Primitive Life Thrives in Ancient Cave
Ecosystem relies on chemical reactions, not sunlight, for food
BOSTON — Bizarre cave creatures and colonies of bacteria the consistency of wet tissue are giving scientists insights on how primitive life may have formed on Earth. And they are catching the eye of at least one NASA engineer researching ways to seek signs of life beyond Earth.
For two decades, biologists have studied unique deep-sea communities that rely on chemical reactions, not sunlight, for the energy that produces their food.
During the past few years, a team of researchers from the University of Cincinnati in Ohio has been exploring the only known fresh-water ecosystem to get its energy the same way. Located deep in a Romanian cave, the ecosystem - from microscopic bacteria and fungi to pill bugs, spiders, and centipedes - has spent at least 100,000 years cut off from the most tenuous nutrients based on photosynthesis.
The ecosystem's significance for space research may stem from its independence from sunlight as an energy source. Conditions on the surface of other bodies in our solar system are extremely harsh. Exobiologists suspect that if organic life exists or existed in our planetary neighborhood, evidence would be found below the surface.
The crawl-space tunnels, small chambers, and subterranean lake that form the Movile Cave - some 800 feet long and 160 feet below ground - were discovered in 1986, while construction crews dug wells for nearby towns.
Located near the Black Sea, the limestone cave became a haven 10 million years ago for some of the region's tiniest inhabitants. They were seeking refuge from one of the planet's many dramatic climate changes, according to Thomas Kane, a cave biologist from the University of Cincinnati and a member of the team studying the cave. At the time, he says, the tropical climate in the area cooled and dried rapidly. The Black Sea became little more than a salty lake.
The cave, by contrast, provided the warm, humid conditions the creatures craved. Water heated deep within the earth welled up, forming a lake whose temperature is a Miami Beach-like 21 degrees C. (70 degrees F.) About 5 million years later, glaciers began to ebb and flow, refilling the sea and leaving layers of material behind that sealed the cave. Some 100,000 years ago, the material atop the cave had grown so thick that its water supply came exclusively from deep underground instead of seeping directly from the surface. This cut the cave's inhabitants off from even highly diluted nutrients based on photosynthesis, Dr. Kane says. He says he suspects that much of the organisms' evolution has occurred during the past 100,000 years.
When the Movile Cave was discovered, it quickly drew the interest of Romanian scientists, including cave biologist Serban Sarbu. But political upheaval halted the research. In 1988, Mr. Sarbu was forced to flee the country, Kane says. The two men joined forces following a meeting of European cave researchers in Hamburg, Germany, that year.
"Serban was working in New York City as a carpenter at the time," Kane recalls, and was invited to the meeting discuss some of his early work at the site. At Kane's urging, Sarbu enrolled at the University of Cincinnati the next year to earn his doctorate.
With the overthrow of the Ceaucescu regime, work at the site resumed with help from the Romanian Academy of Sciences. Since then, the team, which also includes University of Cincinnati microbiologist Brian Kinkle, has conducted extensive studies at the cave.
Climbing down a 50-foot cable ladder, crawling through muddy passages, and diving into a subterranean lake, the team found 48 species of plants and animals that rely on so-called chemoautotrophic processes as the basis for their food chain.
Topside, solar radiation provides the energy plants use to convert carbon dioxide (CO2), inorganic minerals, and water into carbon-based nutrients such as carbohydrates. Other life forms then use these nutrients as food.
In the Movile Cave, grayish mats of bacteria and fungi cling to walls and cover the surface of the water in air pockets. They use chemical energy from a reaction that combines oxygen and hydrogen sulfide to form sulfuric acid. The acid is largely responsible for forming the caves. Methane and CO2 also are plentiful, providing the carbon needed to form the carbon-based nutrients the cave dwellers use.
It's this process that could interest researchers considering early life on an oxygen-poor Earth, as well as the prospect of life elsewhere in the solar system. During last week's briefing on the Galileo mission to Jupiter, Arizona State University geologist Ronald Greeley noted that exobiologists look for three conditions as they seek life-supporting environments: liquid water; heat sufficient to support biological communities; and organic material. Fresh images of the moon Europa, for example, strongly hint at processes on the satellite that are generating sufficient heat to turn the moon's icy crust to slush, if not water, about 1 kilometer (0.62 miles) below the surface.
Indeed, Kane says, at least one engineer from NASA has expressed interest in what the team is finding at the cave as the agency designs its unmanned missions to Mars. "Is there heating from the Martian core? Can you detect hydrogen sulfide? These are some of the signals they might look for," Kane says.
Like many of their counterparts in other caves, Movile's higher creatures are blind and lack coloring. One species of spider spins no web and has no eyes, but uses smell and sound to find its prey. A new species of blind leech swallows its prey whole, rather than sucking blood from it.
Using DNA-sequencing techniques, Dr. Kinkle has been able to date some of the creatures by comparing their genetic structure with those of distant relatives above ground. One cave species, a relative of the pill bug, hasn't seen the sun for 1 million years. Another, a species of water scorpion, dates back to between 2 million and 5 million years ago.
The team now hopes to extend its research from "who eats what" to a detailed analysis that compares the amount of energy produced through the chemical process in the cave with that of photosynthesis and other chemo-autotrophic processes, Kane says. In addition, Kinkle adds, more needs to be done to count species and quantify the diversity of life present in the cave.