One of the more fascinating stories of adaptation to extreme oceanic conditions is the diverse and abundant community of organisms living on or near deep-sea hydrothermal vents. (Refer to Chapter 2 in your textbook.) Geologists first reported these unusual communities in 1977. Hydrothermal vents are hot springs associated with oceanic ridges. Vents are about 3000 m below the surface of the ocean, but the ridges rise 1000 to 2000 m above the abyssal ocean floor. Ridges mark the locations of sea floor spreading (divergent plate boundaries) and are a source of hot basaltic magma emerging from Earth's interior. Chimneys built by hydrothermal vents are up to 20 m high with dark mineral-laden (inorganic sulfide precipitates) water ("black smokers") coming out at 350 °C. The great water pressure at that depth keeps the water from flashing into steam. Water temperatures in the vicinity of vents average about 8 to 10 deg C.
Clustered around the vents are abundant populations of previously unknown large animals. To date, more than 300 new marine species have been found living in this extreme environment at various distances from a vent. The bottom water is rich in hydrogen sulfide (H2S), carbon dioxide, and oxygen. Specialized bacteria function as primary producers via chemosynthesis and form the base of a food chain (or web) that supports large crabs, mussels, clams (some more than 25 cm long), sea anemones, shrimp, and exceptionally large tubeworms. The bacteria use chemosynthesis to fix carbon--an adaptation that replaces photosynthesis as an energy source in the absence of sunlight. For example, tubeworms have no mouth or digestive tract but are hosts to large quantities of chemosynthetic bacteria in dense bunches. The worm's tentacles absorb hydrogen sulfide and transport it to the bacteria, which then use the H2S as an energy source to convert inorganic CO2 to organic molecules. Tubeworms, called pogonophorans, can be 3 to 4 m long, 8 to 10 cm thick, and are blood red in color due to their high hemoglobin content. Clams and shrimp in hydrothermal vent communities also shelter the same specialized bacteria in the cells of their gills. In this way, they derive nutrition from these bacteria. In another adaptation, the shrimp are equipped with special organs that sense heat from the vents. Apparently, the shrimp use this ability for navigation. They swim away from the vent to feed and use their infrared (heat) sensors to find their way back to the vent while avoiding swimming into the 350 deg C water. In effect, the shrimp use heat from the vent as a location beacon on the ocean bottom.
Chemosynthetic bacteria are also found in great numbers in the hot water streaming from the vents. As many as 105 to 109 bacterial cells per milliliter live in the streaming water. They are adapted to the extreme heat and yet these bacteria exist in organisms that cannot tolerate high water temperatures. Chemosynthesis is not limited to the vicinity of hydrothermal vents. There are also places on the ocean floor where cold water containing methane or sulfides seeps upward through the ocean floor. This encourages and fuels the growth of mats of chemosynthetic bacteria utilizing sulfides or methane as fuel.
After some time, perhaps years to tens of years, hydrothermal vents cease operating. When the vent water stops circulating, primary production is cut off and the community of organisms is threatened with extinction. However, these communities have the ability to colonize other active vents, long distances away. Hydrothermal vent colonization is similar to what happens in surface ecosystems, where surface animals and plants island-hop taking as many generations as necessary to find the next island. And as with surface ecosystems, there are numerous misses. Instead of islands, deep-sea vent colonists use "islands" of nutrients or food (e.g., whale carcasses) that have sunk to the bottom of the ocean. For example, chemosynthesis apparently takes place in the bones as a whale carcass is consumed. Most of the colonists do not end up near a vent, but it only takes a few fortunate individuals to successfully colonize a hydrothermal vent. Apparently, the planktonic larvae of vent animals can somehow sense the location of an active vent from its heat or chemical signature.
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Prepared by Joseph M. Moran, Ph.D., H.J. Niebauer, Ph.D. and
Edward J. Hopkins, Ph.D.,
email hopkins@aos.wisc.edu
© Copyright, 2018, The American Meteorological Society.