Deep, diverse and definitely different: unique attributes of the world's largest ecosystem

Abstract

The deep sea, the largest biome on Earth, has a series of characteristics that make this environment both distinct from other marine and land ecosystems and unique for the entire planet. This review describes these patterns and processes, from geological settings to biological processes, biodiversity and biogeographical patterns. It concludes with a brief discussion of current threats from anthropogenic activities to deep-sea habitats and their fauna. <br><br> Investigations of deep-sea habitats and their fauna began in the late 19th Century. In the intervening years, technological developments and stimulating discoveries have promoted deep-sea research and changed our way of understanding life on the planet. Nevertheless, the deep sea is still mostly unknown and current discovery rates of both habitats and species remain high. The geological, physical and geochemical settings of the deep-sea floor and the water column form a series of different habitats with unique characteristics that support specific faunal communities. Since 1840, 27 new habitats/ecosystems have been discovered from the shelf break to the deep trenches and discoveries of new habitats are still happening in the early 21st Century. However, for most of these habitats, the global area covered is unknown or has been only very roughly estimated; an even smaller – indeed, minimal – proportion has actually been sampled and investigated. We currently perceive most of the deep-sea ecosystems as heterotrophic, depending ultimately on the flux on organic matter produced in the overlying surface ocean through photosynthesis. The resulting strong food limitation, thus, shapes deep-sea biota and communities, with exceptions only in reducing ecosystems such as <i>inter alia</i> hydrothermal vents or cold seeps, where chemoautolithotrophic bacteria play the role of primary producers fuelled by chemical energy sources rather than sunlight. Other ecosystems, such as seamounts, canyons or cold-water corals have an increased productivity through specific physical processes, such as topographic modification of currents and enhanced transport of particles and detrital matter. Because of its unique abiotic attributes, the deep sea hosts a specialized fauna. Although there are no phyla unique to deep waters, at lower taxonomic levels the composition of the fauna is distinct from that found in the upper ocean. Amongst other characteristic patterns, deep-sea species may exhibit either gigantism or dwarfism, related to the decrease in food availability with depth. Food limitation on the seafloor and water column is also reflected in the trophic structure of deep-sea communities, which are adapted to low energy availability. In most of the heterotrophic deep-sea settings, the dominant megafauna is composed of detritivores, while filter feeders are abundant in habitats with hard substrata (e.g. mid-ocean ridges, seamounts, canyon walls and coral reefs) and chemoautotrophy through symbiotic relationships is dominant in reducing habitats. <br>...

Biophysical and Population Genetic Models Predict the Presence of “Phantom” Stepping Stones Connecting Mid-Atlantic Ridge Vent Ecosystems

“…In the absence of sunlight, specialized microorganisms oxidize reduced compounds from the hydrothermal fluids to generate energy for carbon fixation [3]. These chemosynthetic microbes often live in symbioses with vent-dwelling invertebrate animals, creating oases of life in the otherwise sparsely populated deep-sea environment [1]. Similar to other marine invertebrates, many vent species pass through a free-swimming larval phase that can establish and maintain connections among geographically separated vent populations.…”

“…The deep ocean hosts one of the most intriguing environments on our planet-hydrothermal vents [1]. These systems result from the penetration of oxygenated seawater into the oceanic crust, where water-rock interactions produce chemically altered fluids that reemerge from the seabed and lead to the precipitation of polymetallic sulfide deposits [2].…”

Vampire squid: detritivores in the oxygen minimum zone

“…In waters over the Monterey Submarine Canyon, off Central California, we have found Vampyroteuthis throughout the depth range between 600 and 900 m and at oxygen concentrations centred around 0.4 ml l 21 . Mesopelagic oxygen minimum zones (OMZs) with concentrations less than 0.5 ml l 21 (22 mMO 2 ) occur commonly beneath areas of upwelling and high surface productivity; particularly where circulation is sluggish and source waters are relatively old [17]. In these upwelling areas, phytoplankton productivity is typically high and carbon availability often exceeds metazoan capability to consume it [18].…”

From microbes to macrofauna: an integrated study of deep benthic communities and their response to environmental variables along the Malta Escarpment (Ionian Sea)

“…On the whole, there is a considerable decrease in meiofauna and macrofauna abundance and biomass with increasing water depth, which represents a wellknown pattern in benthic marine ecology (Rex et al 2006, Ramirez-Llodra et al 2010. However, this general rule does not apply to all size classes of deep-sea communities.…”

“…In addition, at least 80% of the mass of terrigenous materials reaching the ocean is deposited in continental margin environments, and more than 90% of the total organic carbon accumulation in the ocean occurs in continental margin sediments (Mackenzie and Lerman 2006). Furthermore, these sediments host a large proportion of the unknown biodiversity and are repositories of deep-sea biomass (Ramirez-Llodra et al 2010). The enhanced levels of biodiversity along slopes are hypothesized to be a source of biodiversity for continental shelves and deeper basins as well (Danovaro et al 2009).…”