Score another win for “Conan the Bacterium” – a robust bacterium that new research suggests could conquer the solar system.
Better known as Deinococcus radiodurans, this microbe is arguably the toughest organism known to science. Previous studies have shown that it can withstand extreme cold, intense radiation, harsh chemicals and deep dehydration – all evolutionary adaptations, perhaps, to what is believed to be its natural home in the high, dry and sun-scorched deserts of northern Chile.
Now a new experiment by researchers at Johns Hopkins University shows that this hardy “extremophile” can also survive the enormous shocks and mechanical stresses associated with asteroid strikes on Mars.
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D. radio duration “is the closest we can get to what we think a Martian life form might look like without having an alien in our lab,” says Lily Zhao, a graduate student at Johns Hopkins University who led the experiment. “And we tried to kill it, but we couldn’t.”
To do so, the researchers fired a high-velocity projectile from a gas gun at colonies of D. radio duration which was sandwiched between two steel plates. The bacteria could withstand split-second exposure to extreme pressures of up to three gigapascals (GPa). That’s 30 times greater than the pressure at the deepest point in Earth’s oceans – and is similar to the crushing impact of an asteroid cratering into Mars and blasting fragments into space.
KT Ramesh, an impact expert at Johns Hopkins, who oversaw the work, was shocked by the results. “Our expectation was that most of them would die,” he says, because other types of microbes in previous high-pressure studies had survival rates of only about 1 percent or less. Instead, almost everyone D. radio duration microbes survived initial 1-Gpa shots. Even at the highest pressure of 3 Gpa, more than half survived. Subsequent analyses, led by Johns Hopkins microbiologists Cesar A. Perez-Fernandez and Jocelyne DiRuggiero, showed that some of the microbes had perished from ruptured cell walls, but also confirmed that the survivors could repair their damaged DNA, grow back and reproduce.
These pictures show Deinococcus radiodurans microbes before (Again) and after (center, right) high-pressure experiments.
Lisa Orye/Johns Hopkins University/Zhao et al.
Coupled with the microbe’s other feats of strength, its extraordinary resilience suggests it has all the basics required for interplanetary hops aboard a shock-ejected debris that could seed life throughout the solar system, the researchers say. This far-fetched idea, called lithopanspermia, stems from musings in the 1870s about bacteria-filled meteorites. However, scientists began to take it more seriously in the 20th century, as space exploration revealed potentially habitable conditions on other planets and as cell biology showed that some microbes were astonishingly adaptable and robust.
“In terms of the basics—radiation, freezing, desiccation—this ‘pressure’ component was really the last question mark,” says Zhao, “because if life couldn’t even survive shock pressure, then the rest doesn’t matter since cells won’t survive being launched into space in the first place.”
The team’s results, published Tuesday in the journal PNAS NEXUShas weighty implications for understanding the origins of life on Earth—and the search for life elsewhere in the universe.
The discovery strengthens the case that life across a solar system can spread a bit like a cold: if a biofever planet sneezes out debris, others can catch it too. And given that asteroids have been chipping away at the Sun’s wake of worlds for more than 4.5 billion years—and that eons ago Mars was a warmer, wetter, more clement place—the result raises the possibility, however small, that life on Earth began on Mars. Meteorites from the Red Planet routinely pelt our planet, although most burn up completely in the atmosphere or crash into desolate stretches of land or sea.
Scientists do not suggest that homemade D. radio duration microbes are direct emigrants from Mars. Rather, this terrestrial organism’s abilities are a kind of proof of existence. “If you can get a life form, an extremophile, to survive these kinds of conditions, it shows that there is a ‘seed’ for biology to build on,” says Ramesh. “You have DNA, you have cellular structures. And from there biology can move— it doesn’t start in one place and just stay there. Evolutionary adaptation handles the rest.
“This is an amazing experimental representation of how microorganisms can propagate between neighboring planets or out into the universe by hitching a ride in rocks ejected from an impact,” said Moogega Cooper, a planetary protection engineer at NASA’s Jet Propulsion Laboratory who works on the space agency’s Curiosity Mars rover and was part of the latest study.
Planetary protection engineers like Cooper ensure that Earth’s life is not stored on spacecraft to contaminate and obtain on other worlds, where it could confound the search for extraterrestrial life and potentially destroy alien ecosystems. They also protect our own planet from extraterrestrial life forms brought back from space. “Finding signs of life beyond our planet in a way that clearly distinguishes it from Earth life has to be done in a clean way,” says Cooper.
The NASA-led Mars Sample Return (MSR) program – a decades-long effort to bring potentially life-sustaining Martian material back to Earth – is now in shambles, its budget zeroed out just as the space agency’s Perseverance rover had collected the necessary samples, leaving them stranded on the Martian surface. But another effort is quietly waiting in the wings: the Japan Aerospace Exploration Agency’s (JAXA’s) Martian Moons Exploration (MMX) mission is set to launch later this year on a journey to take samples from the Martian moon Phobos for return to Earth in 2031. In light of the Johns Hopkins study, it could suddenly yield an MSR study. had promised.
“Phobos orbits Mars twice daily at an orbital distance of only 3,700 miles and has effectively acted as a vacuum cleaner for ejecta from the Martian surface for the past billion years,” says Michael Daly, a pathologist and extremophile expert at the Uniformed Services University of the Health Sciences in Maryland. “The Johns Hopkins group’s research brings renewed focus to the possibility of detecting not only prebiotic small molecules but also macromolecular remnants of whole cells and viruses ejected from the surface of Mars. Indeed, the remarkable ability (of D. radio duration) to potentially survive both the enormous pressure of meteorite impact and eons of deep space radiation, suggests that the MMX Phobos lunar samples returned to Earth may require a higher level of planetary protection.
Phobos has been deemed uninhabitable and is therefore classified as an “unrestricted” celestial body by the United Nations Joint Committee on Outer Space Research (COSPAR), which dictates planetary protection protocols. This status means that minimal biohazard measures are required for visiting spacecraft. However, Daly notes that recent discoveries of sugars, amino acids and other biological building blocks on asteroids could allow impact-delivered organisms to be planted and fertilized in the Martian soil, enabling long-term survival. This means that COSPAR may have to reconsider its designation – with unclear consequences for the MMX mission that will soon be launched.
“So you go to Phobos, you bring back material, and maybe you bring back (biological) material from Mars,” says Ramesh. “Maybe you need to be worried about this now, right? You might need to be more careful than we previously thought.”
Cooper, NASA’s planetary protection expert, notes that the Martian moon’s “unrestricted” status came in part from an authoritative 2019 report by the National Academies of Sciences, Engineering and Medicine, and that NASA has already funded an ongoing study to assess impact-triggered microbial survival on Phobos. “The burden of proof will require more research,” she says. “However, future experiments can support a deeper conversation that explores unlimited sample return from Mars in an updated safety assessment like the JAXA MMX mission.”
Norman Sleep, a geophysicist at Stanford University and pioneer of modern lithopan sperm studies, rightly believes the MMX Phobos samples will receive extremely careful treatment regardless of the moon’s planetary protection status. “Some people — I won’t say who — think that requiring planetary protection to bring samples back from Phobos is like requiring a lifeguard for a swim test in Death Valley National Park,” he jokes. “But still, it’s worth the Herculean effort to prevent the Phobos samples from being contaminated by terrestrial microbes.”
