The Apollo 12 astronauts were not the only living organisms on the surface of the moon when they landed there in November of 1969. That’s not conspiracy theory looniness. It’s what serious scientists concluded when the crew came back and what a lot of them still believe. The astronauts’ lunar module touched down within walking distance of the unmanned Surveyor 3 probe, which had soft-landed on the airless, windless surface of the moon three years earlier. Part of the reason for choosing that spot was so that the astronauts could retrieve Surveyor’s camera and a few other small parts of the ship and bring them home, where scientists could examine how well terrestrial hardware fares under space conditions.
The machinery aboard Surveyor, they discovered, held up just fine. Much more remarkably, so did the biology. Deep within some of the batting inside the camera was a small colony of streptococcus bacteria which, when cultured, bestirred themselves and resumed growing and dividing. The conclusion: the camera had been contaminated by technicians before it left Earth, and the bacteria had put themselves into a state of suspended animation, waiting for the unlikely day they’d find themselves in a more hospitable environment.
In the years since, some of the luster has come off of this story. A 2011 review of the findings concluded that the camera was contaminated not before it left Earth, but in an imperfectly sterilized clean-room after it returned. Other reviewers dispute that, however, pointing out that the bacteria were very slow to revive upon culturing and that only a small handful of them succeeded, consistent with organisms that had indeed been dormant for a long period of time.
Whatever the true provenance of the Apollo 12 bugs, they were the first hint of what is now the robust study of life in extreme environments—in boiling sea vents, dry desert stones and, significantly, in ancient ice. Viable micro-organisms have been found in Siberian permafrost after 30,000 years of suspended animation. In one case, investigators recovered and revived bacteria from the depths of the Guliya ice cap in western China, where the bugs had been slumbering for an astounding 750,000 years.
“One of the fundamental problems that has been tossed around is how this is possible,” says Brent Christner, professor of biological sciences at Louisiana State University.”These bugs must have some sort of extraordinary DNA remair mechanism, but no one knows what it is.”
Now however, thanks to an imaginative study conducted by Christner and his colleagues, and published in the journal Applied and Environmental Microbiology, we’re a little closer to that knowledge than we were. That has big implications for our basic understanding of both terrestrial biology and, much more tantalizingly, the extraterrestrial kind.
The biggest problem facing organisms consigned to the deep freeze is not so much the low temperature as the high radiation. All living things on Earth are subjected to background levels of natural radiation, which don’t amount to much in any given moment, but accumulate over the years and millennia. Space, without Earth’s protective atmosphere, is a far deadlier environment, with radiation levels climbing to intensities that would quickly kill unshielded humans outright. The worst kind of damage the radiation does is what’s known as double-stranded DNA breaks, in which the molecules fracture into separate pieces, which themselves break up more and more over time. We survive this thanks to a well-understood DNA repair system in our cells. For frozen bacteria to survive an exponentially longer period, a related mechanism must be at work.
To try to determine what that mechanism is, Christen and his colleagues began with a healthy population of Psychrobacter arcticus bacteria which, as their surname suggests, were first discovered in permafrost, in this case in Siberia. The investigators didn’t have the freedom to freeze the bacteria and then study them for 750,000 years, but they were able to accelerate the process a bit by exposing them to a dose of ionizing radiation equivalent to what they’d absorb on Earth over the course of 225,000 years. Then they froze the Psychrobacter at a temperature of 5° F (-15° C), and checked on them periodically over the course of two years, intermittently sampling small bits of their DNA.
No surprise, the DNA strands did not fare well, some of them fragmenting into what the investigators called a “slurry” of smaller bits. But some of the DNA looked just fine, and that meant something was indeed intervening to knit the strands back together. “This isn’t a random process,” Christner said in a release accompanying the paper. “This tells us the cells are repairing their DNA. [Emphasis in the original.] This is important because we don’t typically think of these as being conditions under which complex biological processes are going on.”
Laboratory proof that such repair work does happen in those environments is no small thing. It doesn’t bring Christen and his colleagues much closer to pinpointing the mechanism yet, but they have some ideas. It’s possible, for example, that the structure of ice provides bacteria small, liquid redoubts in which conditions are marginally milder. “When you freeze water you create an ice crystal latticework,” Christen says. “Impurities like salts and the bacteria themselves become segregated there.” And since those tiny pockets can remain liquid, the DNA might repair itself just as it does in organisms living in friendlier places. But, Christen admits, that’s just one theory. “If we did this experiment and we had no evidence that DNA was being repaired, we’d be studying an alternate hypothesis,” he says. “Now we know it can happen and this gives us new avenues to pursue.”
One thing Christen is sure of is what his work suggests about the possibility of life in deep space—including in our own solar system. “It just keeps looking better for conditions of habitability on Mars,” he said in the release. “If these DNA repair mechanisms operate in Earth’s cryosphere, extraterrestrial microbes might be using this survival mechanism to persist on other icy worlds.” Life may or may not be everywhere, but if the new findings show anything, it’s that when the need arises, it can be tough as nails.
(MORE: Aliens Among Us)