Signs of ancient life in deep, dark rock.

A team of Scandinavian scientists has discovered the fossilized remains of microbes deep below Earth's surface, a finding that has broad implications for studies of early life on Earth and other planets.

Although biologists once thought life could not survive far underground, investigators in the last decade have pulled up evidence of microorganisms living 3 kilometers down (SN: 3/29/97, p. 192). It is difficult, however, to prove that such bacteria are actually indigenous residents of the subsurface, not contaminants introduced into the rock by the drilling process.

The discovery of fossils far below the surface bolsters the contention that bacteria live deep underground. "This is a much stronger indication that the microbes existed there before the drilling started," says Karsten Pedersen of Goteborg University in Sweden. His team reports on its work in the September Geology.

While drilling into granite rocks on the coast of the Baltic Sea, Pedersen and his colleagues uncovered the fossils inside fissures located 207 meters below the surface. The objects are rodshaped, and they measure about I micrometer across--similar in shape and size to living bacteria.

The group cites several lines of evidence to support the idea that these rounded forms represent fossilized microorganisms rather than inorganic mineral deposits. X-ray analyses show that the shapes contain primarily carbon and no appreciable amounts of several other elements common to mineral encrustations. Furthermore, the objects have a surplus of light isotopes of carbon, a signature of living organisms.

When viewed under a transmission electron microscope, the fossils appear as a film coating the inside of cracks, a typical way for bacteria to grow.

Granite is a dense rock formed from molten magma. The bacteria and other microbes apparently formed colonies in the rock after it had cooled and fractured, allowing water to flow through the fissures, says Pedersen. The age of the fossils remains uncertain. They could be millions of years old or just a few hundred, he says.

The study of subsurface microbes took on new significance last year, when NASA scientists reported finding bacterialike shapes inside a meteorite from Mars (SN: 3/29/97, p. 190). That report launched a heated debate.

"All this furor over the Martian meteorite is because we really don't know what a fossilized microbial system looks like in hard rock," says Todd O. Stevens of the Battelle Pacific Northwest National Laboratory in Richland, Wash. "We need to learn to recognize them if we're going to look for them here or on other planets."