As no photosynthesis can occur in the Abyssal zone, it would be expected that no primary production occurred at this depth. However, this is not the case – it does occur in the water surrounding hydrothermal vents. The existence of these vents was only discovered in 1977 by the deep sea submersible ‘Alvin’. Specialised bacteria living around hydrothermal vents on the ocean floor have developed a unique way to produce energy without requiring sunlight – chemosynthesis. This process only produces 0.1-1% of the energy that global marine photosynthesis produces (Maruyama et al., 1998), but it is still vital for the communities living there.
Hydrothermal vents release a noxious mix of water containing hydrogen sulphide and heavy metals, including copper, iron, and zinc. This would be toxic to most marine organisms. However some life has adapted to not only survive, but thrive in the presence of these chemicals. Chemosynthetic bacteria oxidise the hydrogen sulphide released by the vents to produce energy (Ruby et al., 1981). These bacteria can live within the tissue of marine meiofauna, forming symbiotic relationships. Due to the scarcity of nutrients and organic carbon in the Abyssal zone, these fauna rely on the chemosynthetic bacteria for their survival.
What lives here?
Life in the hydrothermal vent habitats is incredibly unique – over 80% of the described species living here are endemic (Van Dover, 2000). One of the most studied species in this habitat is the giant tubeworm, Riftia pachyptila.. They were discovered along with the vents themselves in 1977. As their name suggests, these worms are very large – they can grow to 1.5 m in length in only 2 years, making it the fastest growing marine invertebrate (Lutz et al., 1994). These worms were once thought to feed directly on the chemosynthetic bacteria and other organic matter (Lonsdale, 1977). However it was later discovered that the worms lacked a mouth and digestive tract (Jones, 1981), so the worms must survive through symbiosis. In Riftia, these symbiotic bacteria live in the trophosome – a specialised organ in the worm’s body. A gill like structure, called the branchial plume, protrudes out of the chitinous tube that encases the rest of the worm. These plumes extend into the water rising from the vents to obtain sulphide compounds, which are transported to the trophosome. They are then oxidised to produce ATP for carbon fixation (Stewart & Cavanaugh, 2006).
Other organisms found here are the deep-sea mussels of the Bathymodiolus genus. These mussels, like Riftia, harbour chemosynthetic bacteria. They reside in the gills, within specialised cells called bacteriocytes (Fiala-Médioni et al., 1986). However, unlike Riftia, they do not depend completely upon the bacteria for its energy supply. Bathymodiolus sp have a functional mouth and digestive tract, and are able to filter feed (Le Pennec & Prieur, 1984). This enables Bathymodiolus sp. to colonise larger areas, as it is not limited to the sulphide-rich water near the vents (Fisher et al., 1988). But, despite their ability to suspension feed, Bathymodiolus sp. are still limited to the vicinity of hydrothermal vent habitats. When removed from this environment, the symbiotic bacteria are lost and body condition deteriorates (Raulfs et al., 2004). This implies that they rely wholly on chemosynthetically produced organic matter, be it through symbiosis or heterotrophy.