Rich communities of fauna colonize hydrothermal vent habitats. As these habitats occur at thousands of metres depth, beyond the reaches of light and the rain of organic particles, these rich communities cannot rely on photosynthesis to produce organic carbon essential for life. Instead microscopic, chemosynthetic bacteria produce these essential ingredients for life at hydrothermal vents (Ruby et al., 1981; Jannasch & Mottl, 1985; McCollom & Shock, 1997).
Chemosynthesis is the production of cellular material from inorganic carbon derived from chemical energy (McCollom & Shock, 1997). Chemosynthetic bacteria have harnessed the ability to derive chemical energy from the oxidation of hydrogen sulphide H₂S which issues out of vents at high concentrations within the hydrothermal fluid (Ruby et al., 1981). The chemical energy released is used to assimilate carbon dioxide CO₂ and inorganic nitrogen in the form of nitrate NH₃¯ and ammonia NH₃ to produce organic carbon and amino acids required for growth (Lee et al., 1999). Chemosynthetic bacteria are therefore the drivers of primary production at hydrothermal vents and form the base of the food chain (Colaco et al., 2002).
Vent organisms rely on chemosynthetic bacteria as their primary source of nutrition and is obtained through a variety of different mechanisms. Deposit feeding organisms including some alvinellid worms and shrimps feed directly on the bacteria that form dense mats on the walls of the chimneys, while suspension feeding cirripeds, serpulid worms and sea anemones filter out bacteria suspended in the vent fluid (Colaco et al., 2002; Levesque et al., 2005; Bates, 2007).
Some organisms however host symbiotic bacteria within their cells (Jannasch & Mottl, 1985). The best know example of symbiosis with chemosynthetic bacteria is perhaps the large vestimentiferan tube worm Riftia pachyptila (Fig. 5). Cavanaugh et al. (1981) were the first to discover densely packed spherical chemosynthetic bacterial cells (Fig. 6) within the lobes of the trophosome, an organ that amounts up to 60% of the worms total body weight (Jannasch & Mottl, 1985). The trophosome is characteristically red in colour due to the extensive circulatory systems supplying a large volume of blood high in concentration of oxygen, carbon dioxide and hydrogen sulphate to the organ. High concentrations are achieved by having haemoglobin with a high affinity for oxygen and a relative insensitivity to temperature and carbon dioxide concentration changes, as well as the constant diffusion of hydrogen sulphide across the body wall of the worm (Cavanaugh et al., 1981). These adaptions supply the chemosynthetic bacteria with the necessary chemicals to respire and carry out chemosynthesis which in turn supply the worm with organic compounds used for growth (Cavanaugh et al., 1981).
Symbiotic relationships have also been found to exist in many other vent organisms such as the giant white clam Calyptogena magnifica (Fig. 7), which houses chemosynthetic bacteria within their gills (Jannasch & Mottl, 1985). These symbiotic organisms highly depend on chemosynthetic bacteria to provide their nutrition to such extent that ingestive and digestive features have been reduced, or lost completely (Jannasch & Mottl, 1985).
How vent organisms obtain their nutrition from chemosynthetic bacteria influences their distribution at hydrothermal vents (Colaco et al., 2002). Deposit feeders are the first to colonize on the walls of the chimneys while symbiotic organisms are next to colonize the area around the chimneys where hydrogen sulphide is in good supply but out of the main flow of the super-heated venting fluid, while filter feeders are last to colonize at the extremities of venting sites (Colaco et al., 2002).