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Cold seeps and hydrothermal vents are structures found on the ocean floor that support biomes completely independent of the Sun's energy. Cold seeps slowly release hydrogen sulfide, methane and other hydrocarbon-rich fluids, while hydrothermal vents release geothermally heated water rich with the same dissolved minerals. As the ocean floors are typically 2-3 miles (3.2 - 4.8 km) from the surface, these biomes are completely dark for millions of years. They are also heavily pressurized due to the weight of the water above — hydrothermal vent and cold seep biomes usually have an ambient pressure a couple of hundred times greater than at the surface.
Cold seeps and hydrothermal vents are the only long-term biomes whose primary producers do not depend on photosynthesis. Instead of photosynthetic bacteria or plants forming the backbone of the ecosystem, this function is served by chemotrophic bacteria and archaea, which have close symbiotic relationships to heterotrophic organisms which consume them. In turn, larger organisms — giant tube worms, clams, and shrimp -—can consume these to survive.
Besides the diverse biota that hydrothermal vents and cold seeps harbor, they also possess interesting geologic features. Hydrothermal vents may include black smokers, geysers ejecting cloudy water at a temperature of 400 °C (752 °F), or white smokers, around the same temperature but ejecting white clouds instead of black. Because the pressure at these depths is so great, the water doesn't boil, and instead stays in liquid phase. As the superheated solutions come into contact with the cold water, minerals quickly precipitate out, creating chimney structures that may grow as tall as 60 m (197 ft) before collapsing.
Cold seeps resemble lakes on the bottom of the ocean. The cold seeps release their fluids in the form of brine, which, being denser than typical water, settles along the bottom. Along the edges of these brine lakes, we observe many thousands of mussels, living off the chemotrophic bacteria that feed on the sulfides and methane that leaks from the cold seeps. Cold seeps, being more stable than hydrothermal vents, harbor some long-lived life forms, including tubeworms that live between 170 and 250 years, the longest of any non-colonial invertebrate known.