— About 2 miles below the ground in a South African gold mine stands Duane Moser next to the fracture zone (white area) where the one-of-a-kind bacteria were found (Image: Li-Hung Lin)
The diagram shows where the photograph was taken with respect to the discovery (Image: Duane Moser)
The first known organisms that live totally independently of the sun have been discovered deep in a South African gold mine.
The bacteria exist without the benefit of photosynthesis by harvesting the energy of natural radioactivity to create food for themselves. Similar life forms may exist on other planets, experts speculate.
The bacteria live in ancient water trapped in a crack in basalt rock, 3 to 4 kilometres down. Scientists from Princeton University in New Jersey, US, and colleagues analysed water from the fissure after it was penetrated by a narrow exploratory shaft in the Mponeng gold mine near Johannesburg, South Africa. The shaft was then closed.
There were many species of bacteria present, but RNA sequencing showed most were a previously-unknown type of bacteria dubbed Desulfotomaculum.
Similar microbes have been detected in many subsurface environments, study leader Li-Hung Lin, now at National Taiwan University, told New Scientist. What is unique in our study is that this microbial community doesnt depend on photosynthetic products.
All living things require a source of energy which is directly or indirectly from the Sun. Green plants use the Suns energy, in a process called photosynthesis, to make energy-rich chemical bonds, for example, between the carbon and hydrogen atoms in sugar molecules. Some of the energy stored in these bonds is then released when a living organism replaces the hydrogen with oxygen, giving off CO2.
Without the solar energy that plants harness to make those hydrogen-carbon bonds, none of this is possible. But the Mponeng bacteria have another source of energy.
Sulphate and hydrogen gas are generated from geological processes. Microbes use these nutrients to live, explains Lin.
Energy of radiation
Uranium and other radioactive elements in the rock emit radiation that shatters water molecules, producing high-energy hydrogen gas that is able to cleave chemical bonds.
The bacteria exploit this hydrogen gas to turn sulphate (SO4) molecules from the rock into hydrogen sulphide (H2S). It is the energy-trapping equivalent of photosynthesis. The energy of radiation, which makes hydrogen gas energetic enough to form these bonds, replaces the energy of the Sun.
The team examined the sulphur atoms in the hydrogen sulphide they found. The ratio of isotopes different chemical forms of the same element proved the sulphide was produced by living organisms, in a similar way that carbon dating can be used to show whether carbon compounds have been produced by living or non-living processes.
Dying or flourishing
Other sulphate-eating bacteria have been found in ocean sediments, volcanoes and oil deposits. But all have either received some chemicals produced by photosynthesis, or it has not been clear whether they were trapped and dying, or flourishing.
A study of the inert gases in the sample, such as xenon, show that the Mponeng water has been isolated from the surface for 20 million years, says Lin. This shows that the bacteria must live totally independent from surface photosynthesis.
It is not known how widespread these communities are on Earth, he says. But the discovery of a stable, light-independent life form raises hopes of finding similar creatures on other planets.
Research team-member Lisa Pratt at Indiana University at Bloomington, US, also heads a team funded by NASA to design probes to look for just such life under the permafrost of Mars.
Journal reference: Science (vol 314 p 479)