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Can Hitchhiking Earth Microbes Thrive on Mars?

The views expressed are those of the author and are not necessarily those of Scientific American.


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Curiosity rover cleanroom

Engineers swab the Curiosity rover's heat shield to test its cleanliness. Credit: NASA

LOS ANGELES—When the Curiosity rover lifted off toward Mars, the spacecraft carried a few stowaways—278,000 bacterial spores, by NASA’s best estimate. That is sparkling clean, by spacecraft standards—the mission’s components had been sterilized, wiped, baked and coddled in clean rooms to drastically reduce the bacterial burden.

Mars missions such as Curiosity are subject to strict planetary protection policies intended to preserve habitats in the solar system that might harbor life of their own. After all, invasive species are a big enough problem on Earth, and one can only speculate about how terrestrial microorganisms would fare on Mars.

That speculation is getting a bit more grounded, however. At a conference held here this week on the present-day habitability of Mars, numerous researchers described experiments carried out in Mars simulation chambers that can replicate some of the environmental conditions of the Red Planet. Perhaps most intriguingly, a new set of experiments described by Andrew Schuerger of the University of Florida indicate that three of the most hostile elements of the Martian environment—low pressure, low temperature, and a carbon dioxide atmosphere largely devoid of oxygen gas—are not insurmountable blockades for Earth organisms. On the contrary, some microbes don’t just hunker down and hibernate but actually grow under such conditions.

Schuerger, along with University of Florida colleague Wayne Nicholson and their collaborators, collected 24 microbial strains that have been found on spacecraft surfaces, in clean rooms and around Kennedy Space Center in Florida, as well as two extremophile species tolerant of hostile environments. The bacteria included common species such as Bacillus subtilis and Escherichia coli, but “our winner in this set of experiments,” as Schuerger put it, was Serratia liquefaciens, a widespread generalist microbe.

Earth bacteria grown in a Mars simulation chamber
Earth microbes, including Serratia liquefaciens, grown under a variety of environmental conditions, including a control sample (A) and a sample kept under low temperature, low pressure and low oxygen (D). Credit: University of Florida

Most of the selected microbial species shut down at temperatures of zero degrees Celsius (which falls in the upper range of Martian surface temperatures), even without being subjected to low pressure or anoxic conditions. But S. liquefaciens succeeded not only in the low temperatures but also under the simultaneous exposure to a carbon dioxide–dominated atmosphere and Mars-like pressures of only seven millibars. (Sea-level atmospheric pressure on Earth is roughly 1,000 millibars.) The researchers reported their findings in January in the journal Astrobiology.

Whereas S. liquefaciens actually grew under the trio of harsh conditions, the others did not perish—they simply lay dormant. “All of these bacteria were not killed by the conditions they were exposed to,” Schuerger said. When returned to ambient laboratory conditions, the inactive bacterial species all resumed growth.

In a separate study, bacteria pre-adapted to survive in frigid conditions fared even better. In a study published in December in the Proceedings of the National Academy of Sciences, Schuerger, Nicholson and their colleagues reported that bacteria isolated from the Siberian permafrost thrived in Mars-like conditions. Those species, from the genus Carnobacterium, actually seemed to favor the low-pressure conditions. “When they grew at zero [degrees C] under CO2, seven millibar atmospheres, they seemed to grow better, at higher rates, than under CO2 at 1,000 millibars or under oxygen at 1,000 millibars,” Schuerger said.

But bacteria need not hail from extreme habitats to flourish under Mars-like conditions. Schuerger shared preliminary, unpublished research during the conference that indicates that low-pressure, or hypobaric, environments actually stimulated the growth of microbes harvested from an unusual source: human saliva. In petri dishes incubated at low temperature under carbon dioxide atmospheres, the salivary flora failed to grow at Earth-like pressures. “Yet these hypobarophiles have popped out” under Mars-like pressures of seven millibars, he said. The specific organisms that thrived in hypobaric conditions have not yet been identified, Schuerger noted in an email, but “the human oral cavity is not a place that one would expect to find microbes that yield such a strange response.”

Proving that some bacteria fare well under Mars-like pressures, temperatures and atmospheric compositions is nonetheless a long way from proving terrestrial life can flourish on Mars. Schuerger and his colleagues count 17 environmental factors on Mars that could be hostile to life, of which pressure, temperature and anoxia are only three. Two important threats to life that went unaddressed in the two bacterial studies were ultraviolet irradiation from sunlight, which on Earth is thankfully attenuated by ozone in our planet’s atmosphere, and the extreme dryness of the Red Planet’s surface. Schuerger noted that accurately simulating Martian desiccation would rapidly degrade the growth medium for the bacteria. “We had to reduce evaporation to carry out these experiments,” he said.

Which brings us back to those hundreds of thousands of spores on the Curiosity rover and its flight hardware. Even in light of the new research, the rover’s landing site appears extremely unlikely to suffer contamination by terrestrial biology. Direct and reflected sunlight likely sterilized the outside of the rover within the first day or two of the mission, Schuerger said. And any survivors are unlikely to find purchase at the Curiosity landing site, Gale Crater. “Even if the UV radiation doesn’t sterilize or kill off microbes on the outside of the vehicle or on the wheels, even if the microbes are dispersed, the extreme desiccating conditions of Gale Crater argue strongly against” the proliferation of stowaways from Earth, he added.

About the Author: John Matson is an associate editor at Scientific American focusing on space, physics and mathematics. Follow on Twitter @jmtsn.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. lump1 7:02 pm 02/7/2013

    If terrestrial bacteria start growing on Mars, it won’t be because of any rover we send there. All kinds of rocks are dislodged from Earth by various impacts, and some of those have inevitably landed on Mars. If terrestrial microbes really can thrive in Martian conditions, we will probably discover them there soon.

    Link to this
  2. 2. stargene 2:28 am 02/8/2013

    lump1 is right about the probability of ancient
    earth microbes surviving impact generated passage
    ‘up-the-gravity-well’ to Mars. It’s been suggested
    before in the astro literature. Even if that
    turned out to be impossible, for whatever reasons,
    it’s not difficult to imagine a tiny fraction of
    earth microbes on our landers and rovers eventually
    falling into local Martian soil, possibly then
    being transported by the usual Mars dust storms
    or tornadoes, and eventually sinking down into
    rare soil/porous-rock levels (nearer Martian
    poles?) which have water or water ice at least seasonally. Interesting possibilities.

    Link to this
  3. 3. joe poppa 11:31 am 02/8/2013

    What about the return trip? Could Martian microbes hide out, somewhere, on a lander (somewhere hidden from UV rays) and hitchhike back here?

    Link to this
  4. 4. James38 4:32 pm 02/8/2013

    “Proving that some bacteria fare well under Mars-like pressures, temperatures and atmospheric compositions is nonetheless a long way from proving terrestrial life can flourish on Mars.”

    Pardon me, but the image that popped into my mind was “Ducks! Lines of little ducks waddling around on Mars.”

    Link to this
  5. 5. jgrosay 6:31 pm 02/8/2013

    An Exobiology Institute in Southern Spain found organisms able to live an thrive in the extreme environment of heavy metals’ loaded Rio Tinto water. Are photosynthetic bacteria or algae able to thrive in simulated martian conditions, and release Oxygen? What about bacteria or other bugs able to grow and multiply in a gaseous place analog to the Jupiter and Saturn atmospheres, or in the Titan’s methane lakes?

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  6. 6. Ingamas 11:03 pm 02/8/2013

    I think the word is “survive” forget “thrive” until we have evidence.

    Note to self: Don’t lick space probes, Pan-saliva-gensis? .

    Link to this
  7. 7. Quinn the Eskimo 3:24 am 02/9/2013

    They’re already here. Where, exactly, do you think the Tea Baggers are from? Look what they did to the Republicans!!!

    Link to this
  8. 8. WoodHughes 1:58 pm 02/11/2013

    I’m surprised that this article did not even mention the effect of peroxides in the Martian soils on any potential microbial life that should find it’s way from the Earth.

    Link to this
  9. 9. Jerzy v. 3.0. 5:27 am 02/14/2013

    It would be interesting experiment to create in a lab a complete copy of Mars environment, seed it with Earth extremophiles, and try to check whether some Earthling microbes can fully live in Mars conditions.

    If it fails, one could simulate a less harsh version and try breeding Earth extremophiles into Mars-capable strains.

    The purpose would not be to colonize Mars with some Earth bacteria, but to check if life can exist on Mars. I expect it turns, that some existing Earth bacteria can live and breed on Mars, eg. in pockets of wet sub-surface rocks, and that equivalent life exists on Mars.

    Link to this
  10. 10. robertinventor 8:24 pm 08/24/2014

    Actually that idea that Earth and Mars must have the same microbes is a minority view by Zubrin. In the forward contamination direction from Earth to Mars – the problem is – that few impacts on Earth are powerful enough to send debris all through our atmosphere and all the way to Mars. Last one big enough was probably the one that helped make the dinosaurs extinct – and the debris would take abut a century to get there for the fastest trajectories. Then the life has to be able to survive once it gets there – find a suitable habitat – not easy on such a dry planet – and it has to survive the ionizing radiation of the trip there – and vacuum of space – and it has to be ejected from Earth in the first place and survive that process also.

    There are a few microbes, extremophiles able to withstand vacuum and ionizing radiation, that seem to be hardy enough to do all of this, and if they were lucky enough to find a habitat when they get there, could survive on Mars also.

    But many microbes that could survive fine on Mars probably have never made the transfer via meteorite. And is not yet proved that any meteorite has survived this process.

    The NRC examined this in detail in its report on Mars sample return for the other direction from Mars to Earth – and in that direction also – then impacts on Mars able to send material to Earth are more common – but still, only every one or two million years and again quite possibly most microbes on Mars, if there is life there, couldn’t survive the journey. And that way around it has to hit a habitat on Mars in the first place – most recent meteorites from Mars are from the dry volcanic highlands and not the most likely places on Mars to have present day life.

    http://www.science20.com/robert_inventor/blog/could_microbes_transferred_spacecraft_harm_mars_or_earth_zubrins_argument_revisited-121110

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