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  3. Creatures of the Thermal (热量的) Vents The three-person submersible Alvin sank through the cold, dark waters of the Pacific Oc

Creatures of the Thermal (热量的) Vents The three-person submersible Alvin sank through the cold, dark waters of the Pacific Oc

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问题                               Creatures of the Thermal (热量的) Vents
    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 thisstretch 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 boiling, 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 researches 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, making 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 eye-catching 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 strange organisms thriving at a Vent off the Galapagos Islands. Similar communities have since been found at several hundred hot sots around the world. These creatures are like nothing else on Earth.
    Vents form where the planet’s crustal plates are slowly spreading apart and magma 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 basins. Individual vent openings typically range 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, underwater volcanoes that are not located at the intersection of crustal plates.
    Hydrothermal vents are underwater oases (避风港), providing 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℃, the ambient temperature of deep seawater. Most of the creatures that assemble 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 (硫酸盐) in the rocks below the ocean floor. Vent bacteria use hydrogen sulfide as their energy source instead of sunlight. The bacteria in turn sustain large organisms in the vent community.
    The clams, mussels, tube worms, and other creatures at the vent have a symbiotic relationship (共生关系) 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 per ounce of tissue. The plumes at the top of the worm’s body are red because they are filled with blood, which contains hemoglobin that binds hydrogen sulfide and transports it to the bacteria housed inside the worm. In return, the bacteria oxidize the hydrogen sulfide and convert carbon dioxide into carbon compounds that nourish the worm.
    Tube worms reproduce by spawning (产卵). They release sperm (精子) and eggs, which combine in the water to create a new worm. Biologists don’t know how the infant worm acquires its own bacteria. Perhaps the egg comes with a starter set. Scientists also don’t know how tube worms and other organisms locate new vents for colonization. The vents are small, and they are separated like islands. Most vent organisms have a free-swimming larval stage (幼虫期). But scientists aren’t sure whether the larvae float aimlessly or purposely follow clues such as chemical traces in the water--to find new homes.
    Studying the life cycle of vent organisms is difficult. Researchers have visited only a fraction of the ocean’s hot spots. They have been able to observe vent life only by shining bright lights on creatures accustomed to in inky darkness, and many samples die quickly when removed from their unique environment. Underwater cameras are helping scientists make less interfering observations, but diving expeditions are still the most useful way to gather information. The 1993 Alvin expedition to the East Pacific Rise was one in a series of dives to the area.
    The site was first visited in 1989, and scientists observed vent organisms thriving there. But when Alvin returned two years later, its flabbergasted occupants witnessed the birth of a hydrothermal vent. A recent volcanic eruption spread glassy lava across the ocean floor, and the researchers measured temperatures up to 403℃  the hottest ever recorded at a hydrothermal vent. The scientists dubbed the site Tube Worm Barbecue, because the worms they brought back to their ship had burned flesh.
    "The most spectacular sight down there was this massive blinding snowstorm of bacteria," says Rich Lutz, a marine ecologist at Rutgers University, who led the expedition. On the ocean floor, the bacteria formed mats several inches thick, but he scientists saw no other living things. Since the eruption, scientists have been able to watch several stages of colonization at the site. When they returned in March 1992, only a few bacterial mats remained. In their place were colonies of Jericho worms and a variety of small crustaceans. In December 1993, the scientists first observed the giant tube worms there. The scientists named the area Phoenix, because new life had arisen from the ashes of the eruption.
In year ______, there were many dead tube worms found in the East Pacific Rise according to the information from the expeditions.
选项
答案1991
解析 参见文中倒数第2段：“The site was first visited in 1989，and scientists observed vent organisms thriving there．But when Alvin returned two years later．．．the scientists dubbed the site Tube Worm Barbecue，because the worms they brought back to their ship had burned flesh．”综合最后一段可知：1989-flouring organism；1991-dead tube worms；1992- no tube worms；1993-giant tube worms。
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