Art by Breanna Brownson.
Conan the Bacterium may be Earth’s most promising astronaut. Named the world’s “toughest organism,” Deinococcus radiodurans—nicknamed Conan—could survive for a whopping 280 million years if buried ten meters beneath the Martian surface. This resilience suggests that if life ever existed on Mars, it could still exist today.
The surface of Mars is deeply frozen and extremely dry. The atmosphere contains almost no oxygen and is over one hundred times thinner than Earth’s. Any life form released on Mars would essentially be freeze-dried and exposed to intense radiation from the sun. But Conan regularly challenges known limits of survival. The microbe can be frozen, desiccated, and face intense radiation, yet still live to see another day. In a recent study led by Michael Daly, a professor of pathology at Uniformed Services University of the Health Sciences and a member of the National Academies Committee on Planetary Protection, Conan and five other organisms were tested for potential survivability on Mars.
As missions to and from Mars reach fruition, worry over cross-contamination between planets is putting the spotlight on Conan and other hitchhiking microbes. Future manned missions would expose Mars to astronauts and their microbiomes, raising the concern that Earthen microbes could be released and contaminate Mars’ surface. Daly’s study examined six microbes found in the human gut: Conan the Bacterium, E. coli, three spore-forming Bacillus bacteria, and a strain of baker’s yeast called Saccharomyces cerevisiae. All six are representatives of the human microbiome. In this study, Conan and the baker’s yeast broke all previous radiation survival records, even when compared to Bacillus spores, which are renowned for their resistance.
To simulate the conditions on Mars, all six organisms were first dried in a desiccation chamber for five days and then stored on dry ice. The frozen organisms were later placed in an irradiator and exposed to very large doses of ionizing radiation in the form of gamma rays and protons—imitating forms of radiation from the sun.
When charged particles, including protons from the sun, approach Earth, our magnetic field deflects them, and our atmosphere blocks them. But Mars has no magnetosphere and virtually no atmosphere to protect itself: protons are free to crash into the Martian surface and generate additional gamma rays. This is why the most dangerous part of Mars is the top ten centimeters of the Martian surface—Conan could only survive that amount of ionizing radiation for about 1.5 million years. Further below the surface, shielding can protect against main forms of ionizing radiation, leaving only the planet’s low natural background radiation, as it is on Earth. “The deeper you go [into Mars’ surface], the more likely it is that you will find the remnants of life,” Daly said. “The survivability of life is now greater than we had ever thought possible.”
Conan’s mechanisms for survival have previously been characterized, but not in the context of Mars. Past studies looked at radiation under Earthen conditions, representing a planet where life revolves around liquid water. The limits of ionizing radiation survival have traditionally been established by increasing doses of gamma radiation until the last viable microbe is dead. In decades past, Conan’s ‘survival limit’ was approximately 25,000 kGy of gamma radiation under aqueous conditions. The present study found that if first dried and then frozen into a dormant state, Conan could withstand a whopping 140,000 kGy of gamma radiation.
“In the past, folks and scientists considered the survivability of life on Mars to be on the order of perhaps millions of years,” Daly said. “But we now have the evidence to support that life, when dormant, could likely survive hundreds of millions of years.”
There are two essential reasons for Conan’s extreme resistance to radiation: the hyperaccumulation of manganese antioxidants (Mn-antioxidants) coupled with polyploidy and the presence of multiple identical genomes. Mn-antioxidants protect proteins needed to rebuild DNA, and polyploidy provides the cell with backup genomes used in repair.
Extremophiles like Conan accumulate Mn-antioxidants, which are small complexes that consist of manganous ions bound to a variety of common metabolites. Generally, the more Mn-antioxidants accumulated in a cell, the greater the organism’s resistance to ionizing radiation. Ionizing radiation is a high-energy form of radiation that can strip electrons from water, forming unstable molecules called ‘reactive oxygen species’ (ROS). The most toxic ROS in irradiated cells is superoxide, which “fries” the proteome, Daly explained. The proteome is the organism’s set of proteins—including the molecular machinery required to reassemble DNA broken by radiation. Mn-antioxidants in Conan defend the proteome against ROS and thereby preserve the enzymes needed to rebuild its broken genomes after radiation. In contrast, cells like E. coli that lack this Mn-antioxidant defense lose the ability to reassemble DNA damaged by radiation.
Manganese antioxidants do not prevent DNA damage caused by radiation—luckily, the second molecular trick in Conan’s tool kit is polyploidy. Polyploidy means that when one genome is damaged, other undamaged copies can be used to repair the broken one. Conan contains eight identical copies of its genome per cell. The team showed that the organisms with the greatest resistance to radiation are polyploid. E. coli and Bacillus spores typically have only one or two genome copies, while baker’s yeast has four copies. In Conan, the eight genome copies are linked together by interstrand crosslinks called Holliday junctions, further accelerating DNA repair. “When you get a double-strand break caused by radiation in the genome, then the repair templates for homologous recombination are never far away,” Daly said.
The baker’s yeast strain studied is also a polyploid, but this fungus accumulates fewer Mn-antioxidants than Conan. By comparison, E. coli does not accumulate Mn-antioxidants and typically has only one or two copies of its genomes. While the Bacillus spores accumulate Mn-antioxidants, they are merely haploids, containing only a single copy of the genome. In the end, the data showed that dried and frozen Conan would possibly survive 280 million years when buried ten meters below the Martian subsurface, the yeast would survive 48 million years, E. coli would survive sixteen million years, and Bacillus spores would survive relatively less.
The forthcoming ExoMars mission’s Rosalind Franklin rover plans to drill two meters below the surface of Mars and collect samples in search of life. While the surface of Mars has been frozen and desiccated for billions of years, Daly theorized that life could still exist not far beneath the surface. He explained that Mars’ lack of an atmosphere means that meteorites regularly bombard the planet. Upon impact, frozen water beneath a crater will melt, and simple organic compounds delivered by some meteorites could fertilize and fuel cellular recovery. If this theory holds true, Conan could have a Martian doppelganger out there to challenge its title as the world’s toughest organism.
Article IX of the Outer Space Treaty (OST) of 1967 is an international agreement aimed at preventing harmful cross-contamination in the exploration of life across celestial bodies. While Conan’s survivability suggests that forward-contamination of Mars would be essentially permanent over mission time-frames of thousands of years, this would not be considered harmful under the OST because the organisms cannot proliferate when frozen and desiccated. Harmful backward contamination from Mars to Earth is also unlikely because if life ever evolved on Mars, it would now be anaerobic—able to survive without oxygen—and susceptible to the toxic effects of Earth’s oxygen-rich atmosphere.
“It is not considered harmful contamination unless these organisms were dispersed across the planet and somehow found some warmth and water,” Daly explained. “There are good reasons to think that we can explore the surfaces of Mars without harming the science that is dedicated to looking for the possibility of extraterrestrial life. One can speculate that Martian life, if it ever existed there, still exists below the surface.”