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Hydrothermal Vent Communities
(Released May 2006)

 
  by Carolyn Scearce  

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Organism adaptations to vent environments

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For the mussels or giant tubeworms living on an actively venting chimney, environmental conditions vary dramatically from those experienced by organisms just outside the range of the vent's influence. Precipitating heavy metals and other toxic substances can literally rain down on nearby animals. Hydrogen sulfide, an essential component for the chemosynthetic processes that provide energy for vent communities, can have effects similar to cyanide on organisms not adapted to functioning at the high concentrations found at vent sites. Thermally adapted bacteria and archaea may live at temperatures in excess of 100 degrees C. It can take a great deal of adaptation to live in close proximity to hydrothermal vents. Many of the 500 + animals that have so far been discovered at vent sites seem to live exclusively in vent communities (Van Dover, 2000). To gain a better understanding of what it takes to live in a hydrothermal system, the physiology of some of the best studied vent organisms will be discussed.

clump
Riftia patchyptila

The giant tubeworm Riftia patchyptila, which can grow up to 2 meters long, offers one of the most extreme examples of specialization for hydrothermal vent environments. R. pachyptila colonies, which can be found at sites within the Galapagos Rift and the East Pacific Rise, prefer locations with high flow vents (Childress & Fisher, 1992). Vestimentiferans such as R. pachyptila live inside long narrow chitinous tubes that are permanently attached to the substratum. A retractable plume takes up nutrients from the external environment, but the worm possesses no digestive system (Van Dover, 2000). Its growth and metabolism depends on symbiotic bacteria housed in a specialized organ know as the throphosome. R. pachyptilla provides a stable environment and a steady supply of nutrients to the endosymbionts, while the endosymbionts supply abundant organic carbon to their host. The hemoglobin of R. pachyptilla not only binds to oxygen, but is adapted to reversibly bind to sulfide. Unbound sulfide would prove toxic to the tube worm but is essential to the metabolism of the resident symbiotic bacteria (Flores et al, 2005).

bivalves on volcanic rock
Calyptogena magnifica

The Vesicomyid bivalves, mollusks with hinged shells, host symbiotic bacteria within vacuoles of their gills. The vesticomid Calyptogena magnifica, also found at vent sites of the East Pacific Rise and Galapagos Rift, prefer slower venting sites to those chosen by R. pachyptilla. C. magnifica possesses a functional, though highly reduced, mouth and gut, suggesting that these organisms acquire nutrition both through the symbiotic relationship and through feeding (Childress & Fisher, 1992). Studies of endosymbiont and host genomes suggest that C. magnifica clams can pass on bacterial symbionts from one generation to the next in the eggs of their young (Hurtado et al, 2003).

<I>Alvinella</I> worm in its dwelling tube
Alvinella spp.

Though polychaete worms are small and are frequently difficult to detect, they form a large component of hydrothermal vent faunas (Ward et al, 2003). The polychaete Alvinella pompejana, found at the East Pacific Rise, lives in tubes along the sides of black smokers. The worm, which is adapted to deal with varied water temperatures, draws in 20 degree C water at the anterior end but the water temperature at the posterior end of the tube ranges from 30-60 degrees C. A. pompejana possesses a fully functional digestive track and is a deposit feeder (Van Dover, 2000). Mats of filamentous ectosymbionts colonize the epidermis of Alvinella spp. and possibly exchange metabolites with their symbionts.

The vent shrimp Rimicaris exoculata is found swarming over the chimneys of the Mid-Atlantic Ridge and has also recently been discovered at Indian Ocean vent sites (Van Dover, 2001). These shrimp do not have the eyes or eyestocks typical of non-vent shrimp. Instead, they possess a pair of fused eye organs that contain high concentrations of visual pigments (Van Dover, 2000). These photoreceptors are believed to be adapted for the detection of radiation associated with hydrothermal vent activity, and they see in infrared instead of the visible light spectrum. Like A. pompejana, R. exoculata host episymbiotic bacteria.

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