The three-person submersible Alvin sank through the cold, dark waters of the Pacific Ocean for more than an hour, finally touching down on the sea floor more than 8,000 feet below the surface. It was December 1993, and the scientists inside the sub had come to this stretch of the East Pacific Rise, an underwater mountain range about 500 miles southwest of Acapulco, Mexico, to inspect a recently formed hydrothermal vent—a fissure(裂缝) in the ocean bottom that leaks scalding(滚烫的), acidic water.
Peering out through the sub's tiny windows, the visitors were astonished to see thickets of giant tube worms, some four feet tall. The tail ends of the worms were firmly planted on the ocean floor, while red plumes on the other ends swayed like a field of poppies. Alvin had brought researchers to the same spot less than two years earlier, when they had seen none of these strange creatures. Previous measurements showed that individual tube worms could increase in length at a rate of 33 inches per year, malting them the fastest-growing marine invertebrates. That means tube worms can grow more rapidly than scientists once thought.
The giant tube worm is one of the most conspicuous members of a diverse community that forms around hydrothermal vents. Scientists once thought that no living thing could survive the harsh combination of toxic chemicals, high temperatures, high pressures, and total darkness at these vents. But in 1977, researchers diving in Alvin discovered tube worms and other bizarre organisms thriving at a vent off the Galapagos Islands. Similar communities have since been found at several hundred hot spots around the world. These creatures are like nothing else on Earth.
Vents form. where the planet's crystal plates are slowly spreading apart and magma(岩浆) is welling up from below to form. mountain ranges known as mid-ocean ridges. As cracks form. at these spreading centers, seawater seeps a mile or two down into the hot rock. Enriched with minerals leached from the rock, the water heats and rises to the ocean floor to form. a vent. Vents are usually clustered in fields, underwater versions of Yellowstone's geyser(间歇泉) basins. Individual vent openings typically rage from less than a half inch to more than six feet in diameter. Such fields are normally found at a depth of more than a mile. Most have been discovered along the crest of the Mid-Oceanic Ridge, a 46,000-mile-long chain of mountains that wraps around Earth like the seams on a baseball. A few vents have also been found at seamounts(海底山), underwater volcanoes that are not located at the intersection of crystal plates.
Hydrothermal vents provide habitat for many creatures that are not found anywhere else in the ocean. Water pouring out of vents can reach temperatures up to about 400℃; the high pressure keeps the water from boiling. However, the intense heat is limited to a small area. Within less than an inch of the vent opening, the water temperature drops to 2℃. Most of the creatures that congregate around vents live at temperatures just above freezing. Thus, chemicals are the key to vent life, not heat. The most prevalent chemical dissolved in vent water is hydrogen sulfide(硫化氢), which smells like rotten eggs. This chemical is produced when seawater reacts with sulfate in the rocks below the ocean floor. Vent bacteria use hydrogen sulfide as their energy source instead of sunlight. The bacteria in turn sustain larger organisms in the vent community.
The clams(蛤), mussels, tube worms, and other creatures at the vent have a symbiotic relationship with bacteria. The giant tube worms, for example, have no digestive system—no mouth or gut. The worm depends virtually solely on the bacteria for its nutrition and both partners benefit. The brown, spongy tissue filling the inside of a tube worm is packed with bacteria—about 285 billion bacteria pe
A．Y
B．N
C．NG
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