A solitary existence
Deep inside the Mponeng gold mine near Johannesburg, scientists found something unexpected in fracture water 2.8 kilometers (1.7 miles) below the surface. In an environment of total darkness, 60°C (140°F) heat, and a crushing pressure of around 300 atmospheres, they discovered a world with only one inhabitant. Initial analysis of the fluid's DNA expected to find a community of different microbes. Instead, 99.9% of the genetic material belonged to a single, previously unknown species of bacterium.
This lone organism was named Candidatus Desulforudis audaxviator. It is a rod-shaped bacterium, about four micrometers in length, that has everything it needs for life packed into a single genome of 2,157 genes. For years, it was considered the first known example of a single-species ecosystem. Its existence proved that life could sustain itself in complete isolation, without energy from the sun or nutrients from other living things. This discovery changed ideas about where life can exist, both on Earth and potentially on other planets.
Life from radioactivity
Candidatus Desulforudis audaxviator lives a life completely divorced from the sunlit world. Its energy source is radioactivity. Uranium, thorium, and potassium in the surrounding rock formations decay, releasing radiation that splits the surrounding water molecules (H₂O) into hydrogen (H₂) and other chemicals. This process is called radiolysis. The bacterium uses the resulting hydrogen as its fuel.
The bacterium's metabolism is entirely self-contained. It performs sulfate reduction, using the hydrogen from radiolysis to get energy from sulfate minerals dissolved in the ancient water. Its genome contains genes to pull carbon from dissolved carbon dioxide and carbon monoxide, and to fix its own nitrogen from ammonia in the surrounding rock. It can build all the amino acids it needs to survive. Some of its genes, including those for nitrogen fixation, appear to have been acquired from archaea, a different domain of life, through horizontal gene transfer millions of years ago. This genetic toolkit allows it to be the sole producer and consumer in its dark, subterranean world.