Michelle Thaller
Extreme Sports, Bacteria-Style
Human beings seem to have a natural urge to push their limits. Running a marathon doesn't quite cut it these days; why not do a triathalon and tack on some extra miles of swimming and a bicycle road-tip? Sky-diving too tame for you? Try parachuting off the World Trade Center or falling 100,000 feet from the threshold of space via a high-altitude balloon.
We love to take things to extremes, to see how much we can survive. Even before we were doing this for fun, humans demonstrated just how tough they could be, and how adaptable, colonizing the surface of the Earth from the deepest deserts to the Arctic ice floes. But until recently, we never realized what taking things to extremes really meant. Whatever feats of strength and endurance the human race can lay claim to, we are mere amateurs compared to microbes.
In the last ten years or so, scientists have become very interested in microbes. Germs and bacteria have, of course, been carefully studied before, but things got a lot more interesting when scientists first considered, seriously, how to search for life outside the Earth.
We knew that some Earth bacteria could survive in Mars-like conditions from laboratory experiments, so Mars seemed a good bet for microbial life. So, when we sent the Viking Lander to Mars, several tests were conducted on the Martian soil to determine whether there were any small organisms living there.
The results were discouraging (no organic compounds were detected), but also ambiguous. There seemed to be some kind of chemistry going on in the soil, and small changes were noticed as the soil was sifted, wetted, and mixed with bacteria food brought from earth. Were these changes due to some simple chemistry in a clay-like soil, or were there actually microbes on Mars?
We still don't know the answer. So, it became apparent that if we couldn't determine if there was life on Mars or not from the tests we did, then we must not be asking the right questions. How do you search for tiny, invisible life forms whose composition and chemistry may be very different from Earth life? Where, exactly, could life be expected to be found? Scientists began by looking more closely at bacteria on Earth, and that's when the lid blew off a whole new field of biology.
At first, scientists tried to make a census of all the environments on Earth that could support bacterial life. That way, we would at least have some limits. We would know how cold, hot, dry or toxic an environment had to be to kill off all bacteria, and this would guide us in our search for life on other planets.
The dry deserts of Antarctica had long been thought to be lifeless, and in fact, the Viking experiments might well have yielded a negative result there. But when scientists looked more carefully, they found hordes of bacteria just below the surface soil, munching on minerals in the rocks. It turns out that other life forms, besides bacteria, can resist cold as well. In 1997, researchers found 2-inch long worms living in methane ice 1800 feet down at the bottom of the Gulf of Mexico.
How about heat, then? Everyone knows that boiling water kills off germs. Looking for a natural environment with plentiful boiling water, scientists headed off to Yellowstone National Park, to investigate the geysers and hot springs for signs of life. Yup, you guessed it. The boiling pools are home to great globs of heat-loving bacteria, some of which have adapted to high temperatures so well that they die (freeze to death, mind you) if the water they live in cools to room temperature.
And it isn't just the heat at Yellowstone that bacteria like, either. Some of the springs are filled with sulfur deposits, and are scorchingly acidic. Yet one strain of germs, called thermoacidophilic bacteria (which means heat-and-acid-loving) thrives in boiling hot sulfuric acid.
It didn't take biologists long to figure out that they had stumbled onto a trend, and a new name was coined for these impressive survivalists: extremeophiles, meaning organisms that survive, even thrive, in extreme conditions. And the boundaries of life have now been totally re-defined.
Until these discoveries, we still believed that we had to search for Earth-like conditions (liquid water, some kind of atmosphere, moderate temperature) elsewhere in space in order to find life. Once again, it appears we have been too Earth-centric in our view of the universe. Now we're encouraged to search for life in a much wider range of environments.
And just how different might these environments be? The problem now is that as scientists try to find harsher and harsher environments to put a limit on where life can survive, we just keep finding hardier strains of bacteria. We haven't found the limit yet. Here are some examples of newly discovered extremeophiles:
Bacteria have now been found dozens of miles deep in the Earth's hot crust. These bacteria thrive in temperatures about 200 Fahrenheit, and in corrosive acid levels that rival the inside of a car battery. They have no access to (and no use for) air, sunlight, or liquid water. At the other end of our biosphere, bacteria have been found many miles high in our atmosphere, at the very limit of space. They survive in negligible air pressure, and never interact with life-forms from the surface of our planet.
Some microbes love radiation. Bacteria are now known to inhabit the interior chambers of nuclear reactors, as well as nuclear waste dump sites. One strain may prove particularly useful to us; they actually consume nuclear and toxic waste. Research is currently underway on how to use these obliging bugs (and even modify them genetically) to clean up our most difficult messes.
The passage of time is no way to kill extremeophiles either. Bacteria have been cultured (meaning they came back to life when food was introduced) after being trapped inside an insect's stomach for 135 million years. The insect had been sealed in a blob of amber. With nowhere to go and nothing to do, the bacteria just shut down and "hibernated" until they could get food again.
But I have to say that there is one, last kind of extremeophile that really takes my breath away. When the first generations of spacecraft were launched, scientists didn't worry about how to sterilize them. After all, any stow-away germs would be blasted with extreme heat and cold, the hard vacuum of space, radiation from the Sun, not to mention a total lack of water, air, and food. What could possibly survive?
The answer came back to us on Apollo 12. During that mission, astronauts landed on the Moon within walking distance of Surveyor III, an un-manned lunar probe that had been launched in 1967. Bits of Surveyor were brought back to Earth to study the impact the lunar environment had on metals and electronics. To everyone's shocked surprise, there were still bacteria alive inside one of the cameras. The bacteria had survived on the lunar surface for 31 months. They must have been relieved (sorry to anthropomorphize) to get back home.
This last discovery has to give us pause. We've been sending out probes around the solar system for decades now. Some have landed on the terrestrial worlds, while others have been sent careening into the atmospheres of the giant planets. We've got plans to drill through the ice of Jupiter's moon Europa and search for permafrost under the surface of Mars. How can we be sure we haven't already contaminated another planet with Earth bacteria, and what can we do to prevent any further harm?
Earth life, it seems, is surprisingly hard to kill. Is there a chance that bacteria from the Viking Landers have already gotten a foothold on Mars? Will we go there and take soil samples, only to find Earth bacteria happily going about life as usual? The chances of us having done any real harm at this point are vanishingly small, but that doesn't mean we don't have to be more careful.
A lovely ending note to this story is the planned death of the Galileo spacecraft, currently flying around Jupiter and it's diverse brood of moons. At first, scientists didn't much care what happened to the craft after its mission was completed; it could drift around as it pleased, probably crashing into Jupiter or one of the moons at some point in the future.
But that same spacecraft has just discovered that warm saltwater oceans are likely to exist beneath the surface of two of the moons, Europa and Ganymede. What delicate life may have taken hold there, and how will it react to the nuclear batteries and bacteria on our spacecraft? To protect this life we haven't found yet, Galileo will be directed to plunge into the atmosphere of Jupiter, ending in a fiery, germ-killing descent.
And the newly discovered seas, pristine and unexplored, will be protected from our resilient cousins, the extremeophiles.