Hydrothermal Vent Background

Hydrothermal vents are found on the ocean floor at an average depth of 2,100 meters. Hydrothermal vents are formed as a result of shifting tectonic plates, which leave gaps in the ocean floor. Water enters these openings in the sea floor, heats up, and then rises back up to the sea floor surface. Hydrothermal vents emit extremely mineral rich water ranging from 60°C to 400°C. Interestingly, the water does not boil because of the extremely high pressures on the seafloor.

Hydrothermal vents are diverse ecosystems, contrary to the rest of the deep sea. This is a result of the minerals emitted from the vents. Hydrothermal vents release metals, such as copper, iron and zinc, as well as sulfur.

There are two types of vents, “white smokers” and “black smokers”. “Black smokers” are generally hotter and emit iron and sulfide, while “white smokers” are often cooler and emit barium, calcium, and silicone.

Due to the sea floor’s extreme depth, organisms are not able to rely on photosynthesis for energy. Many organisms surrounding hydrothermal vents are chemosynthetic, and the organisms unable to chemosythesize rely on the organisms that can.

Hydrothermal vents are an important part of the ocean, as a vital contributor to the chemistry, temperature, and circulation. The diversity of hydrothermal vent ecosystems is important for the maintenance of the ocean as a whole (Scearce, 2006).

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Hydrothermal vent organisms generate energy through the process of chemosynthesis. During chemosynthesis a chemical compound, often hydrogen sulfide, is used as an energy source. These chemicals act as electron donors and the energy generated is used to fix inorganic carbon into organic compounds.

Hydrogen sulfide chemosynthesis: CO2 + O2 + 4H2S → CH2O + 4S + 3H2O

In the absence of sunlight, organisms are still able to generate energy. Chemolithoautotrophs oxidize hydrogen sulfide to make carbohydrates and other organic compounds. These organisms are consumed by other organisms or form symbiotic relationships and provide their symbiont with organic compounds in exchange for protection and other nutrients (Dublilier, Bergin, and Lott, 2008).

The Encyclopedia of New Zealand, http://www.teara.govt.nz/en/sea-floor/4/4

Ifremeria nautilei and Alviniconcha species (Sea Snail)

Ifremeria nautilei and Alviniconcha species are both gastropods found in high numbers at hydrothermal vents. These sea snails have hypertrophied gills and circulatory systems. I. nautilei have heavily calcified shells and are inactive living in cooler waters. Alvinicocha have uncalcified shells and are active living in higher temperatures (Childress and Girguis, 2010). Alvinicocha species are found in the Western and Southwestern Pacific Ocean and on the Central Indian Ridge (Suzuki et al., 2004), while I. nautilei are found in the Western Pacific (Windoffer and Giere, 1997).

These sea snails have hypertrophied gills and circulatory systems as a result of their symbiotic relationship with chemosynthetic bacteria. These bacteria provide a majority of the snails’ nutrients, evidence of which can be found in the snails' decreased stomach size and radula wear (Suzuki et al., 2004; Windoffer and Giere, 1997).

The bacterial symbionts are found in the host’s gills. Enlarged ctenidial filaments of the sea snail's gills are attached to the mantle of the snail at one end and the other end is free in the snail’s cavity. The flattened sections of theses ctenidial filaments house the bacteria in bacteriocytes (Suzuki et al., 2004). The bacteria are enclosed in vacuoles, these vacuoles only contain one bacterium each and are interconnected in a network. Some vacuoles have openings exposing the bacteria directly to the external water (Windoffer and Giere, 1997). These gills have hemoglobins that play a role in oxygen and sulfide binding and transport to and from the symbionts. Sea snail gills do not function as typical gills, they function to provide their endosymbiont protection, ventilation, and exposure to metabolites (Childress and Girguis, 2010). The enlargement of the gills is a result of the densely packed bacteria and the enlargement of the circulatory system shows the importance and amount of nutrients that must be transported to and from the symbionts to support the high metabolic rates of the symbiosis.

Alviniconcha hessleri in the Mariana Trough harbors have been shown to utilize the Calvin Benson and Rubisco cycles for carbon dioxide fixation, while A. hessleri in the Western Pacific utilize the rTCA cycle. This suggests that the bacterial metabolisms in the two snails may be different (Suzuki et al., 2004).

Ifremeria nautilei- Photographer: C. R. Fisher, http://chess.lifedesks.org/node/1207
Suzuki et al., 2005


The endosymbiotic bacteria are generally gammaproteobacteria or epsilonproteobacteria. These proteobacteria are gram-negative and chemosynthetic.The bacteria reside in the sea snail's gills and provide the snail with vital organic compounds. Analyses suggests that there are at least nine clades of gammaproteobacteria, indicating that the bacteria have evolved separately on multiple occasions. The ecological diversity of hydrothermal vents is a result of these chemosynthetic bacteria (Dubilier, Bergin, and Lott, 2008).

Dubilier, Bergin and Lott, 2008

Symbiotic Relationship

The mutualistic endosymbiotic relationship between sea snails and chemosynthetic bacteria relies on the bacteria’s ability to oxidize hydrogen sulfide and create inorganic molecules. The host must deliver the necessary substrates to the endobacteria for chemosynthesis to occur, hydrogen sulfide and oxygen, as well as excrete the end products, sulfate and hydrogen ions.

Sea snails actively take in hydrogen sulfide and oxygen through their gills and transport the nutrients to the bacteria via specialized hemoglobins. Carbon dioxide is also acquired via the enzyme carbonic anhydrase, which catalyzes the conversion of bicarbonate and carbon dioxide. Hydrogen sulfide is toxic to most organisms, and therefore sea snails oxidize sulfide to thiosulfate, which can then be used by the bacteria in chemosynthesis. Once the bacteria have generated inorganic carbon compounds it has been suggested that the sea snails digest the bacteria to acquire the inorganic carbon. THe endosymbiotic bacteria have a high demand for oxygen and it has been shown that 80% of the snail's oxygen uptake is utilized by the bacteria. The snails sustain these high demands using their enlarged circulatory system (Childress and Girguis, 2010).

Sea snails take in nitrate that is converted to ammonium by the endobacteria to maintain high internal ammonium concentrations. The snails ability to control its intake of nitrogen may be used to control symbiont density (Childress and Girguis, 2010).

One species of sea snail, I. nautilei, is known to be host to two endosymbionts, both a thiotrophic and a methanotrophic symbiont. These sea snails are thought to better adapt to environmental changes due to their ability to generate inorganic compounds in multiple ways (Dubilier, Bergin, and Lott, 2008; Windoffer and Giere, 1997). Different snail species are also thought to host bacteria that utilize the carbon metabolism cycles differently, some bacteria utilize the Calvin-Benson cycle, while others utilize the rTCA cycle (Suzuki et al., 2005).

In this mutualistic symbiotic relationship, the sea snails provide the bacteria with the necessary substrates for chemosynthesis and protection, while the bacteria provide vital organic compounds for the sea snail. The extent of the obligate relationship and nutrient transfer is still unknown.

Childress and Girguis, 2010

Suzuki et al., 2005


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