Biologists have observed a variety of smaller crustaceans around vents, including miniaturelobsters called galatheids, and amphipods resembling sand fleas. They have also seen snail-like limpetsthe size of BBs, sea anemones, snakelike fish with bulging eyes, and even octopuses.
While octopuses are at the upper end of the vent's food chain, bacteria are at the bottom. They are the first organisms to colonize newly formed vents, arriving in a snowlike flurryand then settling to form white mats or tendrils attached to the ocean floor. Bacteria have been found living beneath the ocean's floor, and it seems likely that they emerge from below when the conditions are right. Vent bacteria can withstandhigher temperatures than any other organism. That makes them attractive to researchers who are developing heat-stable enzymes for genetic engineering, and culturing bacteria designed to break down toxic waste.
Water pouring out of vents can reach temperatures up to about 400 C; the high pressure keeps the water from boiling. However, the intenseheat is limited to a small area. Within less than an inch of the vent opening, the water temperature drops to 2 C, the ambienttemperature of deep seawater. Most of the creatures that congregatearound vents live at temperatures just above freezing. Thus chemicals are the key to vent life, not heat.
The most prevalentchemical dissolved in vent water is hydrogen sulfide, 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, mussels, tube worms, and other creatures at the vent have a symbiotic relationship with bacteria. The giant tube worms, for example, have no digestivesystem-no mouth or gut. “The worm depends virtually solelyon the bacteria for its nutrition,” says microbial ecologist Colleen M. Cavanaugh of Harvard University. “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. “It's essentially a bacterial culture,” says Cavanaugh.
The plumes at the top of the worm's body are red because they are filled with blood, which contains hemoglobinthat binds hydrogen sulfide and transports it to the bacteria housed inside the worm. In return the bacteria oxidizethe hydrogen sulfide and convert carbon dioxide into carbon compounds that nourish the worm.
Tube worms reproduce by spawning: They release spermand 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're separated, like island,” says Cindy Lee Van Dover, a biologist and Alvin pilot who studies vent life. Most vent organisms have a free-swimming larval stage. 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 fractionof the ocean's hot spots. They have been able to observe vent life only by shining bright lights on creatures accustomed to inky darkness, and many specimens die quickly when removed from their unique environment. Underwater cameras are helping scientists make less intrusive observations, but diving expeditionsare still the most useful way to gather information. The1993 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 in April 1991, its flabbergasted occupants witnessed the birth of a hydrothermal vent. A recent volcanic eruptionhad spread glassy lava across the ocean floor, and the researchers measured temperatures up to 403C-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 charred flesh.
“The most spectacular sight down there was this massive blinding snowstorm of bacteria,” says Rich Lutz, a marineecologist at Rutgers University, who led the expedition. On the ocean floor, the bacteria formed mats several inches thick, but the 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. The scientists named the area Phoenix, because new life had arisen from the ashes of the eruption.
The scientists first observed the giant tube worms at Phoenix in December 1993. They also noticed a number of mineral deposits, some towering to heights of more than 30 feet. These structures form where hot vent water meets cold seawater, causing metal sulfides to precipitateout. The precipitating sulfides, which look like smoke, amass to form chimneys called black smokers. Like the vent fields, some smokers have names. Smoke and Mirrors, for example, has shelf-like overhangs that trap hot water rising from below, creating upside-down shimmering pools. The largest known black smoker is Godzilla, a 160-foot-tall structure off the coast of Oregon.
During a December 1993 dive to the Phoenix vent field, Alvin accidentally toppled a 33-foot-tall smoker. When the sub returned for a brief visit three months later, the chimney had already grown back 20 feet. Scientists were surprised by the speedy recovery, which seems to parallelthe rapid growth of tube worms and other organisms at the vents. The visits to the Phoenix site “give us a sense of how quickly these vents are colonized,” says Van Dover.
Another expedition is planned for November. By then, the community of organisms now prospering at the vents may already be a ghost town. When the flow of hot, sulfide-rich water slows to a trickle, death also comes quickly.
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