We are fond of posting “Talmudic Questions,” questions we consider to be more interesting than their answers. The very first of them, dated December 1, 2006, reads as follows:
Where on Earth would one expect to find a single species of microbes (a pure culture) for sustained periods of time? Symbioses (mutualistic or parasitic) do not count, neither do Petri dishes.
It turns out there is now a candidate (actually, a Candidatus) nominated for this lonesome role. It is a bacterium, named Candidatus 'Desulforudis audaxviator' , found in the ground water of a South African gold mine, 2.8 km below the surface. Here, in this challenging locale, D. audaxviator accounts for at least 99.9% of all living things (with the remaining fraction thought to be trace contaminants from the mine or lab). A fair amount is known about it, even though it has yet to be cultured. It is a Gram-positive obligate anaerobe, a member of the Firmicutes and somewhat related to the Clostridia. Home is warm and alkaline — around 60 °C and pH 9.3. Its fully sequenced genome tells us that it is probably motile and a sporeformer, and that it makes its living from oxidizing hydrogen using sulfate as the electron acceptor. Both of those substances are relatively abundant in its habitat. Its hydrogen "fuel" is probably formed as a by-product of the radioactive decay of uranium within the earth. Thus D. audaxviator carries the banner for Life Without Sunlight to new depths.
However, this is not life at it’s fullest.
The biomass of D. audaxviator is not overwhelming. By flow cytometry, the authors determined a modest population of about 3.3 x 104 cells/ml in the water of this aquifer. In addition, their best guess is that foodstuff is so limiting that the generation time of these organisms may be reckoned in the hundreds, if not thousands, of years. This is not life in the fast lane. But not having competitors allows for a leisurely existence.
Deep gold mines are great places to set up a geomicrobiological laboratory — if you can stand the heat. But, think of the advantages of not having to drill down those thousands of meters from the surface. It bears noting that this report is based on a lot of work, involving 20 researchers, and with it comes a colossal amount of supplementary data.
The name of the organism tells its story. It stems from Jules Verne's Journey to the Center of the Earth, where the explorers find a secret inscription: Descende, audax viator, et terrestre centrum attinges (Descend, bold traveler, and you will reach the center of the earth).