Bacteria exposed after ice collapse
Antarctic trawl uncovers threatened ecosystem.
A strange ecosystem has been discovered beneath the Antarctic ice sheet. But the collapse of the ice shelf that allowed researchers to find it also threatens the community's future.
The Larsen ice shelf is on the Antarctic Peninsula, the northern tip of the continent, which reaches up towards South America. Possibly as a result of global warming, a 3,250-square-kilometre piece of ice, roughly the size of Long Island or Cornwall, broke away from the peninsula in 2002.
In March this year, researchers from the US Antarctic Program were surveying a patch of sea bed that used to be covered by this ice. The area lies 850 metres below the waves and 100 kilometres from where the tip of the ice shelf once started. The researchers took video and still images of the ocean floor using a submerged sled towed behind their boat. They weren't expecting, or looking for, signs of life.
But they found it. "Seeing these organisms on the ocean bottom... it's like lifting the carpet off the floor and finding a layer that you never knew was there," says Eugene Domack, a geoscientist at Hamilton College in Clinton, New York. "It was like Christmas morning when I was a child!"
The big seep
The pictures show a thick white mat covering the sea bed that resembles 'bacterial mats' seen in other unusual deep-sea communities and has gas bubbles rising from it. The video shows 'mud volcanoes' with large collections of clams clustered around them. The results appear today in Eos, the weekly newspaper of the American Geophysical Union1.
Domack and his colleagues think the site is an example of a cold seep: a deep-sea site where sulphur-rich waters and natural gas leak from the ocean floor. Mud volcanoes form where the waters well up. Thick mats of bacteria thrive on the gas and sulphur, using chemosynthesis to manufacture organic compounds. Similar communities exist at underwater hydrothermal vents, where sulphur-rich waters are volcanically superheated.
The discovery is significant because no seeps have been found at such a latitude before. It may be that there are many more such sites beneath the vast areas of sea covered by ice sheets. "The area beneath ice shelves is large and very, very poorly known," says Domack.
Gateway to the south
Alex Rogers, a biodiversity expert at the British Antarctic Survey in Cambridge, UK, says the discovery could be the missing piece of a puzzle that has stumped scientists studying cold seeps and other chemosynthetic systems.
Sites in different oceans often host the same species, Rogers says, but it has not been clear how the fauna could travel the thousands of miles between the very specialized habitats they need.
"The Southern Ocean forms a natural gateway between all the other oceans," explains Rogers. "If more seeps occurred in the Southern Ocean it could be acting as a dispersal route to sites in other seas."
But the surprising ecosystem's days could be numbered. The overlying ice shelf sheltered the site from physical disturbance and the organic debris that usually supports bottom-dwelling organisms. Now sediment and rocks are building up, and more usual Antarctic species are moving in.
"It could be a decade or less before the sites are buried," says Domack. He says the bacterial mats show definite signs of decline, as species move in to feed on the accumulating organic matter and discover a tasty food source.
If predators of these grazers follow, then the clams and many other species unique to the site could start to disappear.
- Domack E., et al. Eos, 269. 271 - 272 (2005).