Abstract
Abyssal seafloor ecosystems cover more than 50% of the Earth’s surface. Being formed by mainly heterotrophic organisms, they depend on the flux of particulate organic matter (POM) photosynthetically produced in the surface layer of the ocean. As dead phytoplankton sinks from the euphotic to the abyssal zone, the trophic value of POM and the concentration of essential polyunsaturated fatty acids (PUFA) decrease. This results in pronounced food periodicity and limitations for bottom dwellers. Deep-sea invertebrate seston eaters and surface deposit feeders consume the sinking POM. Other invertebrates utilize different food items that have undergone a trophic upgrade, with PUFA synthesized from saturated and monounsaturated FA. Foraminifera and nematodes can synthesize arachidonic acid (AA), eicosapentaenoic acid (EPA), while some barophylic bacteria produce EPA and/or docosahexaenoic acid. FA analysis of deep-sea invertebrates has shown high levels of PUFA including, in particular, arachidonic acid, bacterial FA, and a vast number of new and uncommon fatty acids such as 21:4(n-7), 22:4(n-8), 23:4(n-9), and 22:5(n-5) characteristic of foraminifera. We suppose that bacteria growing on detritus having a low trophic value provide the first trophic upgrading of organic matter for foraminifera and nematodes. In turn, these metazoans perform the second-stage upgrading for megafauna invertebrates. Deep-sea megafauna, including major members of Echinodermata, Mollusca, and Polychaeta display FA markers characteristic of bacteria, foraminifera, and nematodes and reveal new markers in the food chain.
Highlights
General Characteristics of the Abyssal ZoneAbyssal ecosystems located at depths of 3000–6000 m occupy almost 55% of theEarth’s surface
Abyssal bottom ecosystems are characterized by a lack of in situ primary production
polyunsaturated fatty acids (PUFA) (DHA, EPA, and arachidonic acid (AA)) content, 4–10% in sediment and 5–14% in particulate organic matter (POM), and a relatively low bacterial FA content were recorded from deep-sea sediments collected in the Southern Ocean [30,31]
Summary
The POM reaching the seafloor after being subject to complex biological modifications during sinking has a relatively low nutritional value [8,9,10] This concerns, in particular, the level of essential polyunsaturated fatty acids (PUFA) which are necessary for the normal development of abyssal meio-, macro-, and megafauna [11,12,13]. Major essential PUFA for marine invertebrates are eicosapentaenoic (EPA), docosahexaenoic (DHA), and, to a lesser degree, arachidonic acids synthesized mainly by microalgae in the photosynthesis zone of the ocean [11] These PUFA are crucially important for the normal functioning of biological membranes under extreme environmental conditions in the abyssal zone [14,15,16]. Special attention is paid to marker fatty acids and trophic upgrading in abyssal ecosystems
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