Scientists are looking to Earth's nastiest habitats for clues to life on other worlds.
Searing hot water, deep-sea pressure, freezing polar wastelands, or lightless mile-deep rock represent the extremes of Earth's habitable environment. But such areas may be the only living space available for life forms that may have gained a toehold on Mars or one of Jupiter's moons.
Realizing this, the United States National Science Foundation has launched a $6 million program to fund research into the life-on-the-edge biology of these "extremophiles." The agency also is working with the National Aeronautics and Space Administration (NASA) to develop a program to probe Antarctica's Lake Vostok.
The lake lies 2.5 miles beneath the ice at Russia's Vostok station. NASA calls it a Europa simulator because it resembles conditions on the ice- and water-covered jovian moon. It seems an ideal place to see if life on earth can thrive in such a habitat and to test techniques for probing Europa's ice-covered sea.
Looking ever deeper
Biologists have long been aware of a few hardy microbes that live in hot springs or icy environments. But a wave of discoveries over the past 10 to 20 years has opened up a major new area of biological exploration - and made it clear that scientists have yet to determine environmental limits on earthly life.
"It tells me that life is much more extensive than we thought it was," says geochemist James McKinley at the Pacific Northwest Laboratory, which the Battelle Memorial Institute operates for the Department of Energy in Richland, Wash.
Dr. McKinley is part of a DOE Subsurface Science Program team that has been studying bacteria living 3,300 feet deep in the basalt rock that underlies the Columbia Basin in southwestern Washington. There, chemical reaction between the basalt and ground water generates hydrogen. Microbes then use this hydrogen as an energy source rather than sunlight. McKinley's colleague Ray Wildung says that they have appeared to have discovered a "fully functioning ecosystem" that has "evolved entirely on its own over the past 10 million years."
Dr. Wildung notes that biologists had thought that there were few if any microbes much below the root zone. The new discoveries show that microbial living space extends thousands of feet deep. We now see that subsurface life "could constitute more biomass than what is on the surface," he says.
Thriving on heat
When it comes to heat tolerance, the upper limit isn't yet in sight. A German-American team recently reported finding a microbe of the type called archaebacteria that thrives at 113 degrees C (235 F.). That's the hottest microbe yet found. It lives in a "black smoker vent" 5,400 feet deep on the mid-Atlantic Ridge southwest of the Azores. A plume of volcanically heated water darkened by metal sulfides supports the microbes. They, in turn, support a community of larger organisms.
Team member Holger Iannasch at the Woods Hole Oceanographic Institution in Massachusetts says "we don't really know what determines the [eventual upper] temperature" for life. He adds that scientists also have "no real explanation" for how proteins function at such high temperatures. This has practical as well as scientific importance.
Even though scientists don't fully understand how extremophiles do it, their ability to withstand what we consider punishing environments means they have enzymes useful for industry. Enzymes are proteins that facilitate chemical reactions. Dr. Iannasch notes that some of these will probably be useful for laundry applications, where they could condition fabrics and remove many kinds of stains. Enzymes used now can't tolerate really hot water.
Picking up on this theme, microbiologist Michael Madigan at Southern Illinois University at Carbondale says that "it's likely" that extremophiles contain many "molecules of use to humanity."
He explains that the relevant genes from these organisms can be put into ordinary microbes that are easier to handle. Then these genetically engineered microbes can make extremophile proteins with a wide range of uses.
The Energy Department is winding down the subsurface program that discovered the deep-living bacteria. It has yielded useful information on the role microbes may play in possible corrosion in underground radioactive waste storage facilities and in the possible underground transport of radioactive waste.
Meanwhile, the fact that life thrives in such challenging environments on Earth adds credibility to scientists' hopes of finding life on other worlds.
"This is a turnaround in how we view life in the solar system," observes Princeton University geochemist Tullis Onstott, a participant in the subsurface program.