Rock core

A rock sample drilled from Earth’s depths contained a flourishing mix of bacteria despite the harsh conditions deep underground. Credit: CAB/IPBSL

Microbiology

Gone to the dark side: bacteria thrive 600 metres below Earth’s surface

Microbes may have switched from reliance on sunshine to consuming hydrogen.

A teeming community of odd bacteria has been discovered more than half a kilometre below Earth’s surface, where water is scarce and the Sun’s rays cannot reach.

Fernando Puente-Sánchez at the Spanish Centre of Astrobiology in Madrid and his colleagues extracted a long cylinder of rock from an abandoned mining site in Spain. The rock contained an array of bacteria, including members of a group called cyanobacteria, which generally rely on sunlight to photosynthesize.

According to a genomic analysis, these cyanobacteria might manufacture enzymes for transforming hydrogen into energy. The researchers also found that, along the length of the rock cylinder, hydrogen levels were lowest where cyanobacteria were most abundant. These findings led the team to propose that the cyanobacteria make energy from hydrogen and might be the first members of their group capable of colonizing Earth’s rocky depths.