Unique chemistry enables microbes to survive harsh conditions.
Researchers have discovered a new kind of microbial life flourishing in a dark, icy-cold pool underneath an Antarctic glacier. The finding has implications for how life might have survived on Earth during the coldest, grimmest epochs.
Blood Falls, a rusty red discolouration on the face of the Taylor Glacier in Antarctica, occasionally gushes forth a transparent, briny, iron-rich liquid that quickly oxidizes and turns red, staining the ice below.
In 2004, Jill Mikucki, then a PhD student in microbial ecology at Montana State University in Bozeman, was there to catch this outflow. From a nearby hut, she made many trips up the glacier's snout with an ice axe and collecting vessels. "It almost has a sheen to it, it is so dense and rich in salt," she says of the fluid she collected.
Now Mikucki, currently at Dartmouth College in Hanover, New Hampshire, and her colleagues have found that the liquid, which comes from a pool of seawater that has been trapped inside the glacier for at least 1.5 million years, is alive with around 30 different types of bacteria with some unique chemical moves.
The microbes have cycled through a million generations since being cut off, and live by slowly nibbling away at organic material trapped with them — likely to be "a mishmash of old 'skeletons'", according to Mikucki.
To get energy from organic matter, living things break molecules' high-energy bonds. This releases high-energy electrons, which are passed down a chain of molecules, releasing manageable chunks of energy at each step.
In humans and many other large organisms, oxygen is the final acceptor in this electron transport chain. But in the subglacial pool studied by Mikucki and her team, as well as in other extreme environments, there is no free oxygen to use for this purpose. Many microbes use sulphate as their final electron acceptor, producing hydrogen sulphide in the process. But Mikucki and her co-workers found no evidence that this was happening in the mysterious Antarctic pool.
“Life finds a way. Jill Mikucki , Dartmouth College”
When the team looked at sulphate in their samples, however, they found evidence that the microbes had put the sulphate through chemical reactions. The researchers believe that the microbes are using sulphate as a catalyst in a complex chain of reactions in which the final electron acceptor is iron. "We think they are using it to get at the iron oxides," says Mikucki.
Using iron as a final electron acceptor is not unprecedented. But using sulphate to do it is. In fact, many biogeochemists thought it unlikely that any organism could use iron in this way in the presence of a lot of sulphate, because iron and sulphate would just react together to make pyrite. And yet these Antarctic microbes are using iron as the final step in their electron transport chain not only in the presence of a lot of sulphate, but with its assistance.
"This is how an ecosystem has sustained itself despite being covered by lots of ice in cold and darkness," says Mikucki. "Life finds a way."
This new pathway might also be important for understanding how life on Earth survived during possible 'Snowball Earth' episodes of total glaciation hundreds of millions of years ago. During these periods, there was little if any photosynthesis, and the oceans were filled with iron.
"There's been a time in Earth's past where the ocean chemistry has changed and you've built up a lot of iron," says Mikucki. "This sheds light on how a water body might accumulate iron in the presence of sulphate."
Anna-Louise Reysenbach is a microbiologist at Portland State University in Oregon who specializes in microbes from exotic locations, such as deep-sea hydrothermal vents. She agrees that the conditions underneath the glacier might resemble those of a very cold former epoch.
"It's a little closed system with very little organic carbon to be used, and a slightly different sulphur cycle," she says. "It definitely has implications for understanding early-Earth sulphur and iron ratios and how they come about."
And, ultimately, she says, it proves yet again the tenacity of life. "Microbes can do almost anything. If there is a little bit of energy to be had, they will get it."
With life as tough as this, perhaps it isn't restricted to Earth. "If it can survive below this glacier, why not below the ice cap on Mars and on Europa?" asks Mikucki.
Mikucki, J. A. et. al. Science 324, 397–400 (2009).