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Astounding Biodiversity around Deep Sea Vents

Deep sea vents or hydrothermal vents are fissures at the sea floor out of which flows out a stream of mineral and gas-rich geothermally heated seawater. Deep sea vents are a result of sea floor spreading and are mostly found in ocean ridges.

Beneath the Ocean Floor: Discovery of Ocean Ridge

They are formed by the heating up of sea water that permeates the spreading sea floor by volcanic activity (United Nations, 2017) i.e. by contact with the molten crust. The super-hot lava that permeates the basalt meets the sea water that percolates through the ocean floor and the two intermix in a reaction zone where temperatures can reach up to 400°C. Due to this action, hydrothermal fluids are constantly rising back to the ocean through fissures that connect this reaction zone to the ocean floor, warming the water somewhat in the surrounding area. Vents that are known as ‘black smokers’ are the warmest, reaching temperatures of around 400°C.

Fig: Mechanism of a Deep Sea Vent

Source: Hannes Grobe/AWI

The water that comes out of these deep sea vents are rich in dissolved metals and minerals. Among the metals and minerals expelled by the vents, the most common and plentiful is hydrogen sulphide, along with many other heavy metals combined with other elements and emissions of compounds such as methane.

Hydrothermal vents are found all over the globe, and their occurrence is determined by the conditions governing tectonic plates. In places near tectonic plates, the Earth’s magma is close to the sea floor, and can heat the water that has seeped into the earth, like mid-ocean ridges and places with volcanic activity. These are either areas with high tectonic activity or are places where continental plates intersect.

The Sargasso Sea – a unique ecosystem protected by seaweed

Hydrothermal vents were first discovered when marine geologists were studying ocean temperatures by the Galapagos Rift in May, 1976. A vehicle that was subsequently lowered into the about 2,500 meter depth of the spreading rift reported a plume of vent discharge. While collecting samples of the surrounding water, the vehicle also photographed the ocean floor. These photographs revealed a vibrant and dense benthic (deep sea) community of deep sea organisms in close proximity to the geothermal vents. Peter Lonsdale in 1977 used this data to publish the first paper on life around a hydrothermal vent (C. Scearce, 2006).

Geologists returned to the Galapagos Rift in 1977 and undertook a diving mission at Alvin, which was the first instance of live human contact at a hydrothermal vent. This was followed in 1979 by an expedition of scientists along with a film crew from National Geographic who introduced the world to organisms such as crabs and giant tube worms living around hydrothermal vents.

Scientists were at first puzzled by the vibrancy of life in benthic communities around hydrothermal vents, given the toxicity of certain substances emitted by the vents and the variance produced due to the intermittent mixing of sea water and vent discharges. Vent environments also produce temperature and chemical differences in comparison to the much colder deep sea water, where the properties of water are variable intermittently. Scientists deduce that such an environment would require a great amount of adaptability. Moreover, the water from the vent discharges is filled with inorganic substances. Yet the opposite is true, and deep sea vents are rich in biodiversity.

For a food chain to begin in any location on Earth, primary producers must be present to convert inorganic material to organic material. The organic compounds from these primary producers provide the energy requirements for the rest of the food chain. The energy of the Sun is utilized normally by primary producers on Earth to produce organic compounds by a process called photosynthesis. However, a very low amount of photosynthetic fallout is possible in the deep sea, as there is a complete absence of sunlight and most organic compounds are fully consumed before they reach deep into the ocean floor. The plentiful biodiversity of deep sea vents thus baffled scientists.

Sergei Nikolaevich Vinogradskii carried out research in the 1880s on whether microbes could live solely on inorganic matter and returned positive results on sulphur, iron and nitrogen with bacteria. Wilhelm Pfeffer in 1897 coined the term ‘chemosynthesis’ for this process of production of energy by oxidation from inorganic sources (H.G. Schlegel, 1975). Around the time of the discovery of the deep sea vents at the Galapagos rift, Harvard student Colleen Cavanaugh confirmed chemosynthesis by bacteria in deep sea vents by oxidizing sulphides or elemental sulphur into organic compounds utilized by organisms such as tube worms living nearby the deep sea vents, and she is credited with the natural discovery of chemosynthesis.

A number of organisms with habitats nearby deep sea vents such as clams, mussels, tube worms, etc are hosts to symbiotic sulphur-based bacteria inside their bodies. Instead of the carbon utilized by primary producers over the Earth, these bacteria synthesize sulphur through oxidation. A bizarre variety of adaptive qualities thus exist in this ecosystem, such as vent shrimps (Rimicaris exoculata) whose eyes have photoreceptors that are adapted to detect radioactive particles, such that their sight is adapted to view in infrared instead of the visible light spectrum in the high temperatures of vent discharges. In other organisms such as the giant tubeworm (Rifitia patchyptila), the presence of symbiotic chemosynthetic bacteria in their bodies allows, among other things, their haemoglobin to bind not only with oxygen, but also sulphide, normally incredibly toxic for organisms on Earth, which is possible due to the metabolism of the resident bacteria (C. Scearce, 2006). Deep sea vents prove that other sorts of chemical processes are possible for life to thrive.

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