Goldmine Bug DNA May Be Key to Alien Life
Original source: ABC News
October 10, 2008
By CATHERINE BRAHIC
A bug discovered deep in a goldmine and nicknamed "the bold traveller" has got
astrobiologists buzzing with excitement. Its unique ability to live in complete
isolation of any other living species suggests it could be the key to life on
A community of the bacteria Candidatus Desulforudis audaxviator has been
discovered 2.8 kilometres beneath the surface of the Earth in fluid-filled
cracks of the Mponeng goldmine in South Africa. Its 60o
C home is completely
isolated from the rest of the world, and devoid of light and oxygen.
Dylan Chivian of the Lawrence Berkeley National Laboratory, California, studied
the genes found in samples of the fluid to identify the organisms living within
it, expecting to find a mix of species. Instead, he found that 99.9% of the DNA
belonged to one bacterium, a new species. The remaining DNA was contamination
from the mine and the laboratory.
"The fact that the community contains only one species stands one of the basic
tenets of microbial ecology on its head," says Carl Pilcher, director of the
NASA Astrobiology Institute, who was not involved in Chivian's DNA analysis but
whose team made the initial discovery that there were microbes living in this
particular fissure two years ago.
Micrograph by Greg Wanger, J. Craig Venter Institute, and Gordon Southam,
University of Western Ontario A community of the bacteria Candidatus
Desulforudis audaxviator is the first ecosystem ever discovered having only a
single biological species. Evolutionary biologist E. O. Wilson says the
discovery is so important he will at once begin to mention it in his lectures on
Community of one
A community of a single species is almost unheard of in the microbial world. It
means the ecosystem's only species must extract everything it needs from an
otherwise dead environment. "Virtually all other known ecosystems on Earth that
don't use sunlight directly do use some product of photosynthesis," says
Deep-sea vent communities, for instance, are too far down to directly use
sunlight but they do use oxygen dissolved in seawater, and that oxygen is
produced by photosynthesising plankton at the surface.
Chivian's analysis shows that D. audaxviator gets its energy from the
radioactive decay of uranium in the surrounding rocks. It has genes to extract
carbon from dissolved carbon dioxide and other genes to fix nitrogen, which
comes from the surrounding rocks. Both carbon and nitrogen are essential
building blocks for life as we know it, and are used in the building blocks of
proteins, amino acids. D. audaxviator has genes to produce all the amino acids
D. audaxviator can also protect itself from environmental hazards by forming
endospores - tough shells that protect its DNA and RNA from drying out, toxic
chemicals and from starvation. It has a flagellum to help it navigate.
"One question that has arisen when considering the capacity of other planets to
support life is whether organisms can exist independently, without access even
to the Sun," says Chivian. "The answer is yes and here's the proof. It's
philosophically exciting to know that everything necessary for life can be
packed into a single genome."
McKay says D. audaxviator represents the kind or organism that could survive
below the surface of Mars or Saturn's sixth largest moon Enceladus.
Some of the bacterium's genes appear to be inherited from a related species.
Others have been found in archaea, a group of organisms evolutionarily distinct
from bacteria. Chivian says D. audaxviator may have evolved as it travelled down
through the cracks in the rock, and acquired archaea genes through horizontal
gene transfer from populations it crossed on its way down.
"It can't handle oxygen," he says. This suggests it has not been exposed to pure
oxygen for a long time. The water in which D. audaxviator lives has not seen the
light of day in over 3 million years, and this could be an indication of how old
the species is.
In fact, the species got its name from its long journey towards the centre of
the Earth. In Jules Verne's novel by that name, the fictional Professor
Lindenbrock's journey is triggered by the following message in Latin: "descende,
Audax viator, et terrestre centrum attinges" - meaning "descend, bold traveller,
and attain the center of the Earth".