A recent discovery lying 3km in the depths of a South African gold mine could hold the answer to man's search for extraterrestrial life.
Newly discovered but ancient bacteria at the bottom of gold mines near Carletonville have led a team of international scientists to believe there could be life below the surface of Mars.
A study on the find, published recently in the international journal Science, said the microbes appeared to have survived for tens of millions of years living on hydrogen and sulphate, and not oxygen.
The discovery is the result of collaboration between South African and American researchers on life in extreme environments and is part of an international project funded partly by the National Aeronautics and Space Administration and the National Science Foundation.
Speaking to The Star, Tullis Onstott, a geosciences professor at Princeton University and leader of the research team, said the bacteria - which are completely unknown and have no name or recognisable origin - live in conditions that are similar to those on Mars.
"If you look at Mars now, it is geologically inactive. It kinda looks dead - we don't see volcanoes erupting and we don't see obvious signs of life. It's a dead planet. But it's tectonically active. We know what the rocks are like, we know there is water there and uranium radiation there," Onstott said.
The bacteria found at Carletonville live in the fissures of rocks which contain the radioactive elements uranium, potassium and thorium. The bacteria also live in water that is so old it has never been diluted with surface water and is between 3-million and 25-million years old.
Light does not reach here, and photosynthesis - the process whereby plants create food from the sun's energy and create the byproduct oxygen - cannot occur.
Instead the bacteria live off radioactivity from the surrounding rocks. "Breathing" the cancer-causing metal chromium 6 like oxygen and converting it into the safe chromium 3 is what makes them unique compared with other microbes.
"There is no reason why a geologically inactive planet cannot still have a source for energy for an organism," Onstott said.
He said the single-cell, cylindrical bacteria are believed to be tens of millions of years old and are believed to have a life span of between 1 000 and 10 000 years.
"A single cell lives for that long. They are very, very tiny, and in order for them to reproduce it takes a lot of energy," Onstott said.
The bacteria have been cut off from the surface of Earth for many millions of years but thrive under conditions that most organisms would find inhospitable to life, Onstott said in a Princeton University newsletter.
"It is a new species and new genesis bacteria. Its closest relatives are things that you find coming out of vents in the ocean floor. It could possibly be deeply rooted in the tree of life and could be quite ancient."
He said the bacteria survived in a pool of hot, pressurised salt water that stinks of sulphur and noxious gases that humans find unbreathable and are related to a division of microbes found near undersea hydrothermal vents.
"The cool thing about this is that we have been able to show the hydrogen gas is generated by radiation in the rock," Onstott said. Hydrogen is usually created from the decomposition of water.
"What it means in this particular case is that people have begun to realise that life does exist at great depths. The amount of organisms down there will be small. But what we are showing in the South African environment is that they are not energy - poor at all," he said.
Dr Esta van Heerden, a professor of microbiology at the University of the Free State's Department of Microbial, Biochemical and Food Biotechnology, said while people generally applied negative connotations to bacteria, believing that they made people sick, these were useful bacteria.
She said bacteria convert iron, vanadium and arsenic into forms that have a less harmful impact on the environment.