The soil of the rocky ridge in central Antarctica has never contained microorganisms.
        For the first time, scientists have discovered that there appears to be no life in the soil on the Earth’s surface. The soil comes from two windswept, rocky ridges in Antarctica’s interior, 300 miles from the South Pole, where thousands of feet of ice penetrate the mountains.
        “People have always thought that microbes were hardy and could live anywhere,” says Noah Firer, a microbial ecologist at the University of Colorado Boulder whose team studies soil. After all, single-celled organisms have been found living in hydrothermal vents with temperatures exceeding 200 degrees Fahrenheit, in lakes under half a mile of ice in Antarctica, and even 120,000 feet above Earth’s stratosphere. But after a year of work, Ferrer and his doctoral student Nicholas Dragon still have not found any signs of life in the Antarctic soil they collected.
        Firer and Dragone studied soils from 11 different mountain ranges, representing a wide range of conditions. Those that come from lower and less cold mountain areas contain bacteria and fungi. But in some mountains of the two highest, driest and coldest mountain ranges there are no signs of life.
        “We can’t say they’re sterile,” Ferrer said. Microbiologists are accustomed to finding millions of cells in a teaspoon of soil. Therefore, a very small number (eg 100 viable cells) may escape detection. “But as far as we know, they do not contain any microorganisms.”
        Whether some soil is truly devoid of life or is later discovered to contain some surviving cells, new findings recently published in the journal JGR Biogeosciences could help in the search for life on Mars. Antarctic soil is permanently frozen, full of toxic salts, and has not had much liquid water for two million years—similar to Martian soil.
        They were collected during a National Science Foundation-funded expedition in January 2018 to remote areas of the Transantarctic Mountains. They pass through the interior of the continent, separating the high polar plateau in the east from the low-lying ice in the west. The scientists set up camp on the Shackleton Glacier, a 60-mile conveyor belt of ice that flows down a chasm in the mountains. They used helicopters to fly to high altitudes and collect samples up and down the glacier.
        In the warm, wet mountains at the foot of a glacier, just a few hundred feet above sea level, they discovered that the soil was inhabited by animals smaller than a sesame seed: microscopic worms, eight-legged tardigrades, rotifers and tiny worms. called springtails. Winged insects. These bare, sandy soils contain less than one thousandth the amount of bacteria found in a well-manicured lawn, enough to provide food for the small herbivores lurking beneath the surface.
        But these signs of life gradually disappeared as the team visited higher mountains deeper into the glacier. At the top of the glacier, they visited two mountains—Mount Schroeder and Mount Roberts—which are over 7,000 feet high.
        The visits to Schroeder Mountain were brutal, recalls Byron Adams, a biologist at Brigham Young University in Provo, Utah, who led the project. The temperature on this summer day is close to 0°F. The howling wind slowly evaporated the ice and snow, leaving the mountains bare, a constant threat to the lifting and throwing of the garden shovels they had brought to dig up the sand. The land is covered in reddish volcanic rocks that have been eroded over hundreds of millions of years by wind and rain, leaving them pitted and polished.
        When the scientists lifted the rock, they discovered that its base was covered with a crust of white salts—toxic crystals of perchlorate, chlorate, and nitrate. Perchlorates and chlorates, corrosive-reactive salts used in rocket fuel and industrial bleach, are also found in abundance on the surface of Mars. With no water to wash away, salt accumulates on these dry Antarctic mountains.
        “It’s like sampling on Mars,” Adams said. When you stick a shovel in, “you know you are the first thing to disturb the soil in forever—maybe millions of years.”
        The researchers suggested that even at such high altitudes and in the harshest conditions, they would still find living microorganisms in the soil. But those expectations began to fade in late 2018, when Dragon used a technique called polymerase chain reaction (PCR) to detect microbial DNA in dirt. Dragon tested 204 samples from mountains above and below the glacier. Samples from lower, cooler mountains yielded large amounts of DNA; but most samples (20%) from high altitudes, including most from Mount Schroeder and Roberts Massif, were not tested for any results, indicating that they contained very few microorganisms or perhaps none at all.
        “When he first started showing me some results, I thought, ‘Something’s wrong,’” Ferrell said. He thought there must be something wrong with the sample or the lab equipment.
        Dragon then conducted a series of additional experiments to search for signs of life. He treated the soil with glucose to see if certain organisms in the soil converted it to carbon dioxide. He was trying to discover a chemical called ATP, which is used by all life on Earth to store energy. For several months, he cultivated pieces of soil in various nutrient mixtures, trying to convince existing microorganisms to grow into colonies.
        “Nick threw the kitchen sink at these samples,” Ferrell said. Despite all these tests, he still found nothing in some soils. “It’s really amazing.”
        Jacqueline Gurdial, an environmental microbiologist at the University of Guelph in Canada, calls the results “enticing,” especially Dragon’s efforts to determine what factors influence the likelihood of finding microorganisms in a given location. He found that high altitude and high chlorate concentrations were the strongest predictors of failure to detect life. “This is a very interesting discovery,” Goodyear said. “This tells us a lot about the limits of life on Earth.”
       She’s not entirely convinced that their soil is truly lifeless, partly due to her own experiences 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 that may not have had significant moisture or melt temperatures for 120,000 years. When she incubated it for 20 months at 23°F, a typical summer temperature in the valley, the soil showed no signs of life. But when she heated soil samples a few degrees above freezing, some showed bacterial growth.
        For example, scientists have discovered that bacterial cells remain alive even after thousands of years in glaciers. When they become trapped, the cell’s metabolism can slow down a million times. They go into a state in which they no longer grow, but only repair DNA damage caused by cosmic rays penetrating the ice. Goodyear speculates that these “slow survivors” may be the ones she found in College Valley—she suspects that if Dragone and Firer had analyzed 10 times more soil, they might have found them in Roberts Massif or Schroeder Mountain .
       Brent Christner, who studies Antarctic microbes at the University of Florida in Gainesville, believes these high-altitude, dry soils could help improve the search for life on Mars.
        He noted that the Viking 1 and Viking 2 spacecraft, which landed on Mars in 1976, conducted life-detection experiments based in part on studies of low-lying soil near the coast of Antarctica, a region called the Dry Valleys. Some of these soils become wet from meltwater in the summer. They contain not only microorganisms, but in some places also tiny worms and other animals.
       In contrast, the higher, dry soils of Mount Roberts and Mount Schroeder may provide better testing grounds for Martian instruments.
        “The surface of Mars is very bad,” Christner said. “No organism on Earth can survive on the surface”—at least the top inch or two. Any spacecraft going there in search of life must be prepared to operate in some of the harshest places on Earth.
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