For the first time, scientists have found soils on the Earth’s surface that appear to harbor no life at all. The soils came from two windswept, rocky ridges in the interior of Antarctica, 300 miles from the South Pole, where mountains poke through ice that is thousands of feet thick.
”The assumption has always been that microbes are tough, they can live anywhere,” says Noah Fierer, a microbial ecologist at the University of Colorado, Boulder, whose team studied the soils. Single-celled organisms have, after all, been found living in hydrothermal vents over 200 degrees Fahrenheit, alive in lakes trapped beneath half a mile of ice in Antarctica, and even surviving 120,000 feet up in the Earth’s stratosphere. But after a year of trying, Fierer and his PhD student Nicholas Dragone still have not found any signs of life in some of the Antarctic soils they collected.
Fierer and Dragone studied soils from 11 different mountains, representing a wide range of conditions. Those from the lower, less frigid mountains contained bacteria and fungi; but in some of those from the two highest, driest, and coldest mountains, no signs of life stirred.
“We can’t say they’re sterile,” says Fierer. Microbiologists are used to finding millions of cells in a teaspoon of soil; so a minuscule number—say, 100 living cells—could potentially escape detection. “But as far as we can tell, they don’t harbor any microbial life.”
Looking for signs of life
Whether some of the soils are truly lifeless, or later turn out to harbor a few surviving cells, this new discovery—recently published in the journal JGR Biogeosciences—could help guide efforts to find life on Mars. The Antarctic soils are perpetually frozen, saturated in toxic salts, and have not tasted appreciable amounts of liquid water for up to two million years—similar to Martian soils.
They were collected during a January 2018 expedition, funded by the U.S. National Science Foundation, to a remote section of the Transantarctic Mountains. Those cut through the continent’s interior, separating the high polar plateau in the east from the low-lying ice of the west. Scientists camped on Shackleton Glacier, a 60-mile-long conveyor belt of ice that spills down through a gap in the mountains. They used a helicopter to reach the heights and collect samples up and down the glacier.
On the warmer, wetter mountains at the foot of the glacier, just a few hundred feet above sea level, they found soils inhabited by animals smaller than sesame seeds: microscopic worms, eight-legged tardigrades, whiskered rotifers, and wingless insects called springtails. These bare, sandy soils harbored less than one-thousandth the number of bacteria that you’d find in a well-tended lawn —just enough to provide food for the tiny grazing beasts lurking beneath the surface.
But as the team visited higher mountains farther up the glacier, these signs of life gradually dwindled. At the top end of the glacier they visited two mountains—Schroeder Hill and Roberts Massif—which rise more than 7,000 feet above sea level.
The visit to Schroeder Hill was brutal, recalls Byron Adams, a biologist from Brigham Young University in Provo, Utah, who led the project. The temperature on this midsummer day was near 0°F. Screaming winds—which slowly evaporate the snow and ice, keeping the mountains bare—constantly threatened to lift and toss the garden shovel that they’d brought to dig in the sand. The ground was strewn with reddish volcanic rocks, pitted and polished from eons of wind scour.
As the scientists lifted rocks, they found their undersides crusted in white salts—toxic crystals of perchlorates, chlorates, and nitrates. Perchlorates and chlorates are caustic, reactive salts used in rocket fuels and industrial bleaches—and also abundant on the surface of Mars. The salts had accumulated on these parched Antarctic mountains because there was no water to wash them away.
“It felt like sampling on Mars,” says Adams. When you thrust the shovel in, “you know you’re the first thing to disturb that soil in forever—maybe millions of years.”
The researchers assumed that even in these highest, harshest places, they would still find a few live microbes hunkered down in the soil. But that expectation started to crumble in late 2018, as Dragone used a method called polymerase chain reaction (PCR) to detect microbial DNA in the dirt. Dragone tested 204 samples from mountains up and down the glacier. Those from the lower, less frigid mountains yielded plentiful DNA; but a huge portion of the samples from the higher elevations—20 percent— including most of the ones from Schroeder Hill and Roberts Massif, yielded absolutely nothing, suggesting that they contained vanishingly few microbes—or perhaps none at all.
“When he first started showing me some of the results, I’m like, ‘something’s wrong’,” says Fierer. He figured there must be a problem with the samples, or the lab equipment.
Dragone followed up with a battery of additional experiments looking for signs of life. He incubated the soils with glucose, to see if something living in the soils converted it into carbon dioxide. He tried to detect a chemical called ATP, which all life on Earth uses to store energy. And for months, he incubated bits of soil in a variety of nutrient cocktails, trying to coax any microbes present to grow into colonies.
“Nick threw the kitchen sink at these samples,” says Fierer. Despite all of those tests, he still found nothing in some of the soils. “That was really surprising.”
Is there really no life in the soil?
Jacqueline Goordial, an environmental microbiologist at the University of Guelph in Canada, finds the results “tantalizing”—especially Dragone’s efforts to determine what factors influenced the likelihood of finding microbes at a given site. He found that high elevation and high levels of chlorate salts were the strongest predictors of no detectable life. “That’s a really interesting discovery,” says Goordial. “It tells us a lot about the limits of life on Earth.”
She isn’t entirely convinced that their soils are truly devoid of life—in part because of her own experience in another part of Antarctica.
Several years ago she studied soils from a similar environment in the Transantarctic Mountains—a place 500 miles northwest of Shackleton Glacier, called University Valley, which probably hasn’t seen significant moisture or thawing temperatures for 120,000 years. These soils showed no signs of life when she incubated them for 20 months at 23°F—a typical summer temperature in that valley. But when she warmed the soil samples a few degrees above freezing, several of them showed bacterial growth.
Whether these soils are devoid of life depends on how you define it.
Scientists have, for example, found bacterial cells still alive after being trapped for thousands of years in glacial ice. While they’re trapped, the cells may slow their metabolism by a million-fold. They enter a state in which they no longer grow; they do nothing other than repairing the trickle of DNA damage that they experience from cosmic rays that penetrate the ice. Goordial speculates that these “slow survivors” may be what she detected in University Valley—and she suspects that if Dragone and Fierer analyzed 10 times more soil, they might also find them at Roberts Massif or Schroeder Hill.
Helping to look for life on Mars
Brent Christner, who studies Antarctic microbes at the University of Florida in Gainesville, believes these high, dry soils could help to refine the search for life on Mars.
He points out that the Viking I and II probes, which landed on Mars in 1976, carried life detection experiments that were based, in part, on studies of low-lying soils near the Antarctic coast—a region called the Dry Valleys. Some of those soils become damp with melt water during the summer. They harbor not only microbes—but in some places, tiny worms and other animals as well.
By comparison, the higher, drier soils of Roberts Massif and Schroeder Hill might be a better proving ground for Mars-bound instruments.
“The top surface layers of Mars are awful,” says Christner. “We don’t have an organism on Earth that could survive on the surface”—at least not in the top inch or two. Any spacecraft going there to look for life should be prepared in the nastiest place that Earth has to offer.