Geothermal deep-ocean vents

Geothermal deep-ocean vents are undersea hot springs that occur in clusters along the mid-ocean ridges . Nutrients and energy supplied by vents support communities of deep-sea organisms found in no other environment.

Most deep-ocean vent action is powered by the heat of the same bodies of magma that drive sea-floor spreading . A vent forms when seawater seeps downward through cracks in the flanks of a mid-ocean ridge to depths of 1.25–2 mi (2–3 km), about halfway through the thickness of the oceanic crust ,

and is there heated to 750–840°F (400–450°C). This super-heated water then re-ascends to the center of the mid-ocean ridge and emerges as a fast jet at about 350°C.

As this hot jet mixes with cold (34–36°F [1–2°C]) ocean water, hydrogen sulfide conveyed in solution from the deep rocks precipitates instantly, often coloring the jet black. Such jets are termed "black smokers." Hydrogen sulfide is usually poisonous to life, but the specialized communities around black smokers could not live without it.

A vibrant community of bacteria, tubeworms that are unique to the geothermal vent environment, and other creatures exists around hydrothermal vents. The entire ecosystem is possible because of the activity of the bacteria. These bacteria have been shown, principally through the efforts of the Holger Jannasch (1927–1998) of Woods Hole Oceanographic Institution, to accomplish the conversion of sulfur to energy in a process that does not utilize sunlight called chemosynthesis. The energy is then available for use by the other life forms, which directly utilize the energy, consume the bacteria, or consume the organisms that rely directly on the bacteria for nourishment. For example, the tubeworms have no means with which to take in or process nutrients. Their existence relies entirely on the bacteria that live in their tissues.

Sulfur-oxidizing bacteria pervade the waters around the vents and live symbiotically in the tissues of certain species of animals unique to the vent environment, including 5 ft (2 m) red-blooded worms. These bacteria derive energy by oxidizing the sulfur in hydrogen sulfide, and derive carbon from carbon dioxide dissolved in the seawater. The bacteria depend directly on the vent and all the other organisms in the vent's vicinity—including over fifty species of clams, mussels, crabs, worms, tube-dwelling worms, and sea anemones not found in any other environment—depend, directly or indirectly, on the bacteria.

Deep-sea vents and their associated fauna were unknown until the late 1970s. They have aroused keen interest for several reasons. First, they demonstrate that life can thrive on energy from purely geothermal sources, isolated from solar energy . This suggests the possibility of life in places in the solar system where liquid water and heat are available but sunlight is not, such as under the ice-encrusted oceans of Jupiter's moon Europa. Second, some biologists believe that some vent species are living fossils or survivors from earlier periods of geologic time . Third, some molecular biologists have theorized that complex chemical reactions in and around ancient deep-sea vents may have synthesized amino acids and other organic molecules key to the spontaneous origin of life .

Vents remain an active research topic. In 2001, a new class of deep-sea vents was discovered that derives its energy not from volcanic magma but from a chemical reaction between seawater and the rocks of the upper oceanic crust.

See also Deep sea exploration; Geothermal gradient; Mantle plumes; Ocean trenches