Soft-bottom marine benthos

PAN, JERÓNIMO; PRATOLONGO, PAULA D.

Marine Biology: a functional approach to the oceans and their organisms

CRC Press/Taylor & Francis Group

Lugar: Boca Ratón, FL; Año: 2022; p. 180 - 210

1. The benthic environment is the two-dimensional habitat in which bottom-dwellers are found; the term benthos applies to the organisms that inhabit benthic habitats. These range in size and taxonomy from bacteria to vertebrates.2. Organic matter (OM) can be locally (autochthonous) or remotely produced (allochthonous); the greatest portion of marine sedimentary OM is originally produced in the form of particulate matter through photosynthesis. The burial of OM in marine sediments constitutes the main link between ?active? pools of carbon, and those carbon pools that cycle on much longer, geological, timescales.3. In addition to microbially-produced extracellular polymeric substances (EPS), virtually all benthic animals secrete copious amounts of mucus to accompany many physiological processes.4. Benthos distribution is controlled by a combination of factors such as sediment granulometry, dissolved O2, OM content, as well as disturbance and pollution processes. There is little evidence to a single factor being the primary determinant of benthos distributions.5. The transition zone or boundary from predominantly oxidizing to reducing conditions in sediments is called the redox potential discontinuity (RPD) layer. The depth of the RPD layer can vary extensively in time and space.6. The functional classification of soft-bottom marine benthos may follow different criteria, namely, size (micro-, meio- and macrobenthos); habitat preference (epifauna, infauna); feeding strategies (shredders, grazers, suspension- and deposit feeders); type of carnivory (predators and scavengers); or their contribution to sediment stability and/or erodibility (stabilizers and bioturbators).7. Deep-sea benthic ecosystems harbor a high biological diversity; several hypothesis (either focusing on stability or disturbance as determinants) have been invoked to explain such diversity and distribution patterns. In relation to the energy fuelling them, deep-sea ecosystems can be divided into two major groups: heterotrophic and chemosynthetic habitats.8. Hydrothermal vents, methane cold seeps, and communities developing on seafloor oxygen minimum zones (OMZs) are examples of deep-see chemolithotrophic communities, in which sulfide-oxidizing bacteria create trophic endosymbioses with the dominant invertebrates.9. The benthos influences soft sediment properties in a number of ways and at various scales.10. Soft-bottom ecosystems play a major role in nutrient regeneration, through the processing of OM that comes from different sources. Ammonification produces NH4+ and nitrification mediated by distinct groups of nitrifying bacteria yields NO3?. In anaerobic conditions, different groups of bacteria use alternative electron acceptors to decompose OM and liberate energy. Nitrate reduction may follow the denitrification pathway, or dissimilatory nitrate reduction to ammonia (DNRA). Nitrogen reduction under anoxic conditions can also occur through anaerobic ammonium oxidation (anammox). In mudflats, aerobic and anaerobic processes are tightly coupled.11. Sulfate-reducing bacteria and archaea perform anaerobic respiration by reducing SO42? to H2S. Methanogenesis in anaerobic sediments is mediated by archaea and produces CH4 by using CO2 as the terminal electron acceptor at the final step in OM decomposition.12. Epipelic microphytobenthos (MPB) are microscopic, unicellular eukaryotic algae and cyanobacteria that grow in the uppermost mm of illuminated sediments and in association with sedimentary particles. MPB secretes EPS as part of their normal physiology, thus contributing to biostabilization and geomorphological processes in mudflats. Furthermore, when adjusted by the areal extent they cover, these autotrophs contribute the most to active estuarine carbon pools.13. Sediments ingested by benthic animals experience a unique environment controlled by the digestive chemistry of the organism. Deposit-feeding implies several sequential steps initiated with ingestion and trituration, followed by solubilization (mediated by surfactants and enzymes) in the fore- and midgut, and ending with selective absorption in the hindgut and/or defecation of unassimilated solid material. The guts of deposit-feeding invertebrates could be likened to laundry washing machines.14. Seagrass beds are composed of dense populations (mono-specific or more commonly, more than one species) of flowering plants that grow on well-illuminated (often oligotrophic) soft sandy bottoms, across tropical and temperate latitudes worldwide. They offer structural complexity in an otherwise topographically simple, two-dimensional environment and can modify the hydrodynamic regime. Seagrass meadows are highly productive and highly autotrophic; furthermore, OM is sequestered for long periods of time (100s of yr) and preserved forming anaerobic peats, thus being categorized as ?Blue Carbon? ecosystems.15. The pelagic and benthic sub-systems are intimately interconnected. Such connections may be established through physical or biologically-mediated processes, collectively known as benthic-pelagic coupling (BPC). Ecological BPC traditionally focuses on trophic and reproductive interactions, and it generates many ecosystemic benefits.16. Suspension-feeding is an energy-acquisition strategy convergently evolved among many phyla of marine organisms, that targets a profit from the large amount of particles suspended in seawater. There is no analogous mechanism to suspension-feeding in terrestrial ecology.17. Marine organisms have developed unique life-history strategies that differ from those of terrestrial organisms. Many planktonic organisms enter a benthic resting phase and spend weeks to years in the seabed. Reciprocally, most macrozoobenthos in coastal systems have a planktonic larval stage that, after a variable time, settles to the bottom (benthic larval recruitment).