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Life Without Light

by Kirsti Ritalahti

On the deep sea floor, where no sunlight penetrates the thousands of meters of water overhead, life has found a rare, ephemeral environment in which to flourish. It is a world hidden from our view in which the animals have adapted to a way of life only imaginable in a wild science fiction tale. It is a place which is never touched by a ray of light, other than that of the submersible submarines which first encountered them two decades ago. This environment is different from any other on earth, a consequence of the forces responsible for the spreading of the continents.

In both the Pacific and the Atlantic Oceans, there are places where hot magma under the surface of the sea floor causes cracks in the Earth's crust. Sea water seeps into these holes, only to be forced out as mineral rich, warmed geysers into the cold, oxygen rich, deep ocean water. These unique springs are known as hydrothermal vents.

It was in one of these seeps at the Galapagos Rift off the west coast of South America, that Alvin, the tiny submarine, encountered a spectacular site. In front of the porthole were thousands of pale, long tubes, topped by a bright red plume. Like thick-stemmed tulips up to 3 meters long, these long tube worms swayed in the turbulence of the never before encountered spring. As Alvin scouted the area, it came upon a field of giant white clams; wide mouths open to the currents, padded feet embedded in the sediments. Teeming with curiosity, scientists asked the question: What is the food source that drives this ecosystem?

Their search revealed some surprising results. The water springing from the vents is rich in hydrogen sulfide, the same compound which gives rotten eggs their distinctive odor. Although not a very attractive source of food, some tiny free-living bacteria located in the region encircling the vents think otherwise. In a process akin to the way in which green plants on land use the power of sunlight for photosynthesis, these bacteria utilize the hydrogen sulfide emitted from the vent as a source of energy to convert carbon dioxide from the surrounding sea water into sugars that it needs for growth.

The tulip-like worms, however, proved even more astounding. These animals, Riftia pachyptila, have no mouth or gut. No way to eat food, including bacteria, from their surroundings. Instead their red plume rimmed with blood vessels reaches into the vent waters. Oxygen, carbon dioxide and hydrogen sulfide bind to carrier molecules in the blood. As the heart pumps, these three compounds are delivered to the trophosome, a very specialized tissue. A unique structure, the trophosome contains densely packed bacteria, similar to the free living forms found in the surrounding water. In other words, within the body of the tube worm is a farm, well tended, and well worth it. For in return for a stable environment in which to live, and a constant source of energy and carbon, the bacteria provides Riftia with all the organic compounds that it requires for growth. This association of two different living entities is a symbiosis. Both organisms are wholly dependent upon the other for survival. Alvin encountered an ecosystem quite different from the ones in the sun-bathed world above the ocean floor. A place where tube worms and clams provide housing, and the bacteria pay rent.

The hydrothermal vents are oases teeming with life on the dark sea floor. Each vent is transitory in the life of the planet; new ones are born as old ones die. They are distant and isolated from one another by hundreds of miles. Yet there are similarities among the communities surrounding these sites. It is likely that for as long as the geological forces that formed the earth continue, the prolonged success of these symbioses is almost indefinitely insured.

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