More than a mile and a half below the earth, deep in a sunless gold mine, water pools in a fracture heated, just barely, by the slow radioactive decay of uranium and thorium. That radiation is enough to split some water into one of its constituents: hydrogen. By combining this, the lightest element, with geologically produced sulfate, a newly discovered microbe species thrives far from the light of the life sustaining sun. In fact, this newly discovered ecosystem has flourished for at least 20 million years, according to new testing reported in the October 20 Science.
Li-Hung Lin of National Taiwan University and a host of colleagues visited the Mponeng gold mine in South Africa and sampled groundwater exposed by new drilling four times over the course of 54 days. Each sample, uncontaminated by contact with the surface, revealed the same deep-earth ecosystem dominated by a new microbe related closely to the sulfate-reducing Desulfotomaculum kuznetsovii that lives near vents on the ocean floor. Dating the helium and argon isotopes in the water revealed it had been isolated from the surface--and its biological cousins--for millions of years. "The analysis shows that the groundwater age is between 16 [million] and 25 million years," Lin says. "So we have a microbial community isolated from the surface for 20 million years, and this community uses sulfate and H2 produced from inorganic processes."
In fact, the energy chain of this community relies not on photosynthesis from the sun but sulfate reduction, much as anaerobic bacteria have done for at least a billion years. This energy chain allows other types of microbes to survive, primarily Proteobacteria, according to Lin. "They probably represent the relic members of the surface community," he notes. That means millions of years ago these microbes slipped deep into the earth and have survived there ever since, eking out a strange existence far from the light of day. "We don't know the actual life span of these microbes," he explains. "We estimate their doubling time at 45 to 300 years, which represents a very slow microbial activity when compared with surface environments." Nevertheless, such slow, steady, shielded organisms may represent a hardy life form that could equally well persist deep beneath the surface of other worlds, such as Mars or Europa. "It is possible," Lin adds, "that communities like this can sustain themselves indefinitely, given enough input from geological processes."