Living cyanobacteria have been found far beneath Spain. The discovery is surprising because these photosynthesizing life forms were thought to be restricted to places light can reach. The finding may also prove important, as the site bears a resemblance to conditions beneath Mars.

By using sunlight to produce oxygen, cyanobacteria helped make the Earth what it is today. Now, they are known to live in a wide range of environments, including some few other life forms can handle. They've also been found to generate energy other than through the processing of sunlight. Nevertheless, until a borehole dug into the Iberian Pyrite Belt in southwestern Spain was tested for cyanobacteria, it was thought all cyanobacteria were at least partially dependent on sunlight.

Yet Dr Fernando Puente-Sánchez of the Spanish National Centre for Biotechnology has retrieved cyanobacteria from the bottom of a borehole 613 meters (2,000 feet) beneath the Earth's surface, far beyond even the most filtered light. The site was chosen for its combination of a rich sulfide deposit and the presence of a deep aquifer. Some single-celled organisms use sulfur chemistry for energy, and access to water is essential for even the most basic living things.

Bacteria at these depths are rare, so Puente-Sánchez and colleagues needed to use gene amplification to confirm their presence. Once they did, they found organisms that appear to be related to three genera of surface cyanobacteria. In Proceedings of the National Academy of Sciences, Puente-Sánchez reports that the sites where the bacteria were found were lower in hydrogen than those nearby. Influencing the local chemistry shows that these are not dead or dormant organisms that have fallen from the surface.

Having examined the borehole at various depths, the authors found concentrations of life-identifying fatty acids at 420 meters (1,400 feet) and 607 meters (2,000 feet). The shallower deposit coincided with bands of fractured quartz; the lower one was characterized by alternating dark shale and sandstone, indicating the bacteria can live in a range of environments.

The work provided considerable detail about the chemistry of these deep-living organisms, but for non-microbiologists, the greatest interest is what the work may say about the prospects for extraterrestrial life. The site was specifically chosen because so much of the geology resembles that on Mars.

However, the authors think evolution is probably so slow as to be almost non-existent in these conditions. Instead, the organisms are expected to have acquired the traits that allow them to survive far below the surface when their predecessors lived in more energy-rich environments. Whether Martian life forms had the opportunity to develop near the surface when the planet was more inviting, before making a similar migration underground, remains the tantalizing question.