Life under pressure

Cyanobacteria, known informally as “blue-green algae”, were perhaps the first life on Earth. Indeed, the oldest known fossils are 3.5-billion-year-old fossilised cyanobacteria, found in the Archean rocks of Western Australia. Not bad, considering that the oldest rocks ever found are only 3.8 billion years old!

Not only are cyanobacteria the oldest organism, they’re also one of the most versatile. Think of pretty much any habitat, and chances are you’ll find them thriving there:  oceans, rivers, deserts and even the icy wastes of Antarctica – cyanobacteria can live basically anywhere. However, as they produce energy through photosynthesis (chlorophyll gives them their distinctive blue-green tint), it was assumed that they could only survive in environments that had at least occasional exposure to sunlight.

cyanobacteria can live basically anywhere

So it was understandably shocking when cyanobacteria were found living 2,000 feet below the surface of the Iberian Pyrite Belt in Southwestern Spain. The scientific team were looking for bacteria that survive by oxidizing iron and sulfur but instead found cyanobacteria. The blue-green algae were thriving, congregating in tiny pockets of air dotted amongst the subsurface. To do this, the subsurface cyanobacteria had repurposed cellular machinery for photosynthesis to instead ‘digest’ hydrogen.

For the most interesting implications of this discovery, however, we need to look the other way. The discovery of these microorganisms in such a harsh environment dramatically increases the likelihood of finding life in our solar system. Take Mars. The planet is a frozen, barren wasteland on the surface: average daily temperatures reach minus 125 degrees centigrade at the poles, and rarely go above 20 degrees at the equator.  All water is frozen – locked away from life – and solar radiation scorches the surface. Dig a little deeper however, and things become a bit more hospitable.

The Martian subsurface was once ideal for life. Not only would there have been enough dissolved hydrogen to power a global subterranean biosphere, but the temperature was also high enough for pockets of liquid water to exist. Radiation too is minimal just a couple of thousand feet underground, meaning that any organisms would have faced conditions essentially identical to those on Earth. No wonder then, that astrobiologists have their sights set on Mars.

The discovery of these underground cyanobacteria promises to revolutionize our understanding of the limits of life, both on Earth and on Mars. If we extrapolate just a little further, we can see how this could guide our understanding of life on planets orbiting stars other than our own Sun. It’s too early to be sure, but it isn’t unreasonable to imagine colonies of primitive microorganisms, thriving under the surface of some distant planet.

Whether on Earth, Mars or some yet-to-be-discovered planet, it seems that life does always find a way, even if that way is to hide deep underground.

Photograph: Wikimedia Creative Commons

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