Antarctica's McMurdo Dry Valleys may appear to be one of the least hospitable places on Earth. They contain a frigid desert where high winds scour the rocky ground, and the only water present is in the form of ice, some of it left over from when the ocean extended into the valley over a million years ago. The area is so inhospitable that NASA has used it to simulate conditions on Mars.

So biologists were probably very surprised to find that the area hosts a number of distinct ecosystems. Not on the surface; instead, these communities of bacteria live under the ice, in salty lakes that have been isolated from any external sources of energy or chemicals for anywhere from thousands to millions of years. Now, researchers have characterized one of the youngest under-ice lakes, which evidence suggests has been isolated from the air for only a few thousand years. Though estimates would suggest that the bacterial community within should be rapidly running out of food, the contents of the lake water suggest that the organisms are doing just fine, powered by the chemistry of the underlying minerals.

The most dramatic feature of the McMurdo Dry Valleys is probably Blood Falls, where iron both stains the ice red and helps power a community of bacteria that have been trapped within it for about 1.8 million years, ever since an arm of the ocean got cut off and frozen under a glacier. Lake Vida is in a different valley, and probably hasn't been isolated for nearly that long. It's extremely salty, and stays at a chilly -12°C under a sheet of ice that's at least 16m (52.8 feet) thick. But radiocarbon dating seems to indicate that it hasn't been isolated for nearly as long, and probably has exchanged carbon with the atmosphere within the last few thousand years.

Based on energy budget calculations, the authors expected that any communities trapped under the ice would be in what they called the "final steps of decomposition." The few microbes left should be feeding on the last remaining organic compounds and releasing methane in the process.

Sampling the water, however, showed that nothing of the sort was going on. Lake Vida's waters are anoxic (normally a condition that methanogens prefer), but are rich in dissolved organic compounds, including plenty of carbohydrates but little methane. There are also significant amounts of dissolved hydrogen in the waters. Nitrogen compounds were also very common, with supersaturated levels of nitrous oxide and high levels of ammonia. This mix of oxidized and reduced compounds suggested that the chemistry of the lake water was nowhere near the final steps of decomposition.

So, what's living in this complex mix? The authors looked for DNA sequences that help identify species, and found 32 of them from eight different phyla of bacteria. That is a broad enough sampling that it's hard to tell what, exactly, is likely to be powering the metabolism of most of them. However, it is a further indication that methanogens probably aren't dominating the system. It also showed that archaea, which are often common in extreme environments, are completely absent.

Based on the chemicals present in the brine of Lake Vida, the authors speculate that the bacterial community is powered by molecular hydrogen that can be released by a reaction between water and iron silicates in the underlying rocks. That hydrogen could then be incorporated into the complex organic compounds seen in the lake's waters.

If that's accurate, then it suggests that Lake Vida's life is probably powered by a different source of chemical energy than the one that sustains the life under the ice in a nearby valley (the one that creates Blood Falls).

That's a rather important finding, since it suggests that a diversity of simple, naturally occurring chemical reactions could provide the energy needed to sustain life, even in the absence of things like sunlight or plate tectonics. This in turn should influence our thinking about the prospects for finding life in the sub-ice oceans of places like Europa, as well as in possible sub-surface brines on Mars.

The other intriguing prospect is that the different dry valleys could all host distinct ecosystems, with different degrees of isolation and different sources of chemical energy. By sampling a few of them, we could potentially get a greater indication of how these communities could evolve over time. This may tell us something about life on our own planet, where communities appear to have survived several instances of global glaciations that created a "snowball Earth."

PNAS, 2012. DOI: 10.1073/pnas.1208607109 (About DOIs).