Hydrothermal vent fauna

Hydrothermal vents are submarine geysers of superheated water that form along volcanic ridges on the ocean floor. These vents are populated by various bacteria that gain energy through chemosynthesis and form the base of a complex ecosystem that includes many symbiotic relationships. Because there are relatively few species in the deep ocean environment, hydrothermal vents function as isolated islands of biodiversity within the ocean.

The Mid-

The mid-ocean ridge is an underwater mountain range formed along the juncture between tectonic plates. It comprises a number of smaller mountain ranges that combine to cover more than 56,000 kilometers (35,000 miles) of the ocean floor, making it the longest mountain range and the largest volcanic feature on Earth. The mountains that form the ridge can reach as high as 12,000 feet; at places, the range stretches for more than 1,200 miles.

The mid-ocean ridge forms because of convection currents under the Earth's surface, which are created by heat generated by the decay of radioactive elements in the Earth's core. It is along the ridge that most of the Earth's new seafloor crust is produced. The ridge also releases heat generated within the planet.

Ridges are formed in areas where magma rising from the core causes two tectonic plates to push away from one another. This magma then fills the gap left as the tectonic plates push farther apart, generating new sea floor. In areas of the ocean known as subduction zones, two plates collide so that the older of the two plates pushes over the younger, converting the older sediment back into magma.

Hydrothermal vents form in areas surrounding the mid-ocean ridge, where heat from volcanic activity under the surface creates pockets in the sediment along the sea floor. Water seeps into these subterranean voids and becomes heated by molten material beneath the surface. This water then pushes out from these subterranean caverns and creates undersea geysers, where superheated water spews into the environment. Most vents occur at depths of 2,100 meters (7,000 feet) or more below sea level, though researchers have discovered evidence of vent activity at more than 5,000 meters (16,400 feet) below the Caribbean Ocean.

Types of Hydrothermal Vents

Some vents have cylindrical structures (chimneys) around their mouths that form from minerals that accumulate from the vent water. Water spewing from a hydrothermal vent may reach temperatures up to 400 degrees Celsius (750 degrees Fahrenheit). Still, this water does not boil because of the intense pressures at the ocean floor generated from the weight of the much cooler ocean water surrounding the vent.

When the superheated water contacts the surrounding ocean water, which is generally only a few degrees above freezing, a resultant chemical reaction leads to the accumulation of minerals and metals from the surrounding water. These minerals then add to the leading end of the chimney, eventually forming an extended cylinder surrounding the spouting water at the vent. Chimneys may extend more than 40 meters from the ocean floor and grow more than 6 meters yearly.

There are two basic types of hydrothermal vents. The hottest vents are black smokers, which eject superheated compounds containing a high density of sulfides (sulfur-containing compounds) and iron-containing compounds. Just as the superheated liquid leaves the vent chimney, these compounds form iron monosulfide, making the geyser a black color reminiscent of black smoke. White smokers contain cooler water than their black-smoker relatives. The water ejected from white smokers usually contains calcium, barium, and other minerals that lend a white hue to water ejected from the geyser's spout.

Biodiversity of Hydrothermal Vents

Hydrothermal vents were first discovered in 1976 along the Galapagos rift, which lies along the eastern Pacific Ocean basin. Scientists have since discovered more than two hundred vents throughout both the Pacific and the Atlantic oceans. In 2010, scientists found evidence of additional potential vent sites off the coast of Antarctica, an area previously thought to be devoid of vent activity. Soon after the discovery of the first hydrothermal vents, scientists discovered that the vents themselves and the surrounding area were home to rich benthic communities of organisms.

As marine biologists have discovered additional vent sites, it has become clear that vent environments in different areas contain many different species. Marine biologists have encountered organisms in certain vent communities that appear to be endemic to only one system of vents. Deep-sea environments are relatively low in species diversity; therefore, hydrothermal vents function as relatively isolated islands of biodiversity in an otherwise species-poor environment. Vent species generally cannot cross the distance between vent systems, so each vent may contain a unique assemblage of species.

In terrestrial environments, the base of the food chain largely comprises organisms that use photosynthesis to fuel the formation of organic molecules. Other organisms then feed on these photosynthesizing organisms, creating a chain of predator and prey relationships that cycles energy through the ecosystem. In deep-sea environments, photosynthesis is impossible because of vastly reduced light levels on the ocean floor.

The base of the hydrothermal vent ecosystem comprises bacteria that use chemosynthesis, or the energy of inorganic chemical reactions, to fuel the creation of organic molecules. High levels of hydrogen sulfide and methane provide the raw fuel for the chemosynthetic bacteria that form the base of the complex vent ecosystems. Other microorganisms and multicellular animals exist in the vent communities by feeding on chemosynthetic bacteria.

Vent bacteria are examples of extremophiles, organisms that thrive in extreme environments marked by conditions that would make life impossible for most organisms on Earth. Vent organisms must contend with extreme variations in chemical concentrations and pressure and extreme heat generated from volcanic activity. Because vent organisms have evolved to contend with severely high temperatures, they are sometimes classified as thermophilic, or “heat-loving” organisms. Similar types of heat-loving bacteria can be found in hot springs, which develop when volcanic subterranean activity forms heated pools of water at the Earth's surface. Bacterial organisms found in hot springs have similar evolutionary adaptations to those in hydrothermal vent communities.

Vent bacteria are examples of extremophiles, organisms that thrive in extreme environments marked by conditions that would make life impossible for most organisms on Earth. Vent organisms must contend with extreme variations in chemical concentrations and pressure and extreme heat generated from volcanic activity. Because vent organisms have evolved to contend with severely high temperatures, they are sometimes classified as thermophilic, or “heat-loving” organisms. Similar types of heat-loving bacteria can be found in hot springs, which develop when volcanic subterranean activity forms heated pools of water at the Earth's surface. Bacterial organisms found in hot springs have similar evolutionary adaptations to those in hydrothermal vent communities.

Fauna of Hydrothermal Vent Communities

Organisms living near hydrothermal vents have evolved to deal with their environment's extreme temperatures and pressures. Most such species are confined to the area immediately surrounding the vents. Of the more than five hundred species discovered in vent communities, more than 90 percent have yet to be found outside that vent environment. The majority of vent species are single-celled or simple multicellular creatures. However, hundreds of more complex species live in vent communities, including mollusks, echinoderms, annelids, gastropods, and crustaceans.

One of the most characteristic species associated with vent environments is the tubeworm (Riftia pachyptila), a common species that lives near the black smokers along the Pacific Ridge. Tubeworms can grow up to 3 meters (8 feet) in length, and most of their bodies consist of a white sheath composed of chitin, a relatively stiff material made from glucose derivatives that protect the soft parts of the tubeworm's body. Tubeworms have a fleshy protuberance called a plume that extends from the leading end of their chitinous bodies, which is heavily vascularized to facilitate the absorption of chemical substances into the bloodstream. Tubeworms occasionally fall prey to predatory fish and crabs, so the worms have developed the ability to retract their plumes into their chitin sheaths to avoid predation.

In their adult form, tubeworms lack a digestive system and sensory organs, and they subsist entirely on energy and organic molecules provided by millions of chemosynthetic organisms living within their tissues. Marine biologists have found that tubeworms possess functional mouths, allowing them to ingest large numbers of bacteria that later take up residence within their tissues. The worms provide an ideal living environment for the bacteria and fuel bacterial chemosynthesis by absorbing chemicals into their bloodstream. In return, the symbiotic bacteria provide nutrients to the worm as a by-product of their chemosynthesis. The life of a tubeworm provides an example of an obligate symbiotic relationship, because the tubeworm cannot survive without the endosymbiotic bacteria living inside it.

The Pompeii worm (Alvinella pompejana) is a deep-sea polychaete worm that gathers in dense colonies around the vents and can grow up to 5 inches long. The worms appear to have dense fur along their dorsal surface, formed from colonies of chemosynthetic bacteria gathered into hair-like cylinders. Marine biologists believe that the layers of bacteria on the back of Pompeii worms may help insulate the worms, which can tolerate heat up to 80 degrees Celsius (176 degrees Fahrenheit).

The heads of Pompeii worms are fleshy and appear feathered with vascular extensions that are believed to facilitate chemical exchange with the environment. Though Pompeii worms are still poorly understood, marine biologists believe that they have a symbiotic relationship with the colonies of bacteria living on their backs and that these bacteria may provide food for the worm through chemosynthesis.

Another unique species found in vent environments is the scaly-foot gastropod (Crysomallon squamiferum), first discovered in 2001 in Indian Ocean black smokers. While part of the scaly-foot gastropod's body is protected by a shell similar to those of other marine snails, the exposed part of the animal's body, known as the foot, is protected by metallic armor scales formed from iron sulfides and generated from within the animal's cells. The scaly-foot gastropod obtains food because it has a symbiotic relationship with chemosynthetic bacteria living within the animal's esophagus. Marine biologists believe these symbiotic bacteria colonies also facilitate mechanisms within the scaly-foot gastropod's cells, forming its iron sulfide scales. The scaly-foot gastropod is the only known species that forms armor from iron compounds.

Vent communities also contain numerous predatory species that feed on microorganisms and other small animals living around the vents. One of the most common predators in vent communities is the vent crab (Bythograea thermydron), which was first discovered in the 1980s and is known to exist only around hydrothermal vents and other volcanic undersea environments. In 2002, researchers studying both adult vent crabs and the larvae of vent crabs found that the species develops optic systems that allow the crabs to see using infrared light, produced at the vent in the form of infrared radiation. Vent crabs may prey on small animals in their environment, but they also scavenge on the bodies of dead polychaete worms, gastropods, and other species living around the vents.

Significance of Vent Research

Researchers believe that investigations of extreme environments such as deep-sea vents can help biologists understand the limits of life on Earth. It is further believed that organisms like those found at hydrothermal vents are similar to the first types of organisms that evolved on the planet; indeed, many biologists believe that life on Earth began in the hydrothermal vent environment. Given the discovery that life can survive even in conditions of extreme heat, pressure, and chemical concentrations, some astrobiologists believe that the study of hydrothermal vents may provide clues to the way that life might evolve on other planets and may, therefore, help researchers learn the best ways to search for evolving life beyond Earth. For this reason, the National Aeronautics and Space Administration and other research groups studying astrobiology have contributed to the study of hydrothermal vents and their associated communities.

In July 2024, five new hydrothermal vent sites were found along the East Pacific Rise in the Pacific Ocean during a National Science Foundation expedition. This discovery highlights the importance of continuing hydrothermal vent site research and using emerging technologies, such as manned submersibles and autonomous underwater vehicles, to increase the understanding of the depths of the Earth’s oceans. Hydrothermal vents continue to play a critical role in understanding Earth’s biological and geological processes. 

Principal Terms

benthic: that which pertains to the bottom of a lake or sea, or the ecosystem at the bottom of a lake or sea

chemosynthesis: the synthesis of organic compounds using energy derived from chemical reactions, usually in environments where photosynthesis is not possible because of a lack of sunlight

ectosymbiont: a symbiotic organism living on the outer surface of another organism

endemic: that which is unique to a certain geographic location

endosymbiont: a symbiotic organism living within the body of another organism

extremophile: an organism evolved to live in extreme environments, which may include unusual temperature, chemical composition, and other variables of the physical environment

seamount: a mountain rising from the ocean floor that does not reach the water's surface

subduction zone: an area where two tectonic plates come together; the more recently deposited of the two plates pushes along the surface of the older plate

symbiosis: the close association of two or more different species living in the same environment

thermophile: an organism evolved to live in environments marked by extremely high temperatures

Bibliography

Cox, C. Barry, and Peter D. Moore. Biogeography: An Ecological and Evolutionary Approach. New York: John Wiley & Sons, 2010.

Detjen, Philipp Thomas. Hydrothermal Vents. Saarbrucken, Germany: VDM, 2011.

Fulton, Kim. “Biodiversity and Resilience of Deep-Sea Hydrothermal Vent Communities.” Future Oceans, 20 Jan. 2016, futureoceans.earthjournalism.net/spotlight-biodiversity-and-resilience-of-deep-sea-hydrothermal-vent-communities/index.html. Accessed 27 July 2024.

Herring, Peter. The Biology of the Deep Ocean. New York: Oxford University Press, 2002.

Kiel, Steffen. The Vent and Seep Biota: Aspects from Microbes to Ecosystems. New York: Springer, 2010.

McIntyre, Alasdair. Life in the World's Oceans: Diversity, Distribution, and Abundance. New York: Wiley Blackwell, 2010.

Morrissey, John, and James L. Sumich. Introduction to the Biology of Marine Life. Burlington, Mass.: Jones and Bartlett, 2011.

Putol, Rodielon. “Five New Hydrothermal Vents Discovered in the Pacific Ocean.” Earth.com, 5 May 2024, www.earth.com/news/five-new-hydrothermal-vents-discovered-in-the-pacific-ocean. Accessed 27 July 2024

“Researchers Discover Five New Hydrothermal Vents in Pacific Ocean.” Sci.News, 27 Mar. 2024, www.sci.news/othersciences/geoscience/east-pacific-rise-hydrothermal-vents-12799.html. Accessed 27 July 2024.

Van Dover, Cindy. The Ecology of Deep-Sea Hydrothermal Vents. Princeton, N.J.: Princeton University Press, 2000.