The Dark and Mushy Side of A Frozen Continent
06 Jul 2007
BIG SKY, MONTANA—Wetlands might seem incongruous in Antarctica’s frozen wastes. But recent expeditions have uncovered a hidden landscape of lakes, marshes, and apparent rivers sandwiched between ice and rock. These vast wetlands, imprisoned under the ice, may even be teeming with life.
“There’s water everywhere under there,” says John Priscu, a microbiologist at Montana State University in Bozeman. At a meeting* here last month, Priscu and other experts compared notes on the latest tantalizing clues to what this unparalleled and largely unplumbed world might be like–and laid plans for exploring it.
The first big plunge is likely to occur in Lake Vostok, the largest of Antarctica’s 150-and-counting hidden lakes. A Russian-led team is preparing to penetrate and sample Vostok in 2009. The operation may help settle a point of sharp scientific dispute: whether the Connecticut-sized lake, overlain by more than 3.5 kilometers of ice, harbors microbial life. “We never thought life could exist down there,” Priscu says. Now he’s a believer. Other researchers are skeptics.
But experts concur that there’s far more to Antarctica than meets the eye. “We’re seeing a wide range of subglacial environments, from Lake Vostok to shallow, swampy environments,” says Peter Doran, an earth scientist at the University of Illinois at Chicago. For now, the startling wetlands are terra incognita. Robin Bell, a geophysicist at Columbia University, says, “we’ve got a long way to go” before comprehending what’s going on under the ice.
Peeking under the cover
The revelations about Antarctica’s soggy, pitch-black underbelly have come mainly from drilling campaigns and radar mapping over the past decade. Drills that have bottomed out below the ice sheet have often hit water or warm, soft ice.
The ice blanketing the continent traps heat radiating up from Earth’s core. That warmth, combined with intense pressure from the ice bearing down, allows water pockets under the sheet to keep their liquid form at normally freezing temperatures. All told, Antarctica’s subglacial lakes contain around 10,000 cubic kilometers of water–about 10% of the fresh water in all the lakes elsewhere on Earth.
Antarctica’s frigid water world is more dynamic than expected. Two recent studies found that some smaller subglacial lakes can roam around–they burst their banks and fill lower-elevation depressions. These findings hint at the existence of transient rivers, some as large, perhaps, as England’s Thames–and raise the stakes on attempts to tap into the lakes. “We have to take a watershed approach,” Doran says. If pollutants infiltrate a watershed, he says, “we may be contaminating things all the way downstream.”
Although no subglacial lake has yet been pricked, researchers have drilled to within about 90 meters of Vostok’s surface. Ice from this nether region is illuminating. When drilled down into from about 240 meters above the lake, the core changes from glacial ice, composed of compacted snow, to accretion ice, formed when Vostok water freezes to the ice sheet. Researchers have reported that accretion ice contains microbes that could be revived in the lab. Many scientists infer that these microbes were Vostok denizens, and other studies have shown that the microbes are close relatives of those found from Greenland to the Himalayas.
There are other signs of vitality as well. The sole sediment core under the ice sheet tested so far for microbes is brimming with life. In 2004, Brian Lanoil of the University of California, Riverside, and colleagues found that sodden soil under the Kamb Ice Stream in West Antarctica contained 10 million cells per gram–comparable to that of lake sediments found in temperate regions, and similar to sediments found under glaciers in New Zealand and Norway.
Glacial ice from the Vostok core is studded with modest numbers of microbes, around 100 cells per milliliter, according to studies led by Priscu and Brent Christner, a microbiologist at Louisiana State University in Baton Rouge. At the glacial-accretion ice transition, they reported last year in Limnology and Oceanography, the number rises to around 400 cells per milliliter. Accretion ice is also rich in organic carbon, Christner says. “This suggests that the lake is a source of both cells and organic carbon.”
Other researchers think that the ice–and perhaps Vostok’s waters–is largely sterile. Sergey Bulat, a molecular biologist at the Petersburg Nuclear Physics Institute (PNPI) in Russia, and his colleagues have also been probing the Vostok core for microbes and DNA. At the meeting, Bulat reported that his team often finds no cells in samples from both glacial and accretion ice, and never more than 20 cells per milliliter. (Bulat does put stock in one sign of life: His group has found that accretion ice contains DNA of bacteria similar to thermophilic species in vents on the ocean floor. Such microbes, he says, could be clinging to rocks around Vostok Lake and in lake sediments.)
The discrepancy between the Russian and U.S. cell counts could be due to different sampling techniques, says microbial ecologist Warwick Vincent of Laval University in Quebec, Canada. Whereas Bulat’s team uses flow cytometry, Priscu and Christner count cells under a light microscope or scanning electron microscope. Or, says Vincent, “it could be that there’s a lot of heterogeneity in the ice core.” Others argue that Priscu and colleagues have been led astray by an artifact. To keep the Vostok borehole from freezing shut, it’s filled with drilling fluid. The hydrocarbons are a feast for bacteria. Says Christner: “We can think of the borehole as a 65-ton enrichment culture.”
Irina Alekhina and her colleagues at the PNPI found that some microbes in the drilling fluid match species that Christner and others have found inside cores from Vostok and from the Taylor Glacier in Antarctica–microbes that they argued were native to the ice. The primary bacteria in the drilling fluid were Sphingomonas species, known contaminants of jet fuel–like the drilling fluid, mostly kerosene. “There is no indication for indigenous microbes,” Alekhina concludes.
Priscu rebuts this by pointing to a study in Antarctica’s McMurdo Dry Valleys in which his group found hydrocarbon-eating microbes. “The organisms are there in nature,” Priscu says. “Just because we see it in the drilling fluid doesn’t mean it’s not native.”
That debate notwithstanding, it’s a mystery how microbes can survive deep in the Vostok core, which near the bottom could be 1 million to 2 million years old. If the cells had remained frozen all that time, “their genomes would accumulate enough damage that they would effectively be dead,” Christner says. One microbial refuge might be the water channels between the ice crystals, says Buford Price, a physicist at the University of California, Berkeley. Christner and biophysicist James Raymond of the University of Nevada, Las Vegas, are testing whether the microbes are specially adapted to the cold life. Raymond found that one Chryseobacteriumspecies from the Vostok core produces a protein that, in the lab, blocks icecrystal growth. This suggests the bacteria are reshaping the ice around them to minimize damage, says Christner. The protein might work as antifreeze or as a seed for crystal formation to form an ice cocoon around the bacteria.
“This debate will not be resolved until Lake Vostok is sampled directly,” says Vincent. When Russia breaks through, it will be like exploring a different planet. The drilling that has preceded this adventure has been “like putting pinholes in the continent,” Priscu says. “We don’t know what’s on the bottom of that ice sheet.” Well, we do know one thing: It’s wet.