THERMAL VENTS ARE ECOSYSTEMS FAR FROM HUMAN INFLUENCE

by Whit Gibbons

June 16, 2003

When I spent a year as a visiting scientist at the Smithsonian's Natural History Museum, the corridor I took to my office passed an always-open office with jars of what looked like giant earthworms more than six feet long. The office and worms belonged to a discoverer of one of the strangest ecosystems known on Earth. Although research has been pursued on the worms and their habitat, the basic ecology I wrote of at the time remains intriguing. The habitat, known as a hydrothermal vent community, is more than one and half miles beneath the ocean's surface.

A hydrothermal vent is an area where a major fissure occurs between plates making up the earth's crust. As the plates gradually separate, underlying volcanic activity reaches the surface. As the molten volcanic rock encounters ultracold seawater, the physical and chemical reactions are impressive. Even more remarkable was the discovery that these vents were the habitat of deep-sea animals formerly unknown to science.

Among the notable sea creatures are enormous, bright red tube worms. Some are as thick as a child's arm and twice as long. The tube worms stretch up from the ocean floor in clusters, waving like the tendrils of a huge organism from a science fiction movie.

The world's species are disappearing and natural habitats are being eliminated under the assault of modern technology. Ironically, these mysterious communities would not have been revealed without it. In 1977 this new habitat was discovered in the Pacific Ocean during deep-sea exploration using submersibles that can withstand the tremendous pressure of tons of ocean water.

Other natural communities on Earth ultimately receive their energy from the sun. Even animals that live in caves or more conventional deep-sea habitats depend on organic materials from sunlit regions above or beyond. The debris they rely on for food had its origin in green plants.

But in the vent habitats, bacteria form the base of the food chain. The bacteria acquire their energy from chemical sources in the seepage area itself. In sharp distinction to virtually all other life we know, the hydrothermal vent communities function without dependency on the sun.

The submersibles have extendible devices for picking up items, but many of the animals they are tracking are mobile and have never been captured. Hence, some species are still undescribed. Vent communities have also been discovered in the Gulf of Mexico and the Atlantic, and some of these have species not previously observed. Some of the vent communities in the Gulf are not volcanically heated. Instead, they are sulfide seepage areas in which the temperature is no warmer than the typical deep-sea environment. The sulfides serve as the chemical energy source.

One puzzle for ecologists is how the species inhabiting the vent communities manage to persist, since a vent may remain active for a few decades at most. Where does a giant clam or tube worm that depends on the nutrients in a small area of deep ocean go when the energy source disappears? Except for the fishes, most of the organisms are slow-moving at best.

Presumably, each species has a reproductive strategy in which larvae disperse into the outer blackness beyond the vent. Most probably die in the ocean depths, but some eventually reach other vent habitats. Just as any vent eventually ceases to exist, new ones are constantly being formed, setting the stage for colonization. The lifestyle is a precarious one, but it apparently works, as evidenced by the similarity of species composition from the Gulf of Mexico to the Pacific Ocean.

Twenty-five years ago, the idea of communities of numerous, sometimes large, animals living around deep-sea hydrothermal vents would have sounded like something from Jules Verne. In fact, the first reports seemed too fantastic to be believable. Paradoxically, although these appear to be among the most fragile and sensitive ecosystems in the world, they would rank at the top of ones least likely to be destroyed by humans. Simply studying them is job enough. These bizarre habitats also make the important point that in order to explore the mysteries of the universe, we need not look only to outer space. Plenty of opportunities for discovery are right here on Earth.



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