Extremophiles: caves rank among earth's most amazing natural wonders. And they brim with some of the planet's strangest life-forms. (Caves/Microbes).

EXTREMOPHILES refers to microbes that thrive in extreme places. But the word might just as well apply to cavers who go anywhere to find such life-forms, like this ice cave in Greenland.

THE ROPE IS THEIR ONLY LIFELINE. CAVE EXPLORERS NANCY Holler Aulenbach and Hazel Barton Rappel (descend by rope) into an ice shaft, plunging 152.4 meters (500 feet) into frigid waters. They've rappelled into hundreds of caves--but never in Greenland and never an ice cave.

With ice picks in hand and sharp metal spikes called crampons strapped to their insulated boots, the cavers' descent is painstaking. One ill-placed spike can shatter the ice. "The sound is like a shotgun exploding in your face," says Aulenbach. What are they doing here? They're on scientific mission to map glacial caves and probe the ice for extreme forms of life--while their adventures are filmed for the IMAX movie Journey Into Amazing Caves.

Until recently, places like glaciers (moving ice fields) and pitch-dark caves seemed too inhospitable for life. Now, thanks to fearless explorers and powerful electron microscopes, scientists know that most caves teem with invisible life. These extremophiles are microscopic organisms that thrive in places too cold, too salty, too dark, too hot--too extreme--to support human or plant life. Scientists suspect extremophiles may even play a role in speleogenesis, or how caves form. And new research suggests these bugs might one day help cure diseases, break down toxic waste, even reveal the origins of life on Earth.

LIFE BELOW ZERO

SOME OF EARTH'S MOST ISOLATED caves lurk in the vast, moving ice fields that sprawl over Greenland. These glaciers reach a thickness of 3.2 kilometers (2 miles) and are thousands of years old. Most of Greenland is north of the Arctic Circle, yet during the brief summer, temperatures rise above freezing. Meltwater rivers, or freshets, rage over the ice and plunge into deep cracks, or crevasses, carving caves in the ice cap. "Here, ice caves form then disappear quickly,' says Aulenbach. "It's like geology at warp speed."

Since ice caves evolve quickly, a different set of caving rules applies than in rock caves. "In a rock cave, you wouldn't touch the formations," Aulenbach explains. But in an ice cave, it's vital to knock off icicles. "You don't want to be stabbed to death by a falling one."

As the explorers ease down the rope, blueness engulfs them. The whole cave glows blue because ice reflects the blue end of the light spectrum in sunlight. "It's like being inside a blue cathedral," Barton says. The deeper the cavers go, the older the ice. Alternating layers of white and blue ice make up the glacier: White layers form from winter snow, blue from surface ice that melted in summer and refroze.

Gummy Microbe

About 18.3 m (60 ft) down, Barton spies algae (plantlike microorganisms that produce their own food) in the 200-year-old ice. With sterile tools, she chops out a sample and seals the ice in a sample tube so she can study it under the microscope later. Earlier, on the surface, she sampled a gooey puddle and found a tardigrade, a microscopic animal resembling a gummy bear with a tubular mouth for sucking algae (see photo, bottom right). Tardigrades are extremophiles that can boast life spans of 200 years! "They've evolved to survive repeated freezing," Barton says.

Tardigrades produce a kind of "antifreeze" that prevents cell proteins (substances essential for growth) and membranes from breaking down even after years at subzero temperatures. The question is--how? Barton might begin to answer that when she returns to the University of Colorado's Pace Lab. Her scientific mission: "Looking at extremophiles for new disease-fighting medicines."

Barton has already found hundreds of new species so weird they're not even named--or identified on the Pace lab's massive "tree of life" chart. "Many extremophiles have natural lethal weapons that they use against each other in the fight for food," Barton says. "Our hope is to isolate an organism with such a weapon, to use against like tuberculosis or cancer." In fact, medical researchers have discovered extremophiles in New Mexican caves that spew chemicals to keep other extremophile species out of the way. Those chemicals attack and kill any foreign cell--including cancerous leukemia cells.

MEXICO'S HIDDEN WONDER

FROM THE ARCTIC circle to tropical jungles, Barton and Aulenbach trek the globe in search of amazing caves. "What drives me is the unknown," Aulenbach says. "What's around the next shadow?"

In that spirit, Barton squirms into scuba gear and plunges into a hidden pool in a remote Mexican jungle. At the bottom of the pool, or cenote, a tunnel snakes into a pitch-dark network of connected caves called Dos Ojos, the world's third-largest underwater cave system. The submerged caverns have lured hundreds of inexperienced cavers to their deaths.

Barton's headlamp lights the way into the silent depths. As she floats weightlessly, she unreels a white nylon diveline to mark the path, in case a diver accidentally kicks up blinding amounts of clay-like silt and can't see the way back to the cenote. Even veteran cave divers risk getting hopelessly lost and running out of oxygen.

Beneath the lush jungle foliage of Mexico's Yucatan Peninsula is rugged limestone terrain called karst. The karst formed millions of years ago when fossilized shells and skeletons of tiny sea animals piled up on the ancient ocean floor and cemented into limestone rock. Now, more than 500 km (311 mi) of known caves worm through the limestone. "It takes two things to make a cave in limestone--water and time," Aulenbach explains (see diagram, p. 16).

Watery Network

In the Yucatan, underground streams meander through the karst, continually re-carving the vast cave system. When the last ice age ended about 15,000 years ago, sea levels rose and seawater flooded the caves. The only way to enter them now: Dive into one of 1,700 known cenotes that dot the jungle.

Barton must avoid drifting into huge daggerlike stalactites hanging from the cave's ceiling, and spiked stalagmites jutting up from the cave floor. These cave formations, or speleothems, formed long before the cave flooded. As water trickles through limestone, it absorbs calcite, the main mineral in dissolved limestone. The water drips in the cave, and calcite particles stick to the walls; over millions of years, these particles amass, forming stalagmites and stalactites. This cave's speleothems are so odd that Barton wonders if something in the water has impacted their shape.

A clue shimmers in the distance. It's the halocline, an underwater zone where flesh water from subterranean streams meets--but doesn't mix with--salt water from the ocean. Barton hypothesizes that the halocline shelters rare microbes that exist only in this salty zone. "Extremophiles can live anywhere humans find inhospitable," she says. "But each species of extremophile is specialized to live in only one environment."

If extremophiles do thrive here, could they emit chemicals that affect the bizarre formations in the underwater cave? Could these microbes be of medical use? Barton's mind swims with questions as she fills sterile bottles with the elusive water--the first microbiologist ever to do so! "I travel all over the world doing this really cool stuff," she says. "But I have just as much fun in the lab processing samples and getting results. Microbiology is like exploration, you never know what you'll find."

RELATED ARTICLE: How caves form.

1 As rainwater passes through air and soil, it absorbs carbon dioxide gas to form carbonic acid, a weak natural acid, which seeps into limestone.

2 Over many years the acid dissolves the mineral calcium carbonate from the rock, slowly carving deep hollow caves. Sinkholes (or cenotes) form when underground streams weaken, then collapse the rock above.

3 As water trickles through the cave, calcite crystals, a form of calcium carbonate, stick to the walls. Over millions of years the particles amass, forming daggerlike stalagmites and stalactites.

RELATED ARTICLE: Hands-on science.

SODA-BOTTLE CENOTE (SINKHOLE)

See how surface water drains into the groundwater supply.

YOU NEED: two 2-liter plastic soda bottles * tape * scissors * plastic tube, same diameter as bottle neck and long enough to attach bottles * small rocks and sand * food coloring * bowl of water * measuring cup * aluminum foil

TO DO:

1 Bottle #1: Cut 6.5 cm off bottom. Tape to the side. Bottle #2: Cut 8 cm off top; set aside. Pour 6 cm of water into bottle.

2 Place #1 upside down into #2 (see diagram). Fit plastic tube in bottle neck. (For tight fit, wrap tube in foil.)

3 Pack stones into bottle #1. Cover with sand. Invert top of bottle #2 on sand. Fit tube in bottle neck; don't let it extend past neck. Tape.

4 Pour 1/4-cup (59 ml) water into cenote, or sinkhole. Observe.

5 "Pollute" the water. Add food coloring to 1/4-cup water and pour into sinkhole. Observe.

CONCLUSIONS: What happens when polluted water seeps down a sinkhole? How does this affect the groundwater--our drinking water supply?

Cross-Curricular Connection

Social Studies: Responsible cave explorers have a motto: "Walk softly and leave no trace." Find out more about cave codes of conduct and laws at www.caves.org/section/ccms and www.umr.edu/~spelunk/cavecons.html. Report on your research.

Did You Know?

* Anchialine (meaning "near the sea") is the word for flooded coastal caves, like those in the Yucatan Peninsula. In these perpetually dark caves, eyes are useless, so most fulltime cave dwellers (troglobites) lack eyes.

* Karst terrain is at high risk for groundwater pollution. Since karst is riddled with holes from dissolved rock, the terrain doesn't filter water. Pollution can easily seep into the groundwater supply through karst.

* A snottite is a slimy, hanging formation colonized by sulfer-eating bacteria. A soda straw is a hollow stalactite. Bacon is thin, curtain-like rock with dark and light streaks.

[CHART OMITTED]

Extremophiles

Directions: Match the word(s) in the left column with the correct phrase
in the right column.

__1. algae         a. underwater zone where fresh and salt water meet
                      but don't mix

__2. stalactites   b. formation protruding from cave floor

__3. stalagmites   c. meltwater rivers

__4. speleothems   d. cave formations

__5. halocline     e. plantlike microorganisms that produce their own
                      food

__6. crevasses     f. deep cracks

__7. freshets      g. hanging cave formation

Extremophiles

1. e 2. g 3. b 4. d 5. a 6. f 7. c

National Science Education Standards

Grades 5-8: structure of the Earth system * Earth's history * diversity and adaptations of organisms * populations and ecosystems

Grades 9-12: the origin and evolution of the Earth system * geochemical cycles * the behavior of organisms * biological evolution * natural and humaninduced hazards

Resources

Exploring Caves: Journeys into the Earth by Nancy Holler Aulenbach and Hazel Barton with Marfe Ferguson Delano (National Geographic Society, 2001)

"Extremophiles--Life at the Edge" by Carol Stone, ChemMatters Magazine, December 1999

Learn about Hazel's biomedical research at www.hazelsbugs.com