Polar microbes get helping hand
Thaws and meteors could create cradles for life.
You might think that the polar deserts are barren, lifeless places. But you would be wrong. A British microbiologist has shown how the freezing and thawing of ice can turn polar rocks into a haven for microorganisms.
What is more, he believes that meteor impacts, rather than being purely destructive, could also create oases for life by heating rocks and melting ice, either on Earth or on other icy worlds in the Solar System.
Charles Cockell of the British Antarctic Survey, based in Cambridge, travelled to both ends of the globe to see how many microbes make their home at the poles. He and his colleague Dale Stokes, of the Scripps Institution of Oceanography in La Jolla, California, found that light-loving cyanobacteria flourish underneath the rocks.
This seems to be a paradox: how do photosynthetic microbes live underneath opaque rocks? The answer is that repeated freezing and thawing of polar ice creates chinks in the boulders, which let just enough light through for the microbes to survive, the researchers explain in this week's Nature1.
Cockell and Stokes looked under 850 rocks on Cornwallis Island and Devon Island in the Canadian high Arctic. About 95% of them had cyanobacteria living under them. The researchers estimate that these bacterial communities may harness just as much energy from the Sun as the Arctic desert's scrubby lichens and mosses.
If this is true, then the polar desert is a much more energetic and lively place, in biological terms, than ecologists had suspected, Cockell says. These microbes "can fix carbon compounds that then become available for other bacteria in the soil," he says.
Cockell and Stokes examined glacial rock structures called polygons, which are rings of large, cracked boulders surrounding smaller rocks and soil, to show that the bacteria's habitats are created by a freeze-thaw cycle. On Alexander Island in Antarctica, 100% of outer boulders were colonized by cyanobacteria. But in the polygon's centre, where cracks are smaller, only 5% of rocks harboured the microbes.
It is a plausible theory, comments Stjepko Golubic, who studies cyanobacteria at Boston University in Massachusetts. "Cracks are a fact of life," he says. "Freezing and thawing happen every day." In fact, he adds, underneath the rocks is probably the best place for the microbes to live, as water is less likely to be whipped away by the biting wind.
Crack and blast
But cracks are not the only process by which microbes can find their niche, Cockell suggests. He believes their homes can also be created by a much more explosive event: a meteor strike.
In a previous study, he and his colleagues journeyed to Devon Island to study the Haughton crater, a 24-kilometre-wide dent made by an asteroid 23 million years ago. They found cyanobacteria living inside the rocks, at far higher densities than in rocks found elsewhere2.
Cockell suggests that asteroid impacts create the tiny pores by vaporizing certain minerals such as feldspar, leaving silicate-rich, perforated rocks behind. "Asteroid impacts are not always bad for life," he says.
And meteor strikes could have beneficial effects beyond their initial, brutal impact, Cockell adds. An asteroid crater could become an oasis for life, he thinks, because besides modifying rocks it also heats them, an effect that can last thousands of years.
This could benefit polar deserts or even barren, icy worlds such as Mars or Titan, he speculates. An asteroid impact could melt the ice and allow life, if its building blocks are present, to flourish. "And the good thing about a hole in the ground is that water drains into it," he says.
Perhaps those scouring the Solar System for life should focus on impact craters, then. "It might improve our chances of knowing where to look on Mars," he says. "That's assuming that life on Mars is like life on Earth."
- Cockell C. S. & Stokes M. D. Nature, 431. 414 (2004).
- Cockell C. S. & Lee P., Osinski G., Horneck G. & Broady P. Meteoritics Planetary Sci., 37. 1287 - 1298 (2002).