ShareThisBy LEE BOWMAN
Wednesday, November 08, 2006
If life can make it here, it can make it most anywhere.
At least, that's what scientists are saying about a colony of bacteria they found thriving in fractured rock two miles below Earth's surface off a shaft of a South African gold mine.
The bacteria appear to exist completely independently of any support from the surface, relying on the radioactive decay of uranium and other elements in the rock to convert water molecules to usable energy. And they've apparently been living this way for millions of years.
"These bacteria are truly unique, in the purest sense of the word," said Li-Hung Lin, lead author of a report on the single-celled creatures published Friday in the journal Science.
"We know how isolated the bacteria have been because our analyses show that the water they live in is very old and hasn't been diluted by surface water," said Lin. He carried out the research while a doctoral student at Princeton University and as a fellow in astrobiology at the Carnegie Institution in Washington. He is now at National Taiwan University.
The discovery of such a self-sustaining community disconnected from the energy of the sun bolsters the odds that similar life might exist in deep subsurface environments on other planets, such as in groundwater beneath the permafrost of Mars or under frozen oceans on Jupiter's moon, Europa.
"We know surprisingly little about the origin, evolution and limits for life on Earth," said Lisa Pratt, a biogeochemist at Indiana University in Bloomington who took part in the project.
"Scientists are just beginning to study the diverse organisms living in the deepest parts of the ocean. The rocky crust on Earth is virtually unexplored at depths more than a half a kilometer. The organisms we describe in this paper live in a completely different world than the one we know at the surface."
In recent years, scientists have found numerous troves of microbes living in such extreme environments as aquifers, petroleum reservoirs and hydrothermal vents. Yet all those communities depend at least somewhat on nutrients that can be traced back to plants or bacteria that converted sunlight to food through photosynthesis.
That doesn't seem to be the case for the otherworldly bacteria found in water seeping out of a crack in a shaft of the Mponeng gold mine near Johannesburg. Lin said the hydrocarbons found in the fracture zone did not come from living organisms, as they usually do, and that bacteria obtain the hydrogen needed for respiration from the breakdown of water in the course of radioactive decay of uranium and other elements in the rock.
The water and bacteria were seeping from a natural fault in one of the mine's shafts that workers had accidentally drilled into during an expansion of the mine. Tipped to the hidden reservoir by a geologist working for the mine, researchers led by Tullis Onstott of Princeton arrived to study the microbial lost world.
They sampled the water repeatedly over 54 days, measuring the quality of bacteria in each sample and watching for any sign of contamination.
DNA analysis revealed that many bacterial species were present, but the community was dominated by a newly discovered species that's a relative of hydrothermal vent bacteria called Firmicutes that digest sulfur. The researchers say smaller numbers of the other bacteria survive by living off the new species.
The age of the water and comparative DNA analysis suggest that the bacteria in the mine have been cut off from contact with the surface anywhere from 3 million to 25 million years. Yet the rock surrounding the fault is about 2.7 billion years old.
The scientists say it's impossible to tell how the bacteria wound up so deep within Earth's crust, but speculate that they were carried there by groundwater penetrating the fractured rock.
On the Net: www.sciencemag.org
