Geochemists and microbiologists are delving into the details of extreme biochemistry deep within the Earth, where chemical and metabolic processes go at glacial pace, and life appears to be completely disconnected from the photosynthesis-based biological cycles that dominate surface life.
Deep, very deep, beneath the surface of Earth a microbial community dines and thrives. Slowly, but tenaciously, these deep dwellers feed on gases seeping into rock fissures and divide - maybe once every thousand years - to make more of themselves. Geochemists and microbiologists are delving into the details of extreme biochemistry deep within the Earth, where chemical and metabolic processes go at glacial pace, and life appears to be completely disconnected from the photosynthesis-based biological cycles that dominate surface life.
"There is a huge biomass inside the Earth," says David Boone, a microbiologist at Portland State University in Oregon, who isolated from a Virginia well a species of Bacillus he named infernus, or "from hell." B. infernus dwells nearly three kilometers (just under two miles) down.
Boone is gearing up for closer collaboration with the ongoing program of Princeton geologist Tullis Onstott, funded by NASA and the NSF, that seeks to uncover the details of microbial life in rocks brought up from the depths of South African gold mines.
"We have been examining the limit of life," says Onstott. Todayís geochemists, he says, are trying to find out how a creature will "maintain integrity in the face of the deprivations and insults of a high temperature, saline environment" miles below where most species we are familiar with exist. Pressure at this depth is three hundred times that on Earthís surface, and temperatures climb to 60 degrees Celsius (140 degrees Fahrenheit). Fractures contain water and gases such as methane, ammonia, and molecular hydrogen, as well as the organisms that may feed on or contribute to this gas content.
"We get these organisms literally out of solid rock," says Boone, who studied an early sample of iron reducing bacteria culled from the South African mines.
"We've gone to great pains to demonstrate to ourselves and any one else that what we are looking at are really organisms from the deep subsurface," Boone adds.
Samples are collected into sterile bags, and the scientists then pare away to remove any possible contamination. Researchers also check for contamination by looking for remnants of the drilling fluids used in mining operations. These fluids diffuse into the rock faster than the microbes can, and tests show that the fluids are absent from the samples.
"If the solutes can't diffuse into the rocks, then the organisms can't," Boone says.
Yet another check for contamination involves testing with latex microspheres, which have the same surface adhesive properties as bacteria. Scientists coat samples with these microspheres as they collect, and then take the coated samples through the entire clean-up process back at the lab, Boone says. None of the microspheres remain, he says, after the samples have been processed to counter contamination.
Onstott found his first deep microbe in 1996, when a former graduate student, David Phillips, who worked for a gold mining company at that time, helped arrange a visit down a mine in South Africa. Just about that time, a team from the Pacific Northwest National Laboratory in Washington State reported in the journal Science that microbes can live off the hydrogen released when water reacts with the minerals in the basalt of the Columbia River basin.
The discoveries of life deep inside the Earth" greatly enlarges our biosphere," says Michael Meyer, senior scientist for astrobiology at NASA headquarters in Washington, DC. "It doubles our biomass, and opens up where organisms might be in our own solar system."
John Parkes, a geomicrobiologist at the University of Bristol, U.K., says that the evidence for so large a subsurface biomass, so distant from the usual energy source, indicates "there must be other energy available, or more efficient ways to use what is preserved as photosynthetic carbon in sediments." More efficient use of organic matter, Parkes says, may be possible as sediments are carried deep into the Earth, "because the higher temperatures activate the recalcitrant organic matter they contain making it now degradable to bacteria."
Yet perhaps the most fascinating part about deep rock ecosystems is that some may exist completely apart from the photosynthetic cycle of organics. For billions of years, all surface life on Earth has acquired organic material either directly or indirectly from photosynthesis. Even the organisms living at deep sea vents ultimately depend on the oxygen expelled by photosynthetic surface life and on the hydrogen given off by the fermentation of photosynthetically produced organic matter, according to researchers with the Pacific National Lab.
As yet, itís not entirely clear whether or not organics may be present in trace sediments within the rocks at the Columbia River basin and South African gold mine sites. It appears, however, that deep rock microorganisms are producing organic material using only the inorganic chemistry available within the rocks themselves: hydrogen, released when minerals in the rock react with water; and carbon dioxide. Scientists speculate that this may be a model of the original biology of Earth, before photosynthesis evolved. It perhaps is also a model of life on other worlds.
The possibility that life deep in the Earth can fuel itself without recourse to the Sunís energy, says Meyer, means itís more likely that life can be found below the surface on Mars or Europa, or even inside comets. "If you are having foodstuffs for energy available at the subsurface, without any communication from the surface, that greatly increases the chances that if life ever got started on Mars, or transported to Mars, that it could still be in the subsurface today."
Work will continue in the South African gold mine, with increasing participation by South African and European researchers, to further illuminate the evolutionary history of subsurface life. Onstott is seeking further proof that at least some bacteria can exist on just hydrogen gas produced within the rock, apart from any possible organics.
He also wants to see whether microbes actually live within the mineral structure of the solid rock - the porosity is minimal, a mere one percent Ė or reside only in fractures that lace through the rock formation. He intends to profile cores taken out of rock at the face of fluid-filled fracture zones "to see whether or not there is a change in the amount of biomass that might be trapped inside the rock" as one looks more deeply within the solid rock, away from the fracture surfaces. So far, Onstott says, the evidence for microbes inside the rock itself is tenuous, as the amount registered is at the lower detection limits of the measurement techniques.
Field research will also start to explore new sites, including a two-billion-year-old rock formation, the Bushveld complex, several kilometers thick. Onstott says the type of rock that makes up the Bushveld complex, known as mafic rock, "provides us an opportunity to look at the water-rock interaction" in a rock with a different chemical composition than that of the granite in the gold mines.
Unlike the Columbia River basin and the gold mines in South Africa, in the Bushveld complex there is no possibility of sedimentary relicts that could bear organics generated by photosynthesis, says Onstott. "We are able to access pristine water there," through existing platinum mines, "so weíll be able to observe whether the hydrogen is being generated and whether that hydrogen is being utilized" to create organic fatty acids, Onstott says. "Geochemically [the Bushveld complex is] more analogous to what we might expect to find in the subsurface of Mars."