LEVEL 1:
(TAN COLOR)
The top
section of rock at the mine is
andesite, pressing down into layers
of quartzite
shale and quartzite.
LEVEL 2:
(BLUE COLOR)
Aquifers
scattered in thick layers of
dolomite, trap water that works its
way from the
surface through fissures.
LEVEL 3:
(BROWN COLOR)
In the lower
depths of the mine, the
shaft and tunnels were blasted
through hard layers of
andesite.
LEVEL 4:
(BLACK COLOR)
The microbe
hunters struck pay dirt
in the carbon leader, a gold reef
nestled in a mass of
ancient quartzite.
A WORLD OF
BURIED TREASURE:
The East Driefontein gold mine plumbs
mineral-rich reefs
angling down the
buried slope of a 2.9-billion-year-old
sea bottom. The microbe
hunters
venture by elevator to a rail tunnel that
becomes a crawl space as they
near
their objective: the carbon leader.
This seam of pebbly conglomerate is
rich
with gold, as well as bacteria that
feast on minerals in the rock.
The train stops at a dark hole in the wall: the stope, or mining face. Coleske leads the way. The passage dips down at a steep angle; in many sections the ceiling is only 3 feet high, forcing everyone to crab-walk on hands and feet. Sharp-edged rubble coats the floor, and water from cooling hoses used to spray the rock sluices through.
The stope swarms with miners, their headlamps piercing a murk of water vapor and dust. Drills, saws, and jackhammers roar so loudly that hand signals must be used. Amid the pandemonium, the scientists find their objective: a black coal-like vein laced with round stones and sparkly filaments. This mineral-rich seam is dubbed the carbon leader. Often no more than a finger wide, sandwiched in a mass of ancient quartzite, the carbon leader is heavily laced with uranium and holds an extraordinarily precious treasure: gold.
No one is certain how the carbon leader formed. It could be the remains of either algae that settled on an ancient sea bottom or oil that shot through a crack underground. In any case, the rock formation is loaded with carbon compounds potentially nutritious to microbes.
Strong evidence
that microbes could survive in an even more austere environment was
uncovered in 1995, when a team working at the Department of Energy’s
Hanford Reservation in Washington State found rock-eating organisms
that get their sustenance from elemental-mineral energy sources. The
microbes were taken from groundwater sitting in igneous basalt 4,500
feet down. In the lab, microbiologist Todd Stevens of Pacific Northwest
National Laboratory showed that basalt may react with groundwater to
release hydrogen. The microbes combine the hydrogen with carbon dioxide
in the water to make organic compounds. Their main waste product is
methane—a natural gas long thought to be formed near the surface by
swamp-dwelling microbes, and in the depths of the Earth by
nonbiological chemical reactions.
Coleske shouts: “There’s no supports. Pull a piece and the wall might come in!”
But eating is only half the equation. Like us, some bacteria must respire—combine food with some other substance to release energy. Our respiratory agent is oxygen, a by-product of plant photosynthesis. Oxygen is rare below ground, and microbiologists have found subsurface organisms that breathe an astounding variety of alternatives: ferric iron, sulfate, nitrate, nitrite, uranium, and carbon dioxide. On a 1996 visit to another South African mine, microbiologists Jim Fredrickson and Tom Kieft discovered a heat-resistant bacterium that inhales iron, nitrate, manganese, sulfur, chromium, cobalt, or oxygen.
At East Driefontein, Onstott planned to gather more samples to test a hypothesis that some deep bacteria extract energy from elements released as by-products of the natural radioactivity of the rocks. Now, crouched before the carbon leader, he reaches out and touches the seam with a Geiger counter. The instrument lights up, and he grins. Duane Moser starts to pull a chisel from his backpack to take a sample when Coleske slides over and shouts in his ear: “There’s no supports. Pull a piece and the wall might come in!” Suddenly, a drill starts up somewhere, shaking the place like an earthquake. Rock fragments ricochet off helmets. The researchers move on, looking for a safer spot. A few hundred feet down, they come upon a newly blasted section of tunnel where miners are shoving steel hydraulic jacks against the ceiling. “Good!” hollers Onstott. “Means it’s fresh.”
Moser pulls out a ball peen hammer and a sterilized chisel and pounds at the crumbly ore. Working the chisel in, he outlines a fist-size piece and pries it loose. As it falls, Onstott deftly catches it in a plastic sandwich bag. Moser takes a few minutes to catch his breath, then starts again. He loosens a foot-long piece a finger’s length deep and gingerly tips it into a sterile plastic bag that Onstott holds open.
Because surface microbes swept down by the ventilation system could confound the samples, Onstott wants to gauge how far they might penetrate the rock. Using spray cans, he spritzes the rock with a mist of bright chartreuse latex spheres, each the size of a bacterium, and then with an orange-colored chemical tracer. Because cooling-water jets might work bugs farther into the rocks, he wants to simulate the process. But he cannot reach the rock with the hose. So, one by one, several miners take off their helmets, fill them from a spigot, and pass them to Onstott to splash water on the spray-painted spot.
Precious samples in hand, Onstott and his fellow scientists’ grim faces finally break into smiles. They climb slowly down the stope to another railroad tunnel and begin hiking out, pressing tight against the wall a couple of times to avoid passing ore trains.
When life is encountered this deep and distant, one question must be asked: Did it all start in the proverbial scummy surface pool—or down in the groanings of Earth? Some evolutionary scientists argue that subsurface microbes had the best chance of survival on the Earth early in its history. Below ground they were safe from extreme radiation, asteroids, and other hazards. Respirers of iron and like substances may have evolved before oxygen existed on the surface. Deep methane-making microbes could have come from a lineage stretching back even further. Wouldn’t oxygen breathers—including humans—have evolved from them?
The implications go beyond Earth. Planetary scientists long skeptical of finding extraterrestrial life are intrigued by the discovery of deep microbes. Early landing craft showed the surface of Mars to be barren. But a prime objective of future planetary landing missions is to drill downward as fast and far as possible. Onstott and his colleagues have been asked to help design those probes.
At the moment, they have
something less glamorous to accomplish. With bags of chunks from the
carbon leader, the group drives to a sheet-metal mine building. There,
Moser puts the rocks into an airtight plastic tent filled with 98
percent nitrogen and 2 percent hydrogen, to protect organisms that
might be poisoned by our alien atmosphere. Using gloves fixed in the
tent’s side, he sticks a piece of rock in a hydraulic vise and pumps a
few times. Crack. It splits open neatly. With the vise, he pares the
rock into successively smaller pieces that had not been exposed on any
side to air. One pristine piece is ground into powder for conducting
various tests on-site. Then the scientists take turns dropping other
rock pieces into jars and test tubes headed for various labs in the
United States and Europe.
The discovery of deep microbes has made scientists less skeptical of finding extraterrestrial life.
Months later, they were in for a surprise: some stones appeared to house between 100,000 and one million microbes per gram. “That’s 100 times what we expected,” Onstott says. The team’s trove also included mine-water samples containing rod-shaped microbes six times the size of many surface microbes. Some of the bugs breathe iron; some live on methane. Others might eat hydrogen freed when radiation breaks down water molecules. Onstott and his colleagues even speculate that some of the deep microbes may have deposited the filigree of gold in the East Driefontein mine.
After their journey into the mine, Onstott and his colleagues appear relieved to be out of the mine, far from the privations of the deep. While they huddle in the comfort of their makeshift lab, the sky darkens and a violent storm cranks up. Rain cascades through a leak in the roof, and wind, heavy with the smell of wet trees and dirt, gusts through an open door. A “seismic event” shakes the building. No one seems to mind.
DEEP THINKING
Thomas Gold
Deep microorganisms have long remained hidden because it is so difficult to penetrate Earth, much less gather biological samples there. The first inklings of life far below the surface came in the 1920s, when a microbiologist and a geologist at the University of Chicago cultured anaerobic (non-oxygen-breathing) bacteria from Illinois oil wells 2,000 feet deep. Skeptics said drills must have polluted samples with surface organisms. The study went forgotten.
A half-century later, biologists began finding microbes living, inconceivably, at temperatures of nearly 250 degrees in hot springs and ocean vents. That intrigued Thomas Gold, a Cornell astrophysicist known for his rebellious theories. He proposed that if microbes could live in superheated environments, they must be alive beneath the surface in porous sections of rock. Gold made some calculations. Beneath the surface, the temperature of rock increases 20 to 35 degrees per half-mile. Based on a maximum tolerable temperature of about 250 degrees, that would mean microbes could exist more than three miles down. Suppose, he said, that pores in rock account for about 3 percent of Earth’s upper crust and microbes occupy 1 percent of those pores. Suck all those bugs out and they would cover the surface of Earth with slime five feet thick—more than all the insects, plants, people, and everything else alive. Shutting our eyes to creatures of the deep, says Gold, is “surface chauvinism.” —K. K.
