15.8: Hydrothermal Vents

Until 1977, it was thought that all areas of the abyssal oceans were biologically impoverished because of the limited availability of food that rains down from above. However, in 1977 the research submersible Alvin made a number of dives on the Galápagos Ridge that would change those ideas. The purpose of the dives was to study the geology and chemistry at this oceanic ridge. The researchers were looking for evidence that the high heat flow through the seafloor at the center of the ridge creates hydrothermal circulation. In hydrothermal circulation, seawater sinks through sediments or cracks in rocks of the seafloor and is heated, convected upward, and vented to the water column to be replaced by more seawater drawn through the rocks and sediments (Fig. 15-19a, Chap. 6). The heat comes from the upwelling magma and cooling volcanic rocks beneath the seafloor.

Figure 15-19. Black smoker formation and composition. (a) Hydrothermal vents on the oceanic ridges are believed to be formed because heat from magma below the oceanic ridge sets up a convection circulation. In this circulation, water from below the seafloor is heated and rises at the vents and is replaced by water drawn through the cracks in the rocks and sediment on the flanks or an adjacent section of the ridge. (b) At black smoker vents, the discharged water contains no oxygen but does contain high concentrations of iron, manganese, and other sulfides. As water is vented into cold ocean water, metal sulfides are deposited around the vent and often form a chimney-like structure.

The researchers found much more than they expected. In fact, their findings may represent one of the most surprising and profound scientific discoveries ever made. They found not only hydrothermal vents, but also dense communities of marine organisms surrounding those vents. The communities include tube worms, clams, mussels, and many other invertebrates, most of which belonged to previously unknown species and many of which were very large in comparison with similar known species. The biomass in these vent communities is hundreds of thousands of times greater than that in any other community at comparable depths in the ocean. However, even this finding of abundant oases of life in the “desert” of the abyss was to prove less surprising than the subsequent discovery that these communities do not depend on photosynthesis. In fact, they were found to be dependent on chemosynthesis for primary production of their food.

Hydrothermal Vent Environments

Initially, hydrothermal vents were thought to be rare and to occur only on fast-spreading oceanic ridges. We now know that hydrothermal vents and their associated biological communities are present in many locations, dispersed irregularly along the oceanic ridges in all oceans. Vents have even been found on the ultraslow-spreading Gakkel Ridge in the Arctic Ocean.

Vents have also been found on the submerged volcanoes at island arc subduction zones such as the Mariana Arc. These back-arc volcano vents are especially interesting to scientists because many are at much shallower depths than the oceanic ridge vents. The shallower depth makes them much easier to study, and the fluids they discharge are dispersed in the upper layers of the oceans, where they may have greater immediate effects on marine species that live in, or migrate periodically to, the photic zone. Some estimates now suggest that vents are abundant enough that a volume of ocean water equal to the entire volume of the world oceans may be processed through high-temperature vents about every 10 million years, which is a relatively short period in geological time.

Each vent differs from the others in terms of the temperatures and chemical characteristics of the water it discharges. Two general types of vents are known to occur along the oceanic ridge axis. Vents of the first type, called black smokers, discharge hot water, usually about 360°C (but can be up to about 460°C) The second type of vent, called white smokers discharges somewhat cooler water (about 260 to 300°C), usually at flow rates lower than those at black smokers.

The water discharged by black smokers contains no oxygen or nitrate but has high concentrations of hydrogen sulfide and of certain metals, including iron (Fe) and manganese (Mn; Fig. 15-19). As the superhot water is discharged into cold, oxygenated seawater, it does not vaporize because of the high pressure. However, as it cools and mixes with oxygenated seawater, metal sulfides are precipitated to form a cloud of tiny black particles. This cloud gives the black smokers the appearance of a dirty smokestack and hence their name. As the black smoker continuously disgorges, precipitated metal sulfides are deposited in the area around the vent exit and may help to construct a chimney at the vent outlet that can be up to 20 m high and several meters wide (Fig. 15-19b). The deposits formed at these vents are rich in many metals such as copper, zinc, gold, silver and lead with the quantities of these metals varying with temperature and other characteristics of different vents. These deposits are commercially valuable because the sulfide deposits are rich in these metals and they are now often called seafloor massive sulfide (SMS) deposits (Chap. 2). Many of the metal sulfide ores mined on land today are believed to have originated in hydrothermal vent environments and then to have been scraped off and added to the continents at subduction zones (Chap. 4).

White smokers were not discovered until 2000. Unlike black smokers these vents are not located along the ridge axis. Instead they lie on the flank of the ridge, where the underlying rock is about 1.5 million years old. The fluids discharged at white smokers are cooler, have a higher pH, contain less sulfide but more silica, calcium, and magnesium than fluids discharged from black smokers. When these fluids are discharged and mix with seawater, calcium, magnesium, and other sulfates are precipitated out and deposited to form chimneys much like those at black smokers. However, some of these chimneys can be up to about 60 m high, much taller than black smoker chimneys. The populations of larger animals, such as crabs, are much less abundant at these vents than are often found at black smokers. However, the white smoker vents do support abundant microbial communities.

Biological Communities Associated with Hydrothermal Vents

Although individual species vary, biological communities that surround ridge axis black smoker hydrothermal vents in the Atlantic, Pacific, and Indian Oceans are composed of generally similar species, and similar communities form in distinct zones around the vent.

At many of the Pacific vents, numerous giant tube worms and clams (Fig. 15-20) live closest to the vent. Other invertebrates, such as one or more species of limpets, shrimp, and scale worms, are present but less abundant. Farther from the vent are other plume worms, crabs, amphipods, other shrimp species, and several species of snails. Still farther from the vent are a wide variety of hydroids, species of worms, shrimp, anemones, and snails that are different from those closer to the vent. Many of these species are filter feeders, and others are predators. The biomass decreases progressively and rapidly with distance from the vent, and the more normal sparse fauna of the deep seafloor are present a few tens of meters from the vent.

Figure 15-20. Hydrothermal vent communities support many species that are not found elsewhere. (a) A typical biological community near a black smoker, like the one seen here, includes tube worms, crabs, mussels, and other invertebrates. (b) One of the most important members of most hydrothermal communities is the tube worm Riftia pachyptila, which can be a meter or more in length and which sustains itself by feeding on symbiotic chemosynthetic bacteria.

More than 590 new species have been identified in the hydrothermal vent fauna, representing more than 20 new families and more than 100 new genera (plural of genus) and these numbers continue to climb steadily as new species are found at newly discovered vent areas. The discovery of this bewildering array of new species is unique in the history of biological science, rivaled only by the findings of the Challenger expedition in the 1870s (Chap. 2).

The biomass of many Pacific hydrothermal vent communities is dominated by the giant tube worm, Riftia pachyptila, a very strange creature (Fig. 15-20b). This species has no mouth and no digestive system, but it can grow to several centimeters in thickness and more than 1 m in length. Like the clams that live in the same region near the vents, it has red flesh and blood. Both species get their red color from hemoglobin in their blood, the same oxygen-binding molecule that is present in human blood. 

The food source for the giant tube worm is apparently a population of chemosynthetic bacteria that it cultivates within its body in a symbiotic association similar to that between corals and zooxanthellae. The bacteria oxidize sulfide as an energy source to chemosynthesize organic matter (Chap. 12), and the tube worm assimilates either the waste products of this synthesis or the bacterial biomass itself, or both. This partnership is phenomenally successful, because the tube worms apparently grow very quickly. The tube worm also has a unique enzyme that is incorporated in its tissues, particularly in its surface tissues. The enzyme detoxifies hydrogen sulfide and protects the hemoglobin that carries oxygen needed for the worm to respire. However, this enzymatic protection is carefully adapted to allow a route by which the sulfide can be brought from outside the worm into the part of its body where the chemosynthetic bacteria reside.

Although it is certain that some hydrothermal vent species other than the tube worm have similar associations with chemosynthetic bacteria or archaea to provide a portion of their food, most species within the hydrothermal vent community are probably filter feeders. Hence, the bulk of their food must come from suspended particles. The source of these particles appears to be chemosynthetic bacteria and archaea that grow in profusion in the mouth of the vent and deep within the sediments and rocks of the seafloor. Clumps of the microbial biomass are broken loose periodically by the flow of water through the vent. The clumps fragment to form suspended particles that can be captured by the filter feeders. The concentration of these particles quickly declines with distance from the vent as the plume disperses and large particles are deposited.

Unanswered Questions about Hydrothermal Vents

Many questions about hydrothermal vents remain unanswered, not only about the abundance, geographic distribution, and physical/chemical characteristics of the vents, but also about the species that make up vent communities. For example, we do not know how the chemosynthetic bacteria or archaea can survive and grow at temperatures in excess of several hundred degrees. Also we do not fully understand how these species are able to survive the trip across many kilometers of abyssal ocean to colonize new hydrothermal vents. Most vent species are adapted to higher temperatures and different water chemistry than are present in the abyssal ocean through which they would have to travel to colonize a new vent. Thus, vent species, or at least their eggs or larvae, must be able to survive a much greater range of environmental conditions than do most of the living organisms with which we are more familiar.

Most known vents are scattered along the ridges, some separated by substantial distances, and each may operate for a limited period, perhaps less than 20 years or so. However, observations have shown that new vents may be colonized within months or years. Bottom currents that could carry eggs and larvae on some ridges may tend to follow the ridge, but new vents may be upcurrent of the old ones. Thus, the physical mechanisms by which new vents are colonized, which also likely differ for individual species, may be complex and are likely aided by eddies formed as tidal currents flow over the elevated oceanic ridge topography. Also, recent findings have revealed that the types of species present at the vents are influenced greatly by the chemical and physical conditions at each vent so that, contrary to earlier expectations that new vents would be colonized by fauna from nearby vents, they may be colonized by completely different fauna. 

Many vent species produce extraordinarily large numbers of larvae. One possible explanation for the very large size of some vent species in comparison with similar species elsewhere in the oceans may be a need to produce very large numbers of larval offspring. It has been suggested that, in some species, these larvae may have an arrested development phase, so they could be transported by ocean currents for many decades or even centuries before encountering a new vent to colonize. Eggs and larvae of vent species may rise into shallower layers of the ocean, perhaps entrained in megaplumes of water heated slightly above ambient temperature that are known to occur in hydrothermal vent areas. In the shallower layers, larvae may be widely distributed before settling to the seafloor for a chance encounter with a new vent. There is also evidence that the partially decomposed carcasses of whales and other large mammals that fall to the ocean floor, or other slowly decomposing organic matter such as wood that reaches the deep seafloor, may be ideal sulfide-containing environments to support vent species during a “stopover” while being dispersed across the deep oceans. Any, all, or none of these mechanisms may be involved in new vent colonization.

Genetic analysis of vent species has begun to reveal the rates and patterns of colonization of vents in different parts of the world ocean. Genetic and other studies of many more species from many more vents, exploration of the vast areas currently unsearched to catalog the distribution of hydrothermal vents worldwide, and studies to obtain much better knowledge of the current patterns, mixing, and dispersion in the deep oceans are all needed to enable us to unravel the mysteries of hydrothermal vent species life cycles. We are likely to have more surprises as these investigations proceed.

Other Chemosynthetic Communities

Since the discovery of hydrothermal vents on the oceanic ridges, several chemosynthetic communities have been discovered in other locations in the deep sea, as well as in shallow-water anoxic environments such as marshes. Chemosynthetic bacterial mats have been found at the base of the continental slope off the west coast of Florida. These bacteria use hydrogen sulfide in water that seeps out from the limestone underlying sediments of the continental slope. In and around these mats is a diverse community of animals that may obtain some or all of their food from the chemosynthetic bacteria.

Chemosynthetic communities have also been found at oil and gas seeps, also known as cold seeps, at a depth of 600 to 700 m in the Gulf of America (Golfo de México) south of Louisiana and elsewhere. These communities use either hydrogen sulfide and/or hydrocarbons as an energy source for their primary production. In the Juan de Fuca subduction zone and in other subduction zones, methane in pore waters squeezed out of the buried sediments provides the energy source for other chemosynthetic bacteria.

In addition to these ocean chemosynthetic communities, chemosynthetic communities that are generally dominated by archaea have been found to exist deep within the rocks of the Earth’s crust. Studies of the organisms able to live and reproduce in such extreme environments have assumed an important role in the search for the origins of life on the Earth and in the search for life or evidence of life in the past on other planets and moons of the solar system.