in the great elsewhere. To wit: we don’t necessarily have to care if a planet has an atmosphere anymore, or even sits in its star’s Goldilocks zone, because life could arise there, thriving on cosmic rays and/or the planet’s own radioactivity. Rogue planets are even seriously considered in the paper as being able to support life, because if you don’t need photons from a star… you don’t really need a star. Wherever there’s rocks and water and energy, it seems, we have a chance to find life.

Title image of the Morning Glory sulfur pool at Yellowstone by Clément Bardot, via Wikipedia

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