Planet Earth

Deep Sea Rebirth

By Shanti MenonJul 1, 1997 5:00 AM


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In April 1991, under a mile and a half of water, three researchers inside the submersible Alvin were surveying the East Pacific Rise, an undersea volcanic ridge 500 miles southwest of Acapulco, when their sub skirted a cauldron of 400-degree smoky water littered with charred worms and mussels. The research team had stumbled upon the obliterated remnants of a unique ecosystem. Over the next six years they would also have a chance to witness its rebirth.

Richard Lutz, a marine ecologist from Rutgers University in New Brunswick, New Jersey, who was aboard the surface ship accompanying Alvin that day, has since returned to the East Pacific Rise several times. He is the first to document, from time zero, the growth of the peculiar life that develops exclusively along the underwater volcanic fissures called hydrothermal vents. Unlike almost all other life on Earth, vent creatures don’t ultimately depend on sunlight and photosynthesis for their survival. The microbes that form the base of the vent food chain live off mineral- rich water percolating out of the fissures.

Specifically, the vent species depend on hydrogen sulfide dissolved in hot water issuing from the fissures. Hydrogen sulfide is toxic to most species, but some microbes have learned how to use it to fuel their metabolisms. Immediately after the 1991 eruption, Lutz saw thick mats of microbes around the vents. The bacteria, researchers believe, normally live beneath the seafloor and are tossed out when a vent erupts.

A year later this bacterial bloom had attracted assorted grazers: crabs, eel-like fish, and limpets. Small tube worms had also turned up, probably having traveled as larvae from another vent. Mature worms are immobile and anchored to the seafloor. With no guts or mouths, they can’t eat and must acquire sulfur-loving bacteria from seawater. The bacteria produce life-giving energy for the worms and in return have a place to live.

By December 1993, the small tube worms were being replaced by giant tube worms. The smaller worms may appear first because their larvae can better tolerate high hydrogen sulfide concentrations. Once those levels drop, the giant tube worms--which grow up to six feet high--move in. Elongating by one or two millimeters each day, they’re the fastest growing marine invertebrates on the planet.

But this phenomenal rate of biologic growth is surpassed by geologic growth. What had been flat terrain 18 months earlier was now the site of 30-foot-high chimneys. These towers--which had been thought to take decades to form--consist of metallic sulfides that precipitate out of mineral-laden hot water on contact with the cold deep sea.

In October 1994, Lutz saw mussels and small worms called serpulids surrounding the vents. Both of these animals filter food from the water. A year later, the mussel population had expanded and encroached on tube-worm turf, even fixing themselves to tube worms instead of the seafloor. Lutz expects the mussels will soon drive out the worms. Curiously, giant white clams, long thought to have been a dominant and characteristic vent species, have not yet appeared. They must be a late- stage colonizer, indicative of an old hydrothermal field, says Lutz. He expects the clams will have arrived by his next visit this fall, although no one knows how these large creatures, which also depend on symbiotic bacteria for energy, travel from vent to vent.

Lutz says the number of species at the vent more than doubled between his last two visits, from 12 species to 29. Such growth outstrips anything observed on land. On Surtsey, a new volcanic island off the coast of Iceland, only 16 species were recorded after 12 years.

Though the researchers expect to see continued growth on their next visit, what they’re really hoping for is devastation. This would let them see if the colonization pattern repeats. Every time I go back I’m thinking, ‘Please, let it have erupted; please, let it have erupted,’ says grad student Tim Shank. Although I have become attached to some of the things down there.

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