The Deep-sea floor covers the entire oceanic benthos below ~250 m depth, where there is not enough light to support primary productivity through photosynthesis. It extends from the upper bathyal seafloor to the deepest parts of the ocean, at just under 11 km in the Mariana Trench.

Most deep-sea communities are therefore heterotrophic, depending ultimately on allochthonous energy and nutrients from the vertical flux and/or advection down-slopes of organic matter produced in the upper photic layers of ocean waters. Chemosynthetically-based ecosystems, such as those found at hydrothermal vents and cold seeps, are an exception, as chemoautotrophic microorganisms synthesise reduced compounds (e.g. hydrogen sulphide and methane) in hydrothermal and cold-seep fluids as autochthonous source of energy.

Oxygen is not limiting due to global-scale thermohaline circulation via deep ocean currents, except in bathyal (200–1,000 m) areas along the eastern Pacific, southwestern Africa, the Arabian Sea, and the Bay of Bengal, where physical and biological processes result in the formation of oxygen-minimum zones. Depth generates a strong gradient in hydrostatic pressure, increasing 1 atmosphere with every 10 m in depth, excluding fish from depths >8.5 km. Currents, geomorphology, and substrate type also influence ecosystem function.

Geomorphology differentiates several functional groups of ecosystems within the deep seafloor biome because of its influence on both the movement of currents and the vertical flux of resources, with marine canyons, seamounts, and trenches creating resource-rich hotspots. Extensive soft sediments on the abyssal plains support burrowing detritivores and predators, whereas sessile suspension feeders dominate hard substrates. Deep-sea benthic biodiversity is usually very high and mostly composed of meio-fauna and macro-fauna, with high abundances of microbes. Chemosynthetically-based ecosystems are exceptional again, as their biota is characterised by high biomass, low diversity, and high endemism. Organisms are equipped with traits that enable survival in the absence of light, high hydrostatic pressure, and low levels of nutrients and carbon.