The first time Tullis Onstott ventured underground, he squeezed into an elevator with dozens of South African gold miners and descended a mile into a pit called Mponeng. His goal: Finding the bizarre, hardy microbes that survive in sweltering, inhospitable rock. A geologist by training, Onstott spent his early career studying the Earth’s crust—until he heard a talk in 1993 about colonies of bacteria living thousands of feet below the surface. Ever since, he has made dozens of deep expeditions, sometimes paying his own way, and discovered bacteria living more than two miles beneath the surface in 140-degree-Fahrenheit heat. By investigating microbes in these harsh environments, Onstott is gleaning clues about how life could have begun in Earth’s hot, chaotic early days—and about what it might look like on other worlds. Even his office is underground, in the basement of Princeton University’s geology building, where Onstott met with DISCOVER reporter Valerie Ross.
The first time you went underground to look for life, in 1996, you had no idea what to expect. What was that trip like?
The miners took me into the stopes, the tunnels where they mine gold, to sample the rocks. We were looking at an organic rock layer just millimeters thick that had lots of carbon, because we
figured somewhere with a lot of carbon was a good place to look for life. The stopes are a meter high and they tilt downward at a steep angle, so you go down them almost like a slide, passing from one tunnel to the next. I basically slipped into a rabbit hole and got this big chunk of rock. I put it in an autoclave bag [normally used for sterilizing equipment], stuffed it in my knapsack, and then I went down the stope further until I came out the bottom into another, deeper tunnel.
What did you do with the sample you collected?
We measured the rock’s radioactivity. The Geiger counter showed it was hot as a pistol, so we sealed it up in a steel canister and filled the canister with argon gas, which pushed out all the oxygen. Organisms that live deep down are not normally exposed to oxygen, and in fact it could be toxic to them. So we sealed the rock away until we could get it back into the lab. I checked this radioactive rock inside a steel thing as baggage on a plane. This was 1996. Airport security was not like it is today.
When you analyzed the sample back at your lab, did you find any life?
We found one bacterium species similar to one previously identified from a hot spring in New Mexico. But the surprise was that this particular species could do something the other hot spring organisms could not: reduce [i.e., transfer electrons to] iron, which is present in minerals that are abundant in the mine’s rocks, and uranium, part of soluble compounds found in water in the mine. That helped us understand how they got their energy.
Then you found still more perplexing discoveries in other South
African mines—for instance, microbes similar to those previously seen only at the bottom of the ocean.
That’s right. We went back to South Africa in 1998, this time to Driefontein Mine, located about 40 miles southwest of Johannesburg, and took water samples, which are easier to work with than rock and less likely to be contaminated. We started finding the same organisms that people were reporting from deep-sea hydrothermal vents [where hot, mineral-laden fluid flows through volcanic rock into the ocean from deep within the Earth]. We don’t know how the same organisms got to be in both places, because South African crust has not seen ocean water in two-and-a-half billion years. It’s very much a mystery. We published the data, and the National Science Foundation gave us more money to go back again in 2000.
What happened on your third deep excursion in South Africa?
The next time, we purchased a house in one of the villages near the gold mines and set up a semipermanent lab there. Over two years, a rotating team from my lab and six other institutions collected most of the samples that we’re still working on today. One thing we did was expand on our first find and look at more radioactive samples. We began developing an idea that radiation in the rock provides energy for microorganisms. Wherever we had radiation, we tended to see hydrogen gas forming. It made me realize that radiation should produce hydrogen by breaking water bonds. Hydrogen is the key component the bacteria need to make ATP, the molecule they use for energy.
One bacterium we found is entirely self-
sufficient, a one-species ecosystem. Such things aren’t supposed to exist.”
That’s amazing, since we usually think of radioactivity as deadly—but these organisms were actually living on radiation?
Well, not just radiation, but radiation, water, and rock were all that was needed to support life at depth. You don’t need light, food, or anything else from the surface. Plus, it’s a renewable energy source. It turns water into hydrogen and hydrogen peroxide, which helps make the metals that the organisms consume. It is like recharging an electric battery. The radiation keeps on recharging the battery for the bacteria that then do their thing. Those bacteria could then sustain other deep organisms. That finding was really important to NASA because you can imagine any body in the solar system that has liquid water beneath the surface—like Jupiter’s moon Europa, probably—will have energy for organisms as well.
Can we observe these organisms at work in the lab?
The rule of thumb is that when you get back to the lab, you can grow less than 0.1 percent of what actually exists down there. We tried all sorts of ways to grow them, gave them all sorts of nutrients we thought they might want, and we failed miserably.
Since you couldn’t grow the bacteria that you found deep down, how did you learn just how they functioned?
We looked at their DNA instead, which we filtered out of the water, to determine where these things fit in with other sorts of microbial life.