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Antartide: Le sorprese del lago Mercer

Antartide: Le sorprese del lago Mercer Posted on 10 Febbraio 2019


EXCLUSIVE: Tiny animal carcasses found in buried Antarctic lake

The surprise discovery of ancient crustaceans and a tardigrade emerged from a rare mission to drill into a lake sealed off by a kilometre of ice.
The borehole and hot water drill

Researchers used a hot-water drill to bore through a kilometre of ice, creating a portal with a diameter of just 60 centimetres.Credit: Billy Collins/SALSA Science Team

Scientists drilling into a buried Antarctic lake 600 kilometres from the South Pole have found surprising signs of ancient life: the carcasses of tiny animals preserved under a kilometre of ice.

The crustaceans and a tardigrade, or ‘water bear’ — all smaller than poppy seeds — were found in Subglacial Lake Mercer, a body of water that had lain undisturbed for thousands of years. Until now, humans had seen the lake only indirectly, through ice-penetrating radar and other remote-sensing techniques1. But that changed on 26 December when researchers funded by the US National Science Foundation (NSF) succeeded in melting a narrow portal through the ice to the water below.

Discovering the animals there was “fully unexpected”, says David Harwood, a micro-palaeontologist at the University of Nebraska-Lincoln who is part of the expedition — known as SALSA (Subglacial Antarctic Lakes Scientific Access).

The intrigue deepened when biologists realized that at least some of the beasts from Lake Mercer were landlubbers. The eight-legged tardigrade resembles species known to inhabit damp soils. What looked like worms were actually the tendrils of a land-dwelling plant or fungus. And although the scientists couldn’t rule out the possibility that the crustaceans had been ocean denizens, they might just as easily have come from small, ice-covered lakes.

The researchers now think that the creatures inhabited ponds and streams in the Transantarctic Mountains, roughly 50 kilometres from Lake Mercer, during brief warm periods in which the glaciers receded — either in the past 10,000 years, or 120,000 years ago. Later, as the climate cooled, ice smothered these oases of animal life. How the crustaceans and tardigrade reached Lake Mercer is still a matter of debate. Answers could come as the SALSA team tries to determine the age of the material using carbon dating and attempts to sequence the creatures’ DNA. Piecing together that history could reveal more about when, and how far, Antarctica’s glaciers retreated millennia ago.

“This is really cool,” says Slawek Tulaczyk, a glaciologist at the University of California, Santa Cruz, who is not part of the SALSA team. “It’s definitely surprising.” Tulaczyk, who has studied sediments retrieved from beneath glacial ice since the 1990s, says that nothing like that has ever been found before under the ice sheet. He was a co-leader of the only previous expedition to drill into a subglacial Antarctic lake — in 2013 at Lake Whillans, 50 kilometres from Lake Mercer2. Scientists found Lake Whillans brimming with microbes, but saw no signs of higher life.

Sources: SALSA; M. R. Siegfried & H. A. Fricker Ann. Glaciol. 59, 42–55 (2017).

In the case of Lake Mercer, Tulaczyk says, rivers under the ice could have washed the animal carcasses and fungi from the mountains down to the lake. Or the creatures might have frozen onto the bottom of a glacier that dragged them out of the mountains as it advanced. In other words, the key to understanding a long-ago period of the Transantarctic Mountains’ history could be buried at the bottom of a lake 50 kilometres away.

Frozen in time

The saga began on 30 December, as SALSA scientists hoisted up an instrument that had measured the water temperature, and scraped grey-brown lake mud off it.

When Harwood slid the mud under a microscope, he found what he was hoping for: the shells of diatoms, photosynthetic algae that lived and died millions of years ago, when Antarctica was warmer and an ice-free ocean covered the area that is now Lake Mercer. But he spotted something out of place among the glassy diatom shards: the shell of a shrimp-like crustacean with legs still attached. Its carapace was speckled and discoloured “like an old leaf that’s been sitting on the ground for a season”, Harwood says.

The palaeontologist soon found another fragment of a crustacean’s carapace, this one a healthy amber hue, and still bristling with delicate hairs. ”It looked really fresh,” he says. “Like something that had been living.” The idea that live animals might be flitting around in this dark pocket of water, sealed off from the outside world, seemed at once reasonable and outlandish.

Samples of the lake’s water contained enough oxygen to support aquatic animals, and were full of bacteria — at least 10,000 cells per millilitre. Harwood wondered whether small animals, originally from the ocean, might survive there by grazing on the bacteria.

The gravity core returns with sediment

An instrument brings lake mud up to the surface.Credit: Kathy Kasic/SALSA Science Team

There were other reasons to suspect that sea animals could have got into Lake Mercer. Five thousand to ten thousand years ago, the ice sheet briefly thinned, allowing seawater to intrude under floating ice that was hundreds of metres thick and reach what is now Lake Mercer3. Any animals that came in with the ocean could have been trapped in pockets of water when the ice sheet thickened and once more came to rest like a lid on the sea floor.

Ebb and flow

Scientists know that something similar, although less extreme, has happened in other parts of Antarctica. The gradual uplift of the continent transformed shallow ocean bays into isolated lakes. And tiny marine copepod crustaceans seem to have survived for thousands of years in some of those lakes4, likely spending long periods trapped under several metres of permanent ice5.

Those ice-covered lakes still receive dim sunlight — giving crustaceans algae to eat. But subglacial lakes such as Mercer are tougher environments. No sunlight penetrates their frozen caps, so bacteria survive by gnawing on minerals and organic matter from microscopic plankton and diatoms that lived millions of years before, when the lake bed was part of an open ocean6. Most biologists do not believe that bacteria in these lakes can grow quickly enough to supply food for even the smallest aquatic animals.

John Priscu, a lake ecologist at Montana State University in Bozeman and the SALSA project’s leader, was cautious but excited when Nature spoke to him by satellite phone on 3 January, several days after the animal carcasses were found. He worried that the critter bits his team had found in the lake might simply be contamination carried in by dirty equipment. “I’m pretty cautious about making claims,” he said — while allowing that discovering animals alive in Lake Mercer “would be a real wow moment” if it happened.

To rule out contamination, his team re-cleaned its gear and retrieved more mud. Harwood continued to find crustacean shells and organisms that vaguely resembled worms when he peered at the new mud with his microscope. But neither he nor anyone else at the SALSA camp specialized in studying animals. A more reliable interpretation would have to wait until other scientists saw the samples.

Postcards from the past

That happened on 8 January at McMurdo Station, an NSF base 900 kilometres northwest of Lake Mercer on the Antarctic coast. When an animal ecologist named Byron Adams trained his microscope on a bit of the mud retrieved from Lake Mercer, he quickly spotted some familiar organisms. A new understanding of their importance began to emerge.

Adams, a researcher at Brigham Young University in Provo, Utah, who is not part of the SALSA team, recognized the worm-like objects as thread-like plants or fungi. He had seen those, along with the crustaceans and tardigrade, both alive and dead, in a region of Antarctica called the Dry Valleys that is free of glaciers. He had also seen some of these creatures in the Transantarctic Mountains7, which cut through the continent’s interior.

Adams was all but certain that the organisms had been dead for millennia by the time the scientists scooped them out of the lake. He believed that they had once lived in the Transantarctic Mountains and were transported down to Lake Mercer sometime after they died, anywhere from thousands to tens of thousands of years ago. The remains are young compared to those of other ancient organisms found in the lake, such as the diatoms, which are thought to have lived millions of years ago.

“What was sort of stunning about the stuff from Lake Mercer is it’s not super, super-old,” he says. “They’ve not been dead that long.” And such well-preserved carcasses provide an interesting opportunity. By determining how long ago these organisms lived, and what kind of environment they required, biologists can learn something about Antarctica’s past succession of warm and cold spells.

The job of reconstructing the continent’s history often falls to Earth scientists, but not so in this case. “I think it’s really cool when biology starts to weigh in on that question,” says Adams. “Here’s the story that the biology is telling us — why don’t you glaciologists go back and rethink your models.”

The SALSA scientists finished their work at Lake Mercer and sealed off the borehole on 5 January. As they head home with their samples, the project is entering a slower, more methodical phase. In the coming months, the team will attempt to establish the age of the animal remains using radiocarbon dating; this would reveal if they are younger than about 40,000 years.

The scientists will also try to sequence scraps of DNA from the carcasses, the mud and the lake water in the hopes of finding out whether the crustaceans belong to marine or freshwater species. Chemical analyses of carbon in the carcasses could also confirm whether the animals lived in a sunlit ecosystem, fed by photosynthetic algae — or whether some might have survived for a time in a dark, subglacial environment.

Adams, for one, hasn’t completely let go of the possibility that some animal used to live under the ice, or even still does. As he looked through the mud from Lake Mercer, he was “hoping to see something alive — that’s what I wanted to see”. But it was a tiny sample, barely a teaspoon’s worth. If Adams could have examined more of the muck, he says, “it’s possible that you could still find things that are alive”.

Nature 565, 405-406 (2019)

doi: 10.1038/d41586-019-00106-z


di Douglas Fox


The hunt for life below Antarctic ice
In the next few weeks, researchers in Antarctica will drill through 1,100 metres of ice into a lake that has remained sealed for millennia. Here’s what they hope to find.

At a remote camp just 600 kilometres from the South Pole, the race is on to melt 28,000 kilograms of snow. Within the next two weeks, a team of technicians will use that hot water to melt a hole through 1,100 metres of ice, straight down to the bottom of the Antarctic ice sheet. Their quarry is a hidden lake that has been cut off from the rest of the world for thousands of years. The life they expect to find there inhabits one of the most isolated ecosystems on Earth.

The pool of water, known as Subglacial Lake Mercer, covers 160 square kilometres — twice the size of Manhattan — and might be 10–15 metres deep. Despite temperatures that are likely to stay below 0 °C, the lake doesn’t freeze, because of the intense pressure from the ice above. Researchers discovered its ghostly silhouette a little more than a decade ago through satellite observations, but no human has directly observed the lake.

The drillers hope to tap into Mercer sometime around Christmas. Then, a team of researchers from more than a dozen universities will hoist samples of water and mud from its interior. The scientists will also send a skinny, remote-operated vehicle down through the 60-centimetre-wide hole to explore the dark waters with video cameras and grab samples with a claw.

Antarctica conceals more than 400 lakes beneath its ice, and Mercer will be the second that humans have sampled directly. It marks the first time scientists will use a remote vehicle to roam beneath the ice sheet.

Some of the same researchers drilled into a nearby, smaller subglacial pool called Lake Whillans in 2013, and found it teeming with microbes — many more than they had expected in a place cut off from the Sun’s energy. This time, they wonder whether the cameras of the submersible might even spy animals in the black waters. “We don’t know what’s going to be there,” says John Priscu, a lake ecologist at Montana State University in Bozeman and leader of the project. “That’s what makes it so much fun.”

The team retrieves the first terrestrial samples from Subglacial Lake Whillans.

Researchers pull up sediments from the bottom of Lake Whillans during a drilling project in 2013.Credit: J.T. Thomas

Known as SALSA (Subglacial Antarctic Lakes Scientific Access), the expedition is being funded by the US National Science Foundation at a cost of nearly US$4 million. Its aim is to explore the ice-shrouded environment of perpetually sunless rivers, lakes and wetlands that exists in Earth’s polar regions and covers an area as big as the United States and Australia combined. This ecosystem is much more isolated than even the deepest ocean trenches.

The subglacial biosphere provides an analogue for habitats deep inside Mars or on the ice-covered moons of Jupiter and Saturn. The scientists leading the project hope that the Lake Mercer ecosystem will shed light on what kind of life can survive in such remote environments.

And the sediments they pull up from the bottom of Lake Mercer could provide clues about how susceptible the ice sheet covering West Antarctica will be to global warming. Cores drilled from the bottom of the Ross Sea nearby suggest that this ice sheet has collapsed dozens of times over the past 6 million years. Lake Mercer is about 800 kilometres inland of those sites in the Ross Sea and could yield important clues about the ice sheet’s periodic advances and retreats during previous cold and warm spells, says David Harwood, a bio-stratigrapher at the University of Nebraska–Lincoln who will be present for the drilling. “It would be great to get a record of past changes in the West Antarctic Ice Sheet from that particular location.”

Hidden habitat
Mercer sits within a constellation of nine lakes in West Antarctica that was first discovered in 2006, when satellite altimeter measurements revealed that the ice surface in certain places was periodically rising and falling by up to 10 metres over periods of months1. Helen Fricker, a glaciologist at the Scripps Institution of Oceanography in La Jolla, California, realized that they were subglacial lakes filling and emptying, causing the ice overhead to lift and then drop (see ‘Land of invisible lakes’).

Sources: Tristy Vick-Majors/SALSA; M. R. Siegfried & H. A. Fricker Ann. Glaciol. 59, 42–55 (2017).

Evidence pulled up from the drilling project at Lake Whillans has spawned a series of discoveries that have shaped the current programme at Lake Mercer, 40 kilometres to the southeast. The water from Lake Whillans teemed with 130,000 microbial cells per millilitre — a population 10–100 times bigger than some researchers expected. Many of the microorganisms obtained their energy by oxidizing ammonium or methane, probably from deposits at the bottom of the lake3,4. That was a key insight, because it suggested that this ecosystem — seemingly cut off from the Sun and photosynthesis as an energy source — was still dependent on the outside world in an indirect way.

The researchers who studied Lake Whillans suspect that the ammonium and methane seep up from the lake’s muddy floor from the rotting corpses of marine organisms that accumulated during warm periods, millions of years ago, when this region was covered by ocean rather than ice. Evidence of this food source came from Reed Scherer, a micropalaeontologist at Northern Illinois University in DeKalb, who was part of the Whillans project. He found the shells of diatoms (single-celled algae) and the skeletal fragments of sea sponges littered throughout the lake’s mud. “There is a marine-resource legacy that the microbes are still tapping into,” he says.

When they drilled into Lake Whillans, researchers thought it had been covered by ice for at least 120,000 years, or possibly up to 400,000 years, coinciding with the last time the West Antarctic Ice Sheet was thought to have melted so dramatically that the lake area had been exposed to the ocean. But in June, Scherer reported evidence that Lake Whillans was connected to the ocean possibly as few as 5,000–10,000 years ago5.

This relatively recent delivery of food has big implications. “It’s probably part of the reason we saw such a productive ecosystem” in the lake, says Brent Christner, a microbiologist from the University of Florida in Gainesville who was part of the 2013 expedition and is also involved in the current programme.

When the researchers drill into Lake Mercer, they’ll come armed with new instruments to answer some of the questions that emerged from the previous project. Mercer is twice the size of Lake Whillans, and five times deeper — but was probably connected to the ocean at the same time as Whillans, says Scherer. Given the realization that the lakes might not have been cut off for tens of thousands of years, the team hopes to learn what Lake Mercer’s inhabitants are eating. They could subsist on ammonium, methane and other organic compounds from relatively fresh food deposited a few thousand years ago, or they might consume less-easily digested material that is millions of years old. Discovering the microorganisms’ diet could help the team to predict how much life might inhabit other subglacial lakes that have been isolated for a lot longer.

It could also hint at how much life, if any, might survive below the surface of Mars — a planet that used to be much more hospitable billions of years ago, when water was present on its surface. Priscu suspects that if life exists on Mars, much of it would be living off carbon that was laid down by photosynthetic organisms, when the planet was wetter.

The team hopes to get a lot of answers about life in Lake Mercer by extracting a core of mud up to 8 metres long from the base of the lake. Brad Rosenheim, a palaeoclimatologist at the University of South Florida in Tampa, will use an advanced technique to analyse the mud for carbon-14, a radioactive isotope formed in the atmosphere that decays to undetectable levels within about 40,000 years. This could reveal how much of the lake-bottom muck was laid down the last time the ocean reached this spot.

Such evidence would provide an estimate of the amount of fresh food that was deposited and the degree to which microbes are now eating this young material, relatively rich in carbon-14, versus gnawing on older carbon. That could help researchers to determine what kind of life might populate other subglacial lakes that have been cut off for an even greater length of time.

Christner hopes to enhance this picture by analysing the core for scraps of DNA left behind by sea sponges, brittle stars, crustaceans or other marine life that arrived with the most recent ocean invasion, several thousand years ago. This could provide further information on what today’s resident microorganisms are eating.

Harwood and his colleagues will study the microscopic diatom shells that they expect to find at the bottom of Lake Mercer. By matching up the diatom species found in the core with records of when those species went extinct in other areas, they hope to date the movements of the West Antarctic Ice Sheet.

This evidence could indicate when the ice sheet melted significantly in the past and what the climate was like then, says Harwood. That could help researchers who are trying to forecast when global warming will trigger runaway melting of the West Antarctic Ice Sheet, which in turn could raise sea levels around the globe by several metres.

Creatures from the deep
The SALSA team also hopes to learn whether more-complex organisms, such as animals, might inhabit the subglacial world. In fact, some researchers wonder whether they missed something major when they drilled into Lake Whillans in 2013.

At that time, Whillans’s oxygen levels were low — but survivable by a broad range of aquatic animals. And the abundant bacteria in the lake could potentially support microscopic animals such as worms, rotifers or tardigrades. But the researchers were surprised when DNA studies showed no evidence of such creatures2. And a video camera that was lowered into the hole for a few minutes did not record any such life.

Then, in 2015, the team discovered a different kind of surprise. They drilled through the ice at another location, 100 kilometres downstream from Lake Whillans, where the ice sheet begins to float on the ocean. Beneath 755 metres of ice, they encountered a sliver of sea water just 10 metres thick. Because the water at that point is more than 600 kilometres from the sunlit edge of the floating ice shelf, researchers did not expect to find complex life. And when they lowered a camera into the hole, the blank images that streamed back confirmed their suspicions — for eight days. Then, they sent down a remote-operated vehicle, and its video cameras soon caught fish, amphipod crustaceans and other animals milling about — in an environment that should not have been able to feed such creatures, because microbial life was scarce.

Priscu now wonders whether Lake Whillans could have also harboured animals that didn’t show up in the few minutes of video they captured from their static camera. This time, they are coming to Lake Mercer with better equipment: including the thin submersible that discovered the animals beneath the ice shelf in 2015.

With its three video cameras and claw, it will explore Lake Mercer, venturing up to 100 metres from the borehole and sending images to the surface through a tether. Priscu looks forward to that moment. He thinks that the lake should be capable of sustaining animals.

Peter Doran, a polar scientist at Louisiana State University in Baton Rouge, says that researchers are eager to follow up on the discoveries from Lake Whillans and see what lurks in Mercer. “We need to start building our knowledge, because it turns out that this is a vast ecosystem that’s completely unexplored.”

Nature 564, 180-182 (2018)

doi: 10.1038/d41586-018-07669-3

Altri articoli di Douglas Fox per Nature