An ecosystem of one
In 2008, scientists announced a discovery from water samples collected 2.8 kilometers deep in the Mponeng gold mine near Johannesburg. The samples contained only one organism: a new species of bacterium. This was the first known ecosystem on Earth to be composed of a single species. The bacterium lives in total darkness and without oxygen, in geologically ancient water heated to 60°C (140°F).
Named Candidatus Desulforudis audaxviator, the organism survives completely disconnected from the sun-powered biosphere. Instead of photosynthesis, it relies on a process called radiolysis. Radioactive uranium, thorium, and potassium isotopes in the surrounding rock decay, splitting water molecules (H₂O) into hydrogen (H₂) and other reactive chemicals. The bacterium uses the resulting hydrogen as its energy source. It gets carbon from dissolved CO₂ and nitrogen from ammonia in the rock and surrounding fluids. This self-sufficient existence is so unusual that when researchers sequenced the DNA from the water, 99.9% of it belonged to this single organism.
The bold traveler's genome
The bacterium's name shows its unique nature. "Desulforudis" means "sulfur rod," referring to its shape and its metabolism of sulfur compounds. The second part of its name, "audaxviator," means "bold traveler" and comes from a Latin quote in Jules Verne’s novel Journey to the Center of the Earth. The quote reads, "Descende, audax viator, et terrestre centrum attinges" ("Descend, bold traveler, and you will attain the center of the Earth").
Its genome is uniquely adapted for solitary survival. It contains genes to perform every necessary function, from building amino acids to protecting itself from viruses. Analysis of its DNA shows horizontal gene transfer, a process where an organism acquires genetic material from another species, not its parent. Some of D. audaxviator's genes, particularly those for fixing nitrogen, appear to have been acquired from archaea, a different domain of life entirely. This suggests that despite its current isolation, its ancestors once interacted with other deep-earth microbes. To survive harsh periods, it can form an endospore, a dormant state that protects its DNA from heat and lack of water.