Braving
100-degree temperatures two
miles down in South
Africa’s East
Driefontein
gold mine, geologist
T. C. Onstott sheds his shirt to collect
microbe samples
with biologist
Duane Moser. Scientists once thought
life could not be sustained
so far
underground, but Onstott says evidence
now suggests “microbes have been
here for 2 billion years.”
Oxygen does not exist naturally so far underground, and only a constant rush of forced air from the surface disperses poisonous methane and hydrogen seeping from the rock. Pumped-in jets of water help keep workers from cooking to death.
As the elevator plummets, a miner turns on his headlamp, revealing beads of sweat popping out on the face of geologist T. C. Onstott. The Princeton University professor is not scared. He pulls a water bottle from his knapsack and takes a long, slow swig. To him, the heat and claustrophobia are merely aggravation. Real worry here centers on what are euphemistically referred to as seismic events. So much rock gets dragged out of the mine each day that the earth around it resettles frequently. The resulting quakes spur rock bursts—high-pressure cave-ins that kill.
Day-to-day work
in this hellhole requires a toughness and adaptability that few people
can muster. But there are life-forms deep in the mine that are even
tougher: bizarre tiny creatures that Onstott and his colleagues have
come to study. “If it’s not too bad for me down there, it’s probably
not too bad for the bugs,” he says with a laugh.
The East
Driefontein mine is located
some 45 miles from Johannesburg
in South Africa’s Gauteng Province.
In the SeSotho language, Gauteng
means “place of
gold.”
Not long ago, the very idea of finding life at such depths was considered less likely than discovering it on other planets. Life was thought to peter out a little past the depth of a grave. A billion bacteria may exist in a pinch of topsoil, but biologists found that the numbers dropped to the millions, then the thousands, then the hundreds per sample as they dug down farther from the sun, air, and sources of food. Then in the last decade, researchers digging deeper and using new analysis techniques were astonished to find hordes of unknown microbes.
Isolated from the surface for eons, these organisms have been
living in environments presumed impossible—oil wells, aquifers, and
deep rock. Some even thrive in radioactive stone or at temperatures of
200 degrees. And far from depending on organic matter from the surface
world, the deepest-dwelling microbes both eat and breathe geologic
ingredients—like iron, manganese, and sulfur—and possibly hydrogen. In
their wake they may leave massive deposits of metals—including copper
and gold—as well as natural gases such as methane. More than 11,000 new
strains of these subterranean bacteria have been cultured, but only a
few have been studied and named. Some researchers now speculate that
the mass of life below Earth’s crust may equal or exceed the mass of
life on the surface. A few bold scientists even ponder whether the
creatures from the deep could be our ancestors. Their discovery has
also revived hopes of finding life beneath the surface of barren
planets like Mars.
Some microbes thrive in radioactive stone or
at temperatures of 200 degrees
Onstott, a calm, lanky man who smiles easily, has adapted readily to the strange world below his feet. As a child, he walked in awe through the Carlsbad Caverns in New Mexico. As a graduate student in the 1970s, he studied rocks from South Africa’s diamond mines. Then in 1994 he experienced his first serious encounter with deep life when he worked with a group of scientists from the U.S. Department of Energy who analyzed samples taken from a 1.7-mile-deep exploratory natural-gas well dug near Washington, D.C., by Texaco.
Providing the deepest drilling samples the biologists had seen, the hole bottoms out in a shale deposit, the remains of a long-buried lake bed. The well was a bust for Texaco, but Onstott and his colleagues found other riches—previously unknown anaerobic bacteria spread out as thinly as just one per gram of rock, hanging on amid extreme salinity and 167 degree temperatures.
The scientists who studied the well constructed a geologic history showing that the shale could have hosted bacteria as early as 160 million years ago—before flowering plants first began growing on Earth. “The bacteria probably filtered in with groundwater,” Onstott says.They took up residence in rock pores, and impermeable sediment layers formed over them, blocking new migration and the flow of nutrients. Then, about 80 million years ago, when dinosaurs were still walking around upstairs, the pore entries had gotten too small for even the smallest bacteria to squeeze in or out. Onstott says the bacteria must have been trapped in their tiny tombs since that time. Amid such harsh conditions, cells become living fossils, taking hundreds, even tens of thousands, of years to divide. Scientists named one strain discovered in the Texaco well Bacillus infernus—bacterium from hell.
Even with advances in drilling
techniques, it is difficult to obtain samples from wells that have not
been contaminated by air or water from the surface. Onstott decided he
had to find a way to visit the microbes himself. The mine he chose to
explore is part of a sprawling man-made underworld started on a Sunday
morning in 1886 when an Australian prospector found a gold-rich outcrop
on the present site of Johannesburg. Like a nose peeping out of the
covers, that outcrop was just the start; since then, three-quarters of
the gold ever mined has come out of the region. The most recent shaft
at East Driefontein took 15 years to excavate—and that was just the
vertical hole.
In a
makeshift lab, microbiologist
Svetlana Kotelnikova uses an anaerobic
chamber to
handle mine rocks.
Some samples turned out to contain
methane-eating microbes.
For his journey into this biosphere, Onstott rounded up an all-star crew of microbiologists. They included: Bill Ghiorse, who led some Environmental Protection Agency research into subsurface bacteria that eat toxic waste; Duane Moser, a lab wizard and avid spelunker; Svetlana Kotelnikova, a Russian scientist working in Sweden; Mary DeFlaun of Envirogen; and Jim Fredrickson, who has isolated many deep-bacteria cultures while working at the Pacific Northwest National Laboratory.
At
East Driefontein, Onstott and his teammates would spend three months
gathering samples at a depth where no microbe hunters had ventured
before: the Number Five shaft bottoms out at 11,222 feet. On this
scouting mission, the elevator comes to a grinding halt at 6,120 feet
and the researchers walk through a short tunnel to a second elevator
that takes them to the mine’s newest and deepest diggings. A faint
smell of ammonium-nitrate explosive hangs in the air during the second
elevator descent. At the bottom, two escort miners, Marius Coleske and
Raj Nair, direct the group to a miniature train for a half-mile ride
through the dark as a giant air pipe roars overhead. Everyone sweats
freely from every pore, even the miners. “I couldn’t believe how much
fluid I lost,” Onstott says later. “My boots were full of sweat. I
could feel it slosh up against my shins at every step. I had to keep
emptying the perspiration out of my gloves.”
DEEP DINING
Biologists
found a
wide variety of bacteria
in a sample of
mine-fissure water.
The organisms of the deep biosphere are amazingly well-adapted to the tough living conditions of their world. Normal surface protozoans, which eat bacteria, can be ten times the size of their prey. Far underground—where available food amounts to as little as 1 percent or less of that found on the surface—protozoans might be nearly the size of their victims. The bacteria that live much farther down are themselves much smaller than their relatives on the surface and rarely have the fancy buds, appendages, or spiral shapes of microbes in the soil. Most are efficient, resistant spheres and rods.
Sedimentary rocks near the earth’s surface contain interconnected spaces for water to infiltrate and bacteria to grow and move, plus the remains of dead plants and algae once at the surface—the apparent food of many organisms. This is first-come, first-served dining. Those in upper layers grab the most digestible compounds, leaving lower ones fighting for leftovers, including waste and decay products from those above. Pore spaces that shelter subsurface microbes are constricted to the vanishing point in metamorphic rocks buried over eons by further sedimentation, volcanic eruptions, or massive folding of Earth’s crust. And dense crystalline material formed by melting—basic igneous rock—offers little housing or dining. But igneous rock is brittle, and within a few thousand feet of the surface, it develops water-filled fracture systems through which groundwater-hosted nutrients and microbes can travel. Fluids may take tens of thousands of years to get somewhere, and in these narrow corridors, life takes hold. —K. K.
