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Life under ice

Lake Vostok may harbor ingredients for a complex subglacial ecosystem

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Even by Antarctic standards, the Lake Vostok research station is inhospitable. The outpost at the heart of the frozen continent holds the record for the lowest naturally occurring temperature ever observed on Earth. Scientists commonly describe the place as punishing, unforgiving, the most desolate place on the planet.

That’s nothing. Nearly 4,000 meters below the station, beneath the crushing East Antarctic ice sheet, sits an enormous body of water. Lake Vostok has existed for millennia in dark, frigid isolation, presumably harboring nothing but the toughest microbes.

But now a Russian team has drilled through the ice, breaching Lake Vostok for the first time in 15 million years. Bits of genetic material in ice core samples of frozen lake water include DNA not just from microbes, an analysis published in July contends, but hints of much more complex life: a water flea, a mollusk, maybe something related to a sea anemone. Even more intriguing, some genetic sequences appear similar to bacteria and parasites typically found living inside fish, lobsters and shrimp.

Many researchers find the claim hard to believe. Lake Vostok just seems too deep, too cold, too dark for complex life. Maybe the researchers who discovered the DNA accidentally contaminated their samples with genetic material from the surface, some microbiologists suggest. Or maybe the researchers are just pushing the genetic data too far, seeing similarities to complex life in what are really just scraps of bacterial DNA.

But unlike other recent claims for the existence of life in otherworldly environments, most notably the now-debunked discovery of arsenic-based life in California’s Mono Lake (SN: 2/25/12, p. 10), the case for complex life in Lake Vostok doesn’t require rewriting basic rules of biochemistry. It does require creating an intricate ecosystem in the absence of sunlight and photosynthesis, but there is nothing to rule out the possibility that a strange and otherworldly collection of creatures is waiting to be discovered deep beneath one of the most remote places on Earth.

Antarctica’s Great Lake

Lake Vostok is a behemoth, and not just by Antarctic standards. It is nearly as big as Lake Ontario and much deeper — estimated to be more than 900 meters in some places. By volume, Vostok is the seventh largest lake in the world. While there were clues that a large body of water lay beneath Vostok station, the lake’s existence wasn’t confirmed until the mid-1990s. Almost as soon as its existence was confirmed, murmurs began that the buried lake might harbor life (SN: 10/2/99, p. 216).

At that time, a team of Russian, French and American scientists had already begun drilling through the ice sheet. But drilling stopped in 1999, about 130 meters above the lake. The research community was worried that the kerosene-freon mix used in drilling might contaminate Vostok’s untouched waters. More than five years later, a plan had been approved to protect the lake and drilling resumed. After several seasons and several fits and starts, the Russians penetrated the lake in February 2012.

Russian members of the team now have samples of lake water that gushed up into the borehole, and they expect to publish an analysis of it this fall. But Scott Rogers, an expert in ancient DNA at Bowling Green State University in Ohio, got his hands on some of the American share of the core, which is stored at the National Ice Core Laboratory in Denver. More than 3,000 meters long, the core is mostly glacier ice. But near its bottom, the core contains lake ice that has frozen to the bottom of the overlying glacier as it crawls across eastern Antarctica.

Rogers and colleagues, including his student Yury Shtarkman, took more than two years to analyze their samples of this “accretion ice.” As expected, they found genetic material indicating bacterial life (SN: 3/9/13, p. 12). The team had samples from two parts of the lake: In one, about 77 percent of the identifiable genetic sequences were from bacteria, with most of the rest from fungi. In the other, about 95 percent of what the researchers could identify was bacterial.

And then there were the standouts: The researchers detected DNA similar to that of small water animals called tardigrades, or water bears, which are known for withstanding extreme environments. Some genetic material was nearly identical to the little swimmers called rotifers. Other sequences suggested a whole mess of algae (yellow-green, green, red). The real surprise was sequences indicating larger organisms like clams and jellyfish. Strangest of all were genetic signatures resembling parasites or symbiotic partners of large aquatic organisms: a rainbow trout intestinal bacterium, a sponge symbiont, a lobster gut bacterium.

Finding those DNA sequences doesn’t mean that those exact organisms are swimming around kilometers under the Antarctic ice. But the mix of genetic signatures is suggestive of a functioning rudimentary ecosystem: There are what ecologists call primary producers — bacteria and other critters that extract nitrogen and carbon from the environment and make it usable for other organisms. There are also consumers — various amoebae, zooplankton and perhaps others living off them. And completing the loop are decomposers, including fungi, that make a living breaking dead things down. Many of the DNA sequences are similar to those of organisms with a high tolerance for extremes, be it cold, salt or pressure.

In the July report, published in PLOS ONE, Rogers and his colleagues present a Vostok scenario that goes something like this: About 35 million years ago, much of Antarctica was green and free of ice. Sea levels were higher, so Lake Vostok may have been connected to the Southern Ocean. It could have been a saltwater bay that had reverted to brackish. Bring in a glacier, and the top of the lake becomes freshwater, says Rogers. This scenario might lead to a variety of distinct zones within the lake. At different depths and locations there may be vastly different temperatures, salinities and other chemical concentrations. “You could have organisms surviving there and possibly evolving into different niches in the lake,” he says.

Hard to fathom

Even so, some scientists cannot wrap their minds around the idea of a thriving Vostokian ecosystem.

“Here’s probably the most extreme ecosystem on the planet and it’s teeming with life? And we can go to other less extreme environments and these fauna are absent?” says microbiologist Brent Christner of Louisiana State University in Baton Rouge.

It’s not just the numbing cold of Lake Vostok, which at its surface hovers around 0° Celsius. Nor is it the crushing pressure exerted on a lake beneath almost 4,000 meters of ice. The problem is energy, without which life can’t exist. Lake Vostok sits in permanent darkness, far beyond the reach of any photons.

“If you put all these organisms in a lake, they have to eat something. What do they eat?” Christner says. “I’m not saying it’s impossible that more than a bacterium lives in these lakes. Maybe you have a nematode, but that would be an apex predator.”

Earlier this year, Christner and his colleagues reported finding cells containing DNA in Lake Whillans, which lies beneath a mere 800 meters of ice in West Antarctica (SN: 3/9/13, p. 12). But unlike Lake Whillans and many of the continent’s other subglacial lakes, there aren’t known streams and rivers feeding into Vostok that might bring life or nutrients from afar.

“On the extreme-o-meter, Vostok is an order of magnitude more extreme than these other lakes,” says Christner. “It’s a lot to ask of a multicellular organism to be living like that.”

But if Lake Vostok does harbor an energy source, then the existence of multicellular life seems much less far-fetched.

Data from ice-penetrating radar and laser altimetry suggest that Lake Vostok sits on a major geological boundary. “Recent minor tectonic activity could have the potential to introduce small, but significant amounts of thermal energy into the lake,” a team of geoscientists wrote in 2003 in Earth and Planetary Science Letters. In 2006, a research team that included microbial geneticist Sergey Bulat of the Petersburg Nuclear Physics Institute in Russia, one of the more vocal critics of the current study, reported DNA from Vostok ice cores closely resembling that of microbes thriving in the high temperatures and strange chemistry of volcanic vents and hot springs.

For extremophile microbes like those, Lake Vostok might even seem cushy. After all, there are bacteria that live in the Earth’s upper atmosphere, a freezing place with little oxygen that’s bombarded with UV radiation. Other microbes, like Picrophilus torridus, live in highly acidic volcanic vents that spit out scalding-hot gases. And there are fish that survive just fine in toxic, hydrogen sulfide–laden waters inside permanently dark caves.

If those fish can do it, perhaps others can in an Antarctic lake, says Jack Gilbert, an environmental microbiologist at the University of Chicago and the Argonne National Laboratory in Lemont, Ill. “Complex eukaryotic life is unlikely but not impossible. Perhaps maybe even fish,” he says.

Rogers thinks the tectonic activity and other data suggest a hydrothermal vent, probably near a sediment-rich embayment in the southwestern portion of the lake. The most colorful mix of organisms in his analysis came from that area. “I’d really like to see what’s on the southwestern side,” he says. “I think that’s where all the biological action is.”

On the seafloor such vents often support a motley crew of organisms. A 2010 expedition found a hot-water vent near Antarctica crawling with a recently discovered crab species and other creatures, including barnacles, anemones, sea spiders, snails and a predatory starfish (SN: 1/28/12, p. 5). It’s hard to imagine that similar marine creatures could survive 15 million years sealed off from the ocean and the atmosphere, says Alex Rogers, a deep-sea ecologist at the University of Oxford in England who led the Antarctic expedition (and is unrelated to Scott Rogers). “If there’s anything associated with the lake, I would expect it to be microbial.”

Bulat expresses his skepticism more directly: “It is completely contamination,” he says.

When dealing with fragile, ancient DNA, Scott Rogers acknowledges, some contaminating genetic sequences are to be expected. But as a molecular biologist who has been investigating mummified and hard-to-find DNA for more than three decades, he has learned to take every precaution. His team compared the genetic sequences from their Vostok samples with species on a list of organisms found in the drilling fluids used to extract the ice core, for example. There wasn’t any crossover between the two. The researchers also tested water in their lab and discarded all genetic sequences from the ice that matched anything in their lab water. His lab spent four years, he says, ironing out the best way to extract samples from ice cores, including creating cores in the lab with small amounts of known DNA inside them, coating the cores with contaminating genetic material and then developing a treatment to eliminate the unwanted material (a process that includes washing with Clorox bleach seems to do the trick).

Gene trees

There’s been some additional skepticism from scientists concerned about linking the genetic sequences reported in the paper to familiar organisms. Much of the genetic material pointing to multicellular life was just short little bits, 200 to a few thousand letters of genetic code long. The researchers reported the percent similarity of their samples to genetic sequences that have been deposited in public databases. But depending on the stretch of DNA examined, percent similarity can be a lousy way to determine what creature the DNA came from; a very short string of letters from, say, a fly and a human might be very similar if there has been little change in that region of the genome since the two species diverged from their last common ancestor.

In other words, says Jonathan Eisen, an expert on the evolution of microbes and genomes at the University of California, Davis, “you want to make sure you’re not looking at weird, spurious crap.” For a good, quick estimate of relatedness, Eisen says, use sequences of DNA letters to build a family tree that roughly approximates the one evolution has created in the real world. If purported insect DNA, for example, fits best on an insect branch of the tree of life, it’s a much stronger case that the DNA truly came from an insect.

Eisen has done just that. While initially skeptical about the claims of genetic similarity, his analysis, performed at the request of Science News, finds that many of the purported bits of animal DNA actually do nestle in the appropriate animal branches of reconstructed genetic trees.

“The trees look good,” Eisen says. “I’m not finding anything that I would say is weird.”

That doesn’t mean there are lobsters in Lake Vostok. The genetic sequences that are similar to those of known microbes and parasites associated with larger organisms could indeed be relatives, but ones that abandoned the parasitic lifestyle long ago. Such evolutionary reversals are not uncommon, and can’t be discerned from genes alone. And Eisen’s analysis says nothing about the source of the DNA; it could be contamination.

Bulat’s team is now analyzing samples from the 2012 expedition that captured liquid water from Lake Vostok. It isn’t clear what those samples will reveal, especially since they come from the uppermost zone of the lake at a location far from the region that is suspected of harboring volcanic vents. There aren’t any plans to drill into that portion of Lake Vostok anytime soon. In the absence of sending video cameras or similar devices down into the water, the possibility of contamination is likely to remain an issue for any samples that are collected.

“The half-empty side of me says contamination,” says sea vent specialist Alex Rogers. “But who knows? The half-full side of me says, wow, I really hope there is perhaps diverse life. I would be prepared to be delighted; it would provide an environment for all kinds of scientific discoveries.”

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