The McMurdo Dry Valleys of Antarctica are considered one of the least hospitable places on Earth; NASA has used them to simulate conditions that might prevail on Mars. The area gets little precipitation, and the water that is present is locked into ancient, thick glaciers that have been in the area for longer than modern humans have existed. Almost by definition, any living things that manage to scrape out an existence in this environment are going to be pretty unusual. But scientists are now reporting the discovery of an ecosystem buried under the ice of one of these glaciers that stretches the definition of unusual well past the breaking point.

Nobody would be likely to suspect that there is any life underneath the ice, which is hundreds of meters thick in the area, if it weren't for the striking red ice erupting from the glacier's terminus, giving the formation the name Blood Falls. That red color comes from iron concentrated in the ice, which exists primarily in the Fe(II) state favored when oxygen is absent. Thanks to a release event that occurred in 2004, researchers have projected where that iron originates, and suggested its likely source. They've also figured out what the environment at that source looks like.

It seems that the red ice has travelled from a pocket that exists four kilometers from the glacier's end, at a site where the ice is over 400m thick. Based on the chemical composition of the material trapped in the ice, scientists think that the pocket encases the remains of an arm of the ocean that extended into the area during the Pilocene, which ended over 1.8 million years ago. Once encased under the glacier, that pocket became hyper-saline and completely devoid of free oxygen. Consistent with its total isolation from the surface, the amount of 14C in the sample is extremely low.

Despite the fact that the liquid water is below the freezing point of pure water, there appears to be a microbial community that has called it home. Consistent with an origin in the oceans, three quarters of the DNA sequences obtained from Blood Falls appear to come from relatives of marine organisms. Most of the clones come from within the Proteobacteria, and the closest cultured relatives of many of these metabolize sulfur and iron (there are also high levels of sulfur present in the brine).

Unlike sulfur-powered communities present at undersea vents, there's little indication of a hydrogen sulfide metabolism present in the ice at Blood Falls. Instead, it appears that energy is obtained when sulfur is cycled through different oxidation states by reacting it with iron, producing the Fe(II) seen in the brine. The oxidized sulfur is then used to react with carbon compounds, powering the metabolism. All of that is pretty low-energy—the authors suggest that the doubling time for a bacterium in this environment would be roughly 300 days—and requires an external source of Fe(III) to power the system. The authors posit that the glacier itself might provide the source by extracting new iron as it scrapes across the underlying rocks.

Bizarre environments like this help inform scientists as they consider the prospects for life on other planets and moons. But this specific example may tell us something about how life managed to persist on Earth during what appear to have been complete global glaciations that occurred in the Neoproterozoic. These "snowball earth" events may have frozen over the oceans, but the Blood Falls bacteria suggest that life could have eked out a metabolism under these extreme conditions, providing raw material for evolution once the planet warmed again.

Science, 2009. DOI: 10.1126/science.1167350