On a brilliant mid-summer day in December, our helicopter lifts off from McMurdo Station, the largest outpost of the U.S. Antarctic Program, situated some 2,400 miles south of New Zealand. A quick 50-mile flight across frozen McMurdo Sound brings us to a dark rocky beach at the mouth of Taylor Valley, the southernmost of the Mc-Murdo Dry Valleys. These valleys are a unique creation of the Transantarctic Mountains, which form an i,8oo-mile-long spine separating East from West Antarctica and block the advance of the massive East Antarctic ice sheet toward the sea. In a handful of valleys bordering McMurdo Sound, fierce scouring winds conspire with the bulwark of the Transantarctic ridges to create the largest ice-free expanse on a continent largely frozen for 30 million years. The polar deserts of the dry valleys are often touted as the most Mars-like terrain on Earth.
Turning up Taylor Valley, we fly over a landscape of glacial rubble patterned into tortoiseshell polygons by the heaving and sighing of frozen ground. Along the valley wall to our right, glacial tongues lap out between peaks of the Asgard Range, descending to the valley floor and coming to a halt as stark, blue-white ice walls that rise as high as 65 feet above the bleak terrain. We pass the Commonwealth Glacier and advance toward the Canada. Silver ribbons of meltwater stream from these and smaller glaciers, meandering across the valley floor until they disappear into a liquid moat that rings the permanent ice cover of Lake Fryxell below us.
The Taylor Valley glaciers and lakes—first Fryxell, then Lake Hoare, and at the head of this 22-mile-long valley, Lake Bonney— look almost insignificant from the air. But set off to hike among them and you soon realize that the clear air and stark landscape fool the eye. There are no trees, no familiar living shapes to help judge size and distance, no sound but wind. That very starkness, however, is the reason the research team I'm flying with returns here each December at the peak of the austral summer.1
Despite their barren appearance, the dry valleys serve as an oasis for land-based life on a continent 98 percent concealed by ice. Below us, life persists largely unseen in the soils, rock, ice, and streambeds and also in permanently liquid stews of briny water beneath the lake ice. This is a sparse world, largely microbial, but with a smattering of microscopic invertebrate animals to round out a simplified food chain. In the early 1990s, scientists from many disciplines began converging on this stripped-down ecosystem each summer in a coordinated effort to decipher ecological patterns and processes too complex to unravel in livelier, greener places.2
"This is the only place where we can see the effect of a change or disturbance on an individual species in the soil," soil ecologist Diana Wall had told me a week earlier as we waited in Christchurch, New Zealand, for the military cargo plane that would ferry us across the Southern Ocean to McMurdo. "We want to know how human-caused changes in climate could influence members of the soil food web, and what effect the loss of individual soil species might have on ecological processes such as nutrient cycling," Wall said.
Now, as our helicopter banks to land near the shore of Lake Fryxell, Wall can barely contain her excitement. She points down toward rows of translucent plastic cones glinting like lampshades on a nearby slope. Director of the Natural Resource Ecology Laboratory at Colorado State University, Wall is coleader of a research team long
known here as the "Wormherders" because their efforts focus chiefly on the fortunes of nematodes, microbe-munching roundworms about i/20th of an inch long that dominate the food chain of the dry valleys like lions on the savanna. The field of cones—actually, cone-shaped warming chambers—is one of the "worm farms" we've come to tend.
It is fortuitous for Wall that the animals she has studied for more than three decades dominate these valleys, but it is hardly surprising. Nematodes are the most diverse and abundant animals on the planet, outnumbering even ants. Four of every five animals are nematodes.3 These mostly microscopic and transparent creatures live in our gardens and crop fields, in oceans and lakes, inside the bodies of bees and horses, whales and us. Most of the free-living nematodes in soils and sediments graze on bacteria, fungi, and algae, breaking down the organic matter tied up in these microbial hordes and speeding nutrient cycling by releasing key building blocks of life such as carbon and nitrogen that will nurture and fuel new generations of beings above and below the ground.
In complex soil food webs on other continents, nematodes graze amid protozoa, slime molds, springtails (wingless relatives of insects in the animal phylum or grouping known as arthropods), and other invertebrates that also consume microbes. Predators such as mites, pseudoscorpions, centipedes, and spiders feed on the grazers, and in turn serve as food for larger predators. This complexity masks the importance of any single species in the vast business of nutrient cycling. Thus, the very sparseness of the soil food web in Antarctica makes this an attractive place to explore one of the most urgent questions in ecological research: What do we lose in terms of ecological functioning as species disappear?
"Here we have a group of animals in an extreme environment who are involved in decomposition and nutrient cycling just like their peers in other soil ecosystems," Wall explained. "So it's not a stretch of the imagination to take this animal living in the soil in Antarctica, subject it to climate change or other disturbance, and predict that this is what might happen elsewhere."
For more than a decade, Wall and her collaborators have been altering the temperature, moisture, and food supplies inside the worm farm chambers to see how nematodes respond. Ironically, the climate of the dry valleys has subjected nematodes to an even more severe test during this period, and populations have plummeted.
The pilot touches our helicopter down on a flat square of sand outlined with rocks near a blue hut, the headquarters for field camp F6. Three of us pile out, crouching low under the still-turning rotors, and drag a bevy of gear-filled ice chests, plastic buckets, daypacks, and dozens of 5-gallon carboys full of water and sugar solutions safely beyond the propeller wash before the helicopter lifts off.
Antarctica is billed, without exaggeration, as the highest, driest, coldest, windiest place on earth. You don't have to wait long after arriving on "the ice," as everyone here calls the continent, to hear lurid tales of chilling deaths, and not just among the hoary explorers of a century past. Only a few days earlier I'd completed a mandatory overnight survival school with Wall's two postdoctoral researchers— all three of us new to the ice—and listened to cautionary tales of folly as we dug snow shelters, learned to use two-way radios, and practiced rescuing one another in a simulated whiteout. Although Taylor Valley is ice-free and averages less precipitation than the Sahara Desert— a scant 4 inches of snow a year, the equivalent of a fraction of an inch of rain—the mean annual temperature ranges from 3 ° to -6° F, and the winds that sweep down off the ice sheet or up from the sea can quickly drop the wind chill as low as -100° F.
On this morning, however, the sun is piercing and the temperature hovers in the high 20s. Without the brisk wind, the place would seem almost balmy for three people freshly arrived from winter in the Rocky Mountains: Wall from Colorado; Byron Adams, an evolutionary biologist from Brigham Young University in Utah; and me from Montana. We hurry to adjust clothing layers and lace up our hiking boots. Wall is moving quickly, and her sense of excitement and haste on the first field visit of the season is infectious. Time is critical, both for us and for the soil life we've come to monitor. The soil community here endures in suspended half-life through the long, dark polar winter, waiting for golden days like this one each December and January. In this brief polar summer, the sun shines round the clock, the air temperature rises near freezing, the soil surface absorbs enough solar radiation to thaw, glaciers melt a bit, and liquid water brings streams to life. For the scientists who come here, helicopter hours are rationed and field time crowded with repetitive and often exhausting tasks. I quickly learn that little of the soil team's fieldwork is high tech. We will spend the next 5 hours emptying dust traps; scooping up, bagging, and labeling soil; and pouring water and sugar solutions on some long-pampered little communities sheltered inside the chambers of the worm farm.
I hustle to keep up as Wall and Adams grab packs and buckets and move quickly up the slopes beyond the hut, following a well-worn path past a scattering of blue and yellow sleeping tents and, when the path ends, walking through what feels like the deep sand and jumbled rock of a dry streambed.
"Geez, it's awesome; this place is great." Adams, irrepressibly cheerful, is admiring the glaciers and peaks above us as he hurries toward a line of dust traps. This is his second season on the ice. I gawk up at the stunning scenery, too, while trying to keep my footing on the uneven ground.
"I try not to make new footprints," Wall says matter-of-factly, walking carefully along the troughs of the polygon-patterned ground. "There's so much traffic out here now." I take that as a subtle caution and focus on my feet, seeing only rubble where she sees an ecosystem. Indeed, the Wormherders have learned that the centers of the polygons offer the best habitat and host the most abundant nematode populations.4 I try to keep my feet in the narrow troughs that define the polygon boundaries.
At first, Wall seems an unlikely person to be found kneeling for hours in the dirt, hands cracked and bleeding from digging gloveless in near-frozen ground. Students and colleagues fondly describe her as a "type specimen" of an overachiever. Besides directing a major research center and leading multiple international collaborations, Wall has presided over a growing list of professional societies and international panels, committees, and programs that keep her jetting around the globe much of the year. Yet Wall had been captivated by the romance of Antarctic exploration for 20 years before her research interests presented her with a reason to make the journey herself. From her first season on the ice in 1989, she has remained fiercely devoted to the place and its science, returning annually to the dry valleys with her team as well as longtime collaborator Ross Virginia of Dartmouth College or members of his research group. The team quickly learned that life persists perilously close to the edge here, and that, as Wall puts it, "every human footprint is an ecological footprint."
During the 1990s, the Antarctic Treaty nations acknowledged the fragility of the dry valleys by designating them as a special management area and adopting regulations to protect them from pollution, human waste, vehicles, and even footprints where possible. These may be the only soil and sediment communities in the world with such protections, and it explains why we are all carrying "pee bottles" in our packs and why the helicopters that pass overhead are often "slinging" 50-gallon drums of human waste from the field camps back to McMurdo. (The Antarctic Treaty, first signed in 1959, declares that no country owns or rules Antarctica and that the continent is to be dedicated to peaceful purposes such as scientific research. Some 44 nations are now parties to the treaty.)
As the day proceeds, I make myself useful by holding open twisttop plastic bags while Wall carefully pours wind-blown soils from red nylon trays that have been sitting out in open-topped chambers all winter. The trays serve as dust traps that are helping the team test a theory that winds sweep dormant nematodes and other tiny invertebrates and microbes around the valleys and even disperse them far onto the continent.
Wall picks up a palm-sized rock that had been placed as a weight in the center of one tray. The wind has sandblasted its black top into soft curves. The bottom, buried for ages in the soil, is stained a lighter color. Under rocks like this that pave the polygon surfaces, in ancient soils as coarse-textured as beach sand and often salty and alkaline, live single-celled green algae, cyanobacteria, microbes, and other invertebrate animals as well as nematodes.
"I just hate that we move these," Wall says as she puts the rock aside. "There was a community under this rock that took thousands of years to form."
She lifts the tray over the plastic bag I'm holding out. "Okay, let's see where you're from," she says, talking to the creatures she envisions in the accumulated dust as she pours. We won't know who is actually there until we return to the McMurdo lab, flush them from the soil with sugar and water, and examine them under a compound microscope.
"We have so many questions we want to ask now," Wall explains. "But our first years here were very much a discovery process. First we just wanted to find out if these beasts were here."
Her comments remind me that until she and Ross Virginia began their fieldwork, few people believed there were any living creatures out here at all. In and around the lakes and streams, yes, but not out here in the arid soils that cover 95 percent of the dry valleys. This soil was considered as sterile as the dust of Mars or the moon.
British explorer Robert Falcon Scott and two companions became the first people to set foot in the dry valleys when they descended into Taylor Valley from the eastern ice sheet in December 1903. After the hardships of the polar plateau, the party delighted in the novelty of lunching on a sandy beach beside a gurgling stream. The only sign of life they noticed was the skeleton of a Weddell seal that had inexplicably hauled itself 20 miles up from the sea. In his journal, Scott called this place the "valley of the dead."5
Scott was wrong, but he was hardly the last of us to overlook life right under our feet. Another 55 years would pass before anyone took a closer look. Interest in the biology of the valleys began with explorations conducted during the International Geophysical Year in 1957-1958, when researchers first documented a surprising array of life forms. At the edges of lakes and in ephemeral ponds and streams, researchers found mosses, lichens, and mats of green algae and red, orange, and black cyanobacteria. Living among the mats were bacteria, yeasts, molds, and an array of microscopic invertebrates that feed on microbes, algae, and detritus: nematodes, protozoa, rotifers (tiny aquatic invertebrates known as wheel animals because the beating of their hair-like cilia as they move and feed resembles a rotating wheel), tardigrades (chubby creatures variously nicknamed "moss pigs" or "water bears" because of the claws on their four pairs of stumpy legs), and occasionally, mites and springtails.6
Out beyond the watery habitats, however, investigators were drawing a blank in their efforts to detect even microbial life in the soils using the limited techniques of the day—primarily attempting to grow microbes on growth media and broths in Petri dishes, a method that reveals only 0.1-1 percent of the microbes in most soils.7 Since no microbes could be found, biologists saw little reason to look for nematodes and other organisms that feed on microbes.
Much of the early biological research in the dry valleys involved scientists interested in the practical problems of searching for life on Mars. Because the arid soils appeared sterile and microbes from wetter habitats nearby had apparently failed to adapt and actively colonize the arid areas, some scientists suggested that "Martian life could not be built on a terrestrial model."8 Earthly life seemed to have reached its limits here in conditions much less harsh than those on Mars. But other scientists weren't convinced. One member of the Viking mission biology team, Wolf Vishniac, fell to his death from a steep slope in the Asgard Range in 1973 while trying to disprove the sterility theory and develop a better "life detector" to send to Mars.9 Vishniac and other skeptics were soon proven right: Life has learned to cope with conditions here.
The first direct sightings of life in the polar desert away from ponds and streams came in the mid-1970s, when E. Imre Friedmann and Roseli Ocampo reported finding cyanobacteria and later lichens—a partnership of green algae and fungi—growing within rock fissures and even in the pores of sandstone rocks in the mountains of the dry valleys region. Earlier, the two researchers had found similar "cryptoendolithic"—literally, "hidden in rock"—communities secreted within rocks a world away in hot deserts. In both places, it turned out, these microbes had adapted to aridity in the same way: When water becomes scarce, the organisms simply dry up, shut down metabolic activity, and wait in a "cryptobiotic" state until water again becomes available.10 (Cryptobiotic translates literally as "hidden life," but it is used to describe various states of dormancy in which metabolic activity temporarily ceases and life is essentially suspended.) Similarly, the cyanobacteria and algae that form living crusts across the surface of many desert soils pass the dry periods in a dormant state, just like their cousins within the rocks.
Wall and Virginia, too, had done much of their research in hot deserts before they turned to the Antarctic. Working in the Chi-huahuan desert of southern New Mexico, they had already learned that the diversity and abundance of nematodes are not tied to soil moisture levels. The finding seems counterintuitive because nematodes are essentially aquatic animals that live in water films on soil particles and in soil pores. The key to this paradox is that these tiny animals also have cryptobiotic strategies that allow them to shut down their life processes during dry spells.11
It seemed quite possible to Wall that nematodes could have colonized the arid soils of the dry valleys. But why, I wondered, with plenty of hot deserts to study, would a soil ecologist want to look for worms in Antarctica? One answer: to escape from the influence of plants.
In hot deserts, and indeed, in most other land-based ecosystems, green plants rather than water hold the key to where you will find the highest abundance and diversity of soil creatures. Shrubs such as mesquite create fertile islands, building up organic matter and nutrients around themselves thanks to their litter and roots. Even in the relatively barren stretches between mesquite shrubs, an underground network of mesquite roots exerts a powerful influence on the soil community. At some scale, patterns of underground life are also influenced directly by physical and chemical properties of the soil, but that influence—so stark on the frost-patterned ground of the dry valleys —is hard to detect amid the dominating presence of plants and the teeming activities of the soil communities around their roots.
Think of it this way: Life is not randomly scattered throughout the soil. Plant roots, leaf litter, animal burrows, termite mounds, earthworm castings, and other biological detritus as well as physical and chemical factors such as pH and salinity create a patchwork of good and poor neighborhoods underground.12 The good neighborhoods are hotspots for diverse soil life and for the biological activities that drive decomposition, nutrient cycling, and other processes vital to plant growth. In many ecosystems, plants devote as much or more of the carbon they take in through photosynthesis to growing roots as to building new leaves and stems. Roots form a kind of upside-down forest, dominating the soil community with more than their physical presence. Growing roots push through the soil, drawing in water and soluble nutrients and at the same time sloughing dead cells and leaking significant amounts of sugars and amino acids into the "rhizosphere"—the neighborhood immediately adjacent to the roots. Microbes feast and flourish in the rhizosphere, growing tens or hundreds of times more numerous than microbial populations living in the bulk soil that often begins only i/ioth of an inch away. Protozoa, nematodes, and other consumers of microbes flourish, too, along with their predators and the rest of the soil food web. The rhizosphere is the place where symbiotic (mutually beneficial) interactions such as nitrogen fixation—a process by which microbes capture plant-fertilizing nitrogen from the air—take place, as does competition, predation, grazing, and other interactions between plants and the soil commu-nity.13 Tree roots may plunge 25 feet or more, creating a three-dimensional ecosystem by moving carbon deep into the soil profile. Even the leafy canopies of plants alter the characteristics of the soil habitat by shading it and creating a layer of litter over the surface.14
"We got to thinking, what if you could just take the plant out of the system and have only the chemical and physical structure of the soil," Ross Virginia recalled one day as we sat in the third-floor library of the science lab building at McMurdo. "What would structure these nematode communities and how would they work?" This question is part of a larger mission to find out what individual soil species need, what they do in the soil, how they're vulnerable, and—more urgently, given the array of human threats to soil life—whether the loss of species can cause vital ecological processes to falter.
Most often, researchers approach such questions by using chemicals to knock out certain life forms—say, all plants or plant-feeding nematodes or all fungi—from a field plot. Or they resort to small-scale replicas called microcosms or mesocosms filled with sterilized soil to which they add manageable numbers of microbes, soil animal species, and perhaps plants. In the 1980s, Wall was using artificial systems such as these to look at how nematodes influence the movement of carbon through a system. At the suggestion of a colleague, she began to think about finding a real ecosystem that was not only naturally devoid of plants but harbored a limited number of soil animal species as well. Antarctica came to mind.
Wall contacted a colleague who was already working in Antarctica, and he mailed her three bottles of soil scooped from somewhere in Taylor Valley. She was able to extract a few nematodes from the samples, and on that basis, she and Virginia got their first grant to come to the ice.
Wall and Virginia already knew that Antarctica lacks higher plants, the green, rooted kind we're familiar with (except for a handful on the Antarctic Peninsula, which juts north above the Antarctic Circle). Soils here are nearly two-dimensional habitats, with most biological activity limited to the top 4 or 5 inches by the permanently frozen ground below. But before they turned from hot to cold deserts, Wall and Virginia needed to know just how much biological activity was actually taking place in Antarctic soils. Would they find enough of a soil community to make the dry valleys a worthwhile place to study? After all, they were looking for a place to study life as it works on Earth, not a surrogate for Mars.
In their first season in Antarctica and several to follow, Wall and Virginia and their research teams sampled hundreds of sites, wet and dry, in Taylor and several other dry valleys. What they found and what they didn't find were equally surprising. First, they were able to extract nematodes from nearly two-thirds of their samples—firm proof that most of the dry valley soils aren't sterile and that soil food webs exist. The average 2-pound bag of dry valley soil yielded 700 nematodes, and the liveliest soils they sampled yielded 4,ooo.15
Their second finding was that more than a third of their samples contained no nematodes at all—a phenomenon unique on earth.16
"This is probably the only place on earth where you can pick up a handful of soil and not find a nematode in it, then march several steps and pick up another handful and find nematodes," Virginia said. "In almost every other system, they're ubiquitous, and the numbers and the diversity overwhelm you even in trying to characterize one sample."
It's difficult to grasp how ubiquitous and varied nematodes are in the world beyond Antarctica. In a square yard of pasture soil, for instance, you could expect to find 10 million nematodes, along with similarly overwhelming numbers of microbes and myriad other soil organisms.17 Pioneering nematode researcher Nathan Cobb wrote in 1914: "If all the matter in the universe except nematodes were swept away, our world would still be recognizable, . . . its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes."18 In the dry valleys of Antarctica, that thin and patchy film would be composed of only three species of nematodes, all of them unique to this continent: Scottnema lindsayae, Plectus antarcticus, and Eudorylai-mus antarcticus.
Worldwide, some 25,000 nematode species have been named, and more than 10,000 of these live in the soil or seabed or freshwater sediments. But the named species are just a fraction of the world's nematode diversity. Anywhere from an estimated half million to 100 million more nematode species are still awaiting discovery.19 One soil nematode has become a celebrity of sorts: Caenorhabditis elegans is widely used as a "laboratory rat" and became the first multicellular organism to have its full complement of genes sequenced. A team of developmental biologists won a Nobel Prize in 2002 for work that revealed how the genes of C. elegans regulate the development of a single fertilized egg into an adult. Not surprisingly, however, the best-known nematodes are not the ubiquitous, microbe-eating decomposers but the small minority that parasitize us and our livestock and pets—intestinal roundworms, hookworms, and nematodes that cause elephantiasis and African river blindness—or those that cause substantial damage by feeding on crop plants.20 Wall still works on nematode diseases of alfalfa and other crops as well as nematode roles in larger soil processes.
The Wormherders have been returning to the dry valleys for 15 years to learn which of the three worms live where, what conditions each requires, and what makes some of these barren-looking soils better neighborhoods than others.
Scottnema is by far the most abundant nematode and makes its living eating bacteria and yeast out in the dry, salty soils that dominate the valleys. In these arid reaches, the team usually finds Scottnema or nothing. Because of Scottnema's abundance, nematode numbers are three times higher in the dry polygon surfaces than in moist habitats near streams and lakes.21
"Scottnema is king, he's just lovely," Wall said as she opened a greatly enlarged mug shot of the worm—only i/25th of an inch (a millimeter) long—on her laptop one day. "Look at those probolae!" She pointed to wavy tentacle-like extensions encircling the head end. "And ruffles! Imagine if you had ruffles!" The wrinkly cuticle on the beast's "neck" resembles a stack of Elizabethan ruffs—or to be less charitable, the worm equivalent of a triple chin. Scottnema is indisputably a dandy among worms. The other two dry valley nematodes, by comparison, are plain as spaghetti noodles.
Byron Adams likes to ask his students to guess the function of Scottnema's probolae. "You'll get answers like 'oh, they're feelers' or 'chicks dig the guys with the big long wavy things.' The truth is, we don't know."
Plectus, like Scottnema, eats bacteria, but it prefers living in ephemeral streams. Eudorylaimus is rarer than the other two and prefers damp places.22 For years, Eudorylaimus was labeled an omnivore-predator and suspected of feeding on its fellow worms. Then last season, Adams photographed one of these transparent creatures with a gut full of algae, confirming instead that Eudorylaimus is a vegetarian. So far, in 20,000 soil samples examined over the years, the team has yet to see anything preying on a nematode. That's why Wall has dubbed them lions, kings of the food chain on this harsh plain.
As for the rest of the soil community, the Wormherders have found some tardigrades and rotifers in the wetter sites, and New Zealand researchers have found springtails and mites under surface rocks.23 These findings confirm that the soils here, although certainly
not sterile, host the simplest food webs and lowest biological activity of any soils on earth. Biological activity refers to the daily business of breathing, moving, growing, eating, and being eaten that drives the process of rot and renewal that we call nutrient cycling. Microbial decomposition, or rot, for instance, proceeds so slowly that the dry valleys are littered with freeze-dried carcasses of seals like the one Scott's party saw, including some that died hundreds of years ago. The Wormherders themselves see many more carcasses, ones Scott couldn't have seen.
"One of the things that shocked me when we ran the first samples down here was that we'd see so many dead bodies in the soil," Wall had told me. She meant dead nematodes, tiny morsels that are quickly reduced to recyclable carbon, nitrogen, and nutrients in more amenable climates.
What scant biological activity there is in Antarctica drops to nothing when the sun and liquid water of summer disappear. Even in the sum mer, Wall pointed out, anywhere from 30 to 80 percent of the nematodes extracted from a dry valley soil sample will be coiled and dormant in a cryptobiotic state known as "anhydrobiosis"—literally, "life without water."24
Antoni van Leeuwenhoek, who devised the first microscope in the 17 th century, was apparently the first person to witness a rotifer— he called it a "wheeled animalcule"—awakening from this dormant state. In the 18 th and 19th centuries, the phenomenon of anhydro-biosis prompted a debate about whether the creatures were actually experiencing death and resurrection.25 Anhydrobiosis is a drastic but reversible state triggered by dehydration. In the late 1970s, Wall's research showed for the first time that virtually all nematode species in hot deserts could undergo anhydrobiosis. When soil moisture levels drop below 2 percent, water films on soil particles dry up and desert nematodes begin jettisoning 99 percent of their body water. As the worms dry, the rings or annulus of their body draw closer together like a slinky toy recoiling, and they curl into a characteristic Cheerio shape. (Tardigrades collapse into a dried ball known as a "tun," and rotifers morph into tiny mushroom shapes during anhydrobiosis.) Internally, the worms begin producing an antifreeze solution such as trehalose or glycerol, which protects their membranes during desiccation. All detectable metabolic activity and respiration cease. Just add water, however—a dusting of melting snow or thawing of wet, frozen soil—and the worms begin to swell and uncoil; within 24 hours they are wiggling blindly around and turning their sensory powers to the search for food.26
Nematodes have been resurrected from this state from soil left on a shelf for 60 years or more, Wall pointed out, but no one knows the upper limit to such time travel. It confounds our sense of time and lifespan that a relatively brief life cycle—for dry valley nematodes, about 7 months in a warm, moist lab environment—can stretch for decades, perhaps centuries, of golden days interrupted by long, ageless sleeps through hard times. Nor is this talent for time travel unique to Antarctic or desert worms. Wall found nematodes coiled and dormant in agricultural soil, too, which helps explains why farmers cannot count on ridding their fields of plant parasites simply by leaving the fields fallow. Nematodes in the soil of an Iowa cornfield or an Amazonian forest or your garden can enter anhydrobiosis, although some may surrender and go dormant under far less water stress than natives of arid regions. In fact, a large fraction of the life forms in any soil community may be dormant at any given time, waiting for a growing root tip to shove past or a favored bit of detritus to fall into their sphere or environmental conditions to change at the soil-pore level. The business of life underground everywhere varies seasonally and minute to minute.
How did Antarctic nematodes learn to survive not just drying but freeze-drying? Adams envisions them wiggling beneath the feet of dinosaurs in a beech and conifer forest 200 million years ago when the chunk of the earth's crust that is now Antarctica formed the heart of the Gondwana supercontinent and enjoyed a climate more like that of Oregon: "I think these nematodes actually evolved here," he says. "I think that at one time Antarctica was extremely diverse, just like the northern and southern hemispheres. Then it moved down here where it got colder. But I think it got dry first and then cold. Most people down here think, 'isn't it amazing, these nematodes have evolved to live in cold temperatures.' But I think the opposite is true. I think what really happens is that they do what nematodes in deserts in California do. And it turns out that if you're able to dehydrate yourself in order to survive in a desert, you don't care what the temperature is. It's a key innovation that allows you to survive more than one type of extreme."
Adams and Wall are building a collection of DNA from nematode populations across Antarctica, from sites with different geologic histories and soils and varying degrees of isolation. The two hope to track the evolution of genes that affect the creatures' survival, their responses to the environment, and their contributions to ecological processes. With any luck, they may even come across ancient carcasses of nematodes or long-dormant worms locked in permafrost or glacial formations.
"It's like looking for dinosaur DNA," Wall says. From microscopic dinosaurs.
The two have been asking geologists bound for sites deep on the continent to bring back bags of soil from exposed patches of earth. Their prize acquisition so far has been a single Scottnema pulled from a sample from the Beardmore Glacier, which flows from the Transantarctics onto the Ross Ice Shelf at 83° south latitude. The genetic work done so far in Wall's and Adams' labs, however, has shown that Scottnema is essentially the same beast throughout the continent.27
"I was a little bit disappointed and also a little astonished, given the distances between the sites, to see that they were virtually identical genetically," Adams tells me one day out in the field. "How could this be? And the best I can come up with is they're either incredibly slowly evolving or there's this rampant dispersal." Dispersal of individuals would keep genes flowing between populations and overcome the genetic isolation that often allows new species to evolve.
A few days after that first field outing to F6, we are working in the worm farms at the south end of Lake Hoare. There are six of us this time, the original three plus Emma Broos and Johnson Nkem from Colorado State and Jeb Barrett from Dartmouth. Adams and Barrett—a veteran of multiple seasons on the ice—have been showing Broos and Nkem where to sample in one of the plots. It is another clear, brilliantly sunny day, but a brisk wind sweeps down the valley, chilling our bare hands as we scoop soil into bottles and bags.
The Wormherders have long believed that these winds blow dormant nematodes around the valleys like freeze-dried Cheerios. That's the reason for the dust traps at each field site. But Adams tells us he thinks wind dispersal of nematodes could occur on a much larger scale.
"The circumpolar winds could act just like a big toilet bowl, swirling them around the continent," he says, clearly peeved by the prospect because it bodes relatively uniform genetics.
Barrett sees it in another light: "It may not be an interesting result for an evolutionary biologist if they're all genetically identical, but for an ecologist it's great. It shows this is one tough little worm."
Tough because the same genes seem to equip Scottnema to live across a wide range of habitats.
"Isn't it an amazing beast," Wall says, passing by with two carboys of water. It is not a question.
Tough and amazing they may be, but how will nematodes the world over respond to shifts in their environment that are likely to accompany human-driven changes in atmosphere and climate? Since their first season on the ice, Wall and Virginia have been manipulating temperature, water, and carbon—the essential food stock of earthly life— in their worm farm plots in a search for answers. The cone-shaped chambers, for example, which now number more than 100 at three sites, act like miniature greenhouses, raising the temperature of the soil inside by 1-2° F. The researchers quickly learned that even this minor warming, far from being a boon to the worms, simply dries out the soil and knocks back nematode numbers.
Emma Broos and I follow Wall to one of the long-term plots and measure out portions of water or solutions of carbon-rich sucrose or mannitol—a sugar alcohol that worms would get eating cyanobacte-ria. Wall and Barrett then move along the rows sprinkling water or one of the sugar solutions on the gravelly soil of some sites, with or without chambers, and leaving others as is. After nearly a decade of these annual boosters, the water has done surprisingly little to benefit nematode populations. The sugar amendments, on the other hand, have indirectly boosted nematode numbers, except inside the chambers where increased drying caused by warmer temperatures cancelled the benefit. It took 8 years, but the sugar has supplied enough carbon to fuel a microbial population explosion that provides a feast for the worms.
In the green regions of the earth, soils are usually chock-full of carbon. Some of it is in highly edible forms that cycle quickly through the tissues of plants to the bodies of grazing animals to the decomposer microbes and out through the soil food web of consumers and predators until it eventually returns to the atmosphere. Most of the organic carbon in soils, however, is locked up long term in recalcitrant forms such as lignin and cellulose and the cakey, decay-resistant black humus that gardeners and farmers recognize as the sign of a rich and fertile soil. The capacity of soils to store carbon is particularly important at a time when human societies are pumping unprecedented amounts of carbon into the atmosphere from the burning of fossil fuels, enough carbon to alter the global climate. The soils of the earth harbor more than three times as much carbon as the atmosphere, and four times as much as the bodies of all living plants and animals.28
Little of that carbon, however, is here in the soils of the dry valleys. In a place with no trees, shrubs, or grasses, the task of assimilating carbon from the air through photosynthesis falls to microscopic green algae and chlorophyll-containing cyanobacteria. Consequently, these soils not only host the lowest biological activity but also the lowest organic carbon reserves of any soil on earth. Carbon, in fact, appears to be more important than moisture in defining good neighborhoods for nematodes and other soil life in the dry valleys.29 Organic carbon stocks are richest in the centers of soil polygons where the most abundant Scottnema populations are found.30
One of the first questions Virginia and his students asked was, where did the meager carbon stocks in these soils come from? The long-standing assumption had been that the soils were passive beneficiaries of organic carbon blown out on the wind from eroding cryp-toendolithic communities—the cyanobacteria living inside rock pores—and algal mats rimming today's streambeds and lakeshores. But it turned out to be a much older bounty. By reading the isotopic signatures of the carbon in valley soils, the researchers discovered that most of it is "legacy carbon" left over from the Pleistocene epoch (2 million-10,000 years ago) when a glacial lake known as Lake Washburn inundated Taylor Valley to a depth of 1,000 feet. Carbon had been pulled from the air by lush algal mats that rimmed the shorelines and carpeted the bottom of that ancient lake, and the carbon-rich mats got left behind as lake water receded.31
The realization that most of the carbon is an ancient legacy prompted another question: How much carbon are today's soil organisms using? All living things, from plants to nematodes to humans,
"burn" carbohydrates and other carbon-based molecules to extract energy to fuel their life processes. This metabolic activity generates water and carbon dioxide as wastes, and organisms respire or exhale the CO back to the air.
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