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Life on Earth's final frontier

G. Wanger / JCVI / G. Southam / UWO  
 The rod-shaped bacterium known as Desulforudis
 audaxviator was recovered from water collected deep
 in the Mponeng Mine in South Africa.

A strange breed of bacteria that has been found living alone, nearly two miles underground, is just the kind of creature suited to survive far beneath the surface of Mars, scientists say.

The rod-shaped microbe, dubbed Desulforudis audaxviator, can survive in complete darkness, without oxygen, in temperatures around 140 degrees Fahrenheit (60 degrees Celsius) - as long as it has a trickle of water flowing through radioactive rocks. It was found living under such conditions in a 1.75-mile-deep (2.8-kilometer-deep) gold mine in South Africa.

"I would guess that an organism like this would be ideally suited for the Martian subsurface," said Princeton University microbiologist T.C. Onstott, one of the microbe's discoverers.

The strange case of D. audaxviator, which takes its species name from a message in the Jules Verne classic "Journey to the Center of the Earth," is described in the current issue of the journal Science. The research is significant not only for what it says about the resilience of life on Earth's most extreme frontiers, but also for what it says about the prospects for finding life elsewhere in the universe, said Carl Pilcher, director of NASA's Astrobiology Institute.

"This work is of profound importance," he told me.

The study's lead author, biologist Dylan Chivian of Lawrence Berkeley National Laboratory, said the microbe's existence isn't the only thing surprising about the bacterium. He and his colleagues decoded all the genetic material contained in a water sample taken from the gold mine, and expected to identify multiple types of bacteria. Instead, they found that D. audaxviator was the only species represented in the sample, if you exclude what appear to be trace amounts of lab contamination.

When scientists analyzed the microbe's 2,157 protein-coding genes, they found that it had all the machinery required to create everything it needed, including a complete set of 20 amino acids. "We ourselves can synthesize only 10," Chivian said. "We have to eat all the others."

D. audaxviator didn't require any products from photosynthesis - which sets it apart from deep-sea organisms that live in darkness but nevertheless depend on oxygen and nutrients filtering down from above. The metabolic cycle relies on radioactive elements in the surrounding rocks to break down water molecules, providing hydrogen and sulfate for the microbes to munch on.

"This was a microbial community that appeared to be using a source of energy that no other life on Earth had used, and that is radioactive decay," Pilcher said.

Chivian said the species can live completely independent of any other species - and based on an analysis of chemical isotopes in the water, it has apparently been doing so for millions of years.

"The last time any of the water saw the surface was between 3 million and 10 million years ago," he said.

The bacteria are even equipped with tiny tails, or flagella, enabling them to move around in the water. And if times get too tough, they can turn into spores. That hardiness led Chivian to select the name "audaxviator," which is Latin for "bold traveler." The term appears in one of the clues deciphered by the main character in Jules Verne's tale: "Descende, Audax viator, et terrestre centrum attinges" - that is, "Descend, Bold traveler, and attain the center of the earth."

Once the researchers heard the name and its etymology, "it was a head-slapper," Chivian recalled. "We said, 'That's it!'" 

All this may make it sound as if D. audaxviator is a science-fiction plot device come true: a creature from the primordial lagoon, perhaps, or an invader from Mars. But Chivian said the genetic analysis shows that the bacterium has borrowed bits of DNA from other strains of deep-living microbes in order to boost its capabilities.

"It's not the progenitor organism, and it's not from another planet. ... Earth is its cradle," Chivian said. Over the course of millions of years, D. audaxviator may have picked up useful pieces of genetic code from other microbial species and basically beat them at their own game.

Onstott said the microbe's genes showed that its ancestors came from a low branch on the family tree of bacterial life - and that has implications for a deep question about life's origins: Did microbes first multiply in pools of primordial ooze sitting on Earth's surface, or did they get their start far below the surface? "Finding these deeply rooted bacterial species in the tree is consistent with the idea that life may have originated in the deep subsurface," he said.

Onstott's analysis also hints that the radioactive rocks beneath the Martian surface could sustain such bacteria, although the metabolic rate would have to be significantly slower. "When you do the calculation, you find out that the rate of chemical recharge on Mars is about 10 times less than it is on Earth," he said.

Ah, but how far down would you have to dig to find the hypothetical Martian cousins of D. audaxviator?

"On Mars, it may not be as deep," Pilcher speculated. But Onstott said that the microbe would probably have to find a home under Mars' permafrost, which is thought to extend 1.2 to 3.6 miles (2 to 6 kilometers) beneath the surface. The best way to sniff out such microbes closer to the Martian surface would be to find a place where water may have erupted onto the surface - for instance, in one of the gully deposits detected by NASA's Mars orbiters.

"The only other opportunity you'd have is if there were regions on Mars that have been identified where there were signs of recent volcanic activity," Onstott said.

In any case, astrobiologists are anxious to find the deeper meaning of D. audaxviator's discovery.

Chris McKay, a planetary scientist at NASA's Ames Research Center, said the research is "highly relevant to the search for life below the surface of Mars, Europa, or Enceladus."

Pilcher said it could be relevant beyond the solar system as well, because there's "nothing unusual" about the deep-down conditions where D. audaxviator dominates. "Virtually any rocky planet is going to have those conditions at some point," he said.

In addition to Chivian and Onstott, authors of the Science paper include Eoin Brodie, Paramvir Dehal, Todd DeSantis, Gary Andersen, Terry Hazen and Adam Arkin of Lawrence Berkeley National Laboratory; Eric Alm of the Massachusetts Institute of Technology; David Culley and Fred Brockman of Pacific Northwest National Laboratory; Thomas Gihring of Florida State University; Alla Lapidus, Stephen Lowry and Paul Richardson of the U.S. Department of Energy's Joint Genomics Institute; Li-Hung Lin of National Taiwan University; Duane Moser of the Desert Research Institute; Gordon Southam and Greg Wanger of the University of Western Ontario; and Lisa Pratt of Indiana University.

The work was supported by the Energy Department's Office of Science and by NASA through the Indiana Princeton Tennessee Astrobiology Initiative of NASA's Astrobiology Institute.