The floor of any
marine or
freshwater ecosystem is known as the
benthic zone.
Organisms who live in the benthic
zone are called benthos. In
shallow areas, the benthos live as most organisms: on
oxygen and
energy provided by
sunlight.
In the abyssal zone, the bottom of the ocean where oxygen is scarce and light cannot penetrate, the benthic communities must find an alternate source of energy; they also must bear the high water pressure, low nutrient concentrations, the extreme cold (usually around 3 degrees Celsius). Many communities are associated with deep-sea hydrothermal vents (created by underwater volcanic activity), which provides heat energy. The food producers here are chemoautotrophic bacteria rather than the standard photosynthetic plankton in the photic zone (where there is light). These bacteria live as symbionts within other animals, supporting a diverse and unique community that includes giant tube-dwelling worms and echinoderms.