Life at Deep-Sea Hydrothermal Vents 

Deep below the surface of the oceans, beyond the levels reached by sunlight, live abundant and diverse communities of animals. Tiny microbes or bacteria called Archaea, fantastic red-tipped, white tube worms, gigantic clams, mussels, crabs, shrimp, starfish, and deep-water skates all live at deep-sea hydrothermal vents. All of these animals depend on a food chain that does not rely directly on the sun or on photosynthesis (the production of food by plants using sunlight), as most other life on Earth does. Instead, the tiny Archaea, which form the basis for this unique food chain derive energy and nutrition from the hot, mineral-rich waters venting from the sulfide structures. The Archaea  use sulfide instead of sunlight to create food, a process analogous to photosynthesis called chemosynethesis .

Autotrophs are organisms that get energy directly from sunlight. These include green plants. Archaea, in contrast, are chemoautotrophs. They get their energy by oxidizing the minerals in the black smoke. Most animals on Earth are heterotrophs, which means that they get their energy by eating autotrophs, chemoautotrophs, or other heterotrophs. Humans, cats, dogs, tube worms, and clams are all heterotrophs.

In 1977, scientists were totally surprised to discover dense communities of animals living at deep-sea hydrothermal vents. In fact, the very idea of communities of large animals based on the process of chemosynthesis rather than photosynthesis was beyond the imagination of most people. Since their discovery, biological research has resulted in a good basic understanding of the physiology and ecology of these hydrothermal life forms. More than three hundred species of vent animals have been identified, and the list grows with the discovery of every new vent. There is still a great deal to learn, and this expedition, which combines the expertise of biologists and geologists, will provide key observations that will advance our understanding of these animals and ecosystems.

Life in the Extreme 
A key characteristic of the bacteria that support the food chain at deep-sea hydrothermal vent communities is a tolerance for high temperatures that is much greater than ours. Some species of vent bacteria can exist at temperatures as high as 110°C! This is why these bacteria are called thermophyllic or heat-loving. Some of the larger animals also are observed to live at warm temperatures. For example, the temperatures inside some tube worms, built directly on vent chimneys, have been measured at 40°C, and another kind of worm, the Pompeii worm, has been photographed leaving its tube and swimming near a temperature probe that recorded 110°C. These temperatures are wildly extreme by any standard for marine life. We don't know how the Pompeii worm can withstand such high heat. Other vent animals live on chimney walls but not at the hottest spots. Instead they are bathed in warm water that is the product of mixing between the hot waters from the chimneys and the cold ocean bottom water.  

Do these animals really exist totally independent of the sun?  Not really. Although these vent communities don't depend directly on sunlight for their energy and nutrition, the sun does play a critical role. The bacteria require oxygen to synthesize organic matter from the inorganic compounds present in the vent fluids. This oxygen, which is present in abundance throughout the ocean, is a waste product of photosynthesis, which uses the sun's energy. 

The Three Best-Understood Deep-Sea Hydrothermal Vent Life-Forms

Tube worms: Vestimentiferan Tube Worms are one of the most abundant and best-understood of the animals that live in deep sea hydrothermal vent communities. Baby tube worms are called larvae, and they can swim. Adults, on the other hand, are sessile which means that they stay in one place and are attached to the ground beneath them. Adult tube worms have no mouth, gut, or anus, and thus they do not eat, digest, and eliminate waste the way that we do. Instead the tube worms rely on bacteria living within them (symbionts) to provide them with their nutrition. The distinctive bright red plume or tip of the tube worm is the only part of the worm's anatomy that is exposed to the environment, and it can be fully withdrawn into the protective tube when the animal is disturbed. Tube worms use this plume to take up sulfide and oxygen from vent waters. The sulfide and oxygen are transported, along with other nutrients, by the blood of the tube worm to the bacteria living within the worm's tissue. These bacteria combine these nutrients and chemicals into food for themselves and their host tube worm.  

Clams: Vescomyid Clams  are deep-sea hydrothermal vent cousins to the clams that we're all familiar with. Although these chalky white clams retain some ability to filter-feed, like the tube worms these clams rely on their bacterial symbionts that live within their gills for the major source of their nutrition. The symbionts receive nutrients and oxygen, which flow into the clam through the uppermost end of the shell, and sulfide which is incorporated through the bright red foot that can be extended several times the body length of the clam down into the sulfide-rich vent water below their shells.

Mussels: Bathymodiolid Mussels  are deep-sea hydrothermal vent cousins to the mussels that we're all familiar with. Although better filter feeders than the deep-sea clams, like the tube worms these mussels also rely on bacterial symbionts that live in the cells that cover the surfaces of their gill filaments for some of their nutrition. Unlike the clams and the tube worms, these mussels harbor more than one type of bacterial symbiont. The combination of an enhanced ability to filter-feed and the presence of multiple types of bacterial symbionts enables these mussels to survive farther from the direct sources of vent water relative to the clams and the tube worms.


© 1997, American Museum of Natural History