Organic Substance, Virioplankton, Bacterioplankton

‭ORGANIC SUBSTANCE, VIRIOPLANKTON, BACTERIOPLANKTON‬ ‭ presence at the basis of trophic relationships in pelagic environments but not only is the organic substance‬ A ‭in the water column.‬ ‭Carbon cycle‬ ‭Carbon dioxide fuels the carbon cycle in the sea: the CO2 taken‬ ‭from the atmosphere is used by plants, especially phytoplankton‬ ‭but also by macroalgae and seagrasses, which organicate the‬ ‭CO2 through photosynthesis, transforming it into plant biomass.‬ ‭This can be consumed by herbivores and then in turn by‬ ‭carnivores, following the trophic chain and feeding the pelagic and‬ ‭benthic trophic networks. But all marine organisms in turn produce‬ ‭CO2 through respiration, used by primary producers; a further‬ ‭source of CO2 is the mineralization processes of non-living‬ ‭organic substances, both animal and plant remains degraded by‬ ‭bacteria. Especially at great depths, carbonate dissolution‬ ‭processes can further increase the availability of dissolved and‬ ‭therefore potentially available CO2, thanks to the mixing of the‬ ‭water column for photosynthesis which occurs in illuminated‬ ‭surface waters, thus closing a cycle which is based on‬ ‭contributions of atmospheric carbon, carbon of biological origin‬ ‭from respiration and degradation processes and carbon of‬ ‭sedimentary origin from carbonate dissolution processes.‬ ‭On the left there is the biological pump which, starting from the intake of CO2 by the‬ ‭phytoplankton, transfers carbon through the marine food web up to the highest trophic levels,‬ ‭to then release a part of CO2 from respiration or processes into the atmosphere of‬ ‭degradation. But above all, the biological pump can transfer carbon to deep waters through‬ ‭slow degradation that occurs in the column or through the accumulation of organic material at‬ ‭the bottom. At the bottom, this material may still undergo mineralization by the bacteria, but‬ ‭first of all it will feed the components of the deep trophic networks. On the left we highlight the‬ ‭ ction of the physical pump which acts in synergy with the biological pump, and through the‬ a ‭physical pump, cold surface waters rich in CO2 (because its solubility increases at low temperatures);‬ ‭These waters sink and thus enrich the deep water masses with CO2. The CO2 can thus be‬ ‭surfaced again through upwelling or mixing processes in the column.‬ ‭In marine waters organic substances can be present in dissolved or particulate form. The‬ ‭dissolved fraction is called DOM (dissolved organic matter), the particulate fraction POM‬ ‭(particulate organic matter). By convention, POM is all that organic material that is in suspension‬ ‭and can be retained with a porosity of 0.5 microns. DOM and POM can also be quantified in‬ ‭terms of organic carbon and we speak of DOC and POC. There are also other names such as‬ ‭TOC (total organic carbon) which expresses the sum of DOC and POC, as opposed to TIC (total‬ ‭inorganic carbon) which represents the inorganic component in all its forms. TSP (total suspended‬ ‭particulate) indicates all the particles suspended in the column, both organic and inorganic.‬ ‭The‬‭quantity‬‭of‬‭DOM‬‭in‬‭the‬‭sea‬‭is‬‭very‬‭variable:‬‭it‬‭varies‬‭from‬‭a‬‭few‬‭milligrams‬‭per‬‭liter‬‭to‬‭tenths‬ ‭of‬‭a‬‭milligram‬‭per‬‭liter‬‭in‬‭open‬‭sea‬‭waters.‬‭However,‬‭particularly‬‭high‬‭values‬‭of‬‭a‬‭few‬‭tens‬‭of‬‭m/l‬ ‭are‬ ‭often‬ ‭detectable‬ ‭in‬ ‭coastal‬ ‭waters.‬ ‭In‬ ‭the‬ ‭composition‬ ‭of‬ ‭DOM,‬ ‭low‬ ‭molecular‬ ‭weight‬ ‭carbohydrates,‬‭amino‬‭acids‬‭and‬‭fatty‬‭acids‬‭always‬‭prevail.‬‭DOM‬‭constitutes‬‭the‬‭most‬‭important‬ ‭reserve of organic carbon in the sea: it is 10 to 20 times greater than the POM content.‬ ‭The main sources of DOM are, especially near the coasts, river inputs; as we move away from the‬ ‭coasts, the autochthonous origin becomes more and more decisive, and is represented by the‬ ‭dissolution of the remains of both plant and animal organisms, excretion products, exudates‬ ‭released by phytoplankton, but also by substances released by phytoplankton, bacteria and‬ ‭protozoa following viral lysis or bacterial degradation. The production of DOM in the sea is‬ ‭practically continuous but this does not explain how its quantity in the open sea where there is no‬ ‭variability due to the changing coastal inputs, always remains more or less constant. This‬ ‭condition is explained by the activity of bacterioplankton which uses DOM as a food substrate.‬ ‭POM includes both a living component and organic debris and its range is from a few micrograms‬ ‭to milligrams * liter. The composition of the living part of the pom varies depending on the depth‬ ‭and distance from the coasts. In general we can say that in surface waters this component is‬ ‭mainly represented by phytoplankton. In unlit waters the living component progressively reduces in‬ ‭quantity and is made up of bacterioplankton and zooplankton.‬ ‭Then there is the fraction of POM made up of organic debris: they are plant and animal remains,‬ ‭such as cell walls, exoskeletons, fecal pellets, etc. Fecal pellets in particular are important in the‬ ‭transfer of energy towards the bottom, especially in waters far from land, where it eats up the‬ ‭detrimental component of terrestrial origin.‬ ‭The sun's presence tends to progressively decrease‬ ‭with increasing depth until it reaches almost constant‬ ‭values. It is estimated that only 5/10% of the carbon‬ ‭produced on the surface is exported to depth through‬ ‭sedimentation processes, but only a small fraction‬ ‭reaches the bottom sediments, because everything else‬ ‭is mineralized or consumed in the column during the‬ ‭sedimentation process.‬ ‭Virioplankton‬‭: the study of viruses in the marine‬‭environment is one of the most recent fields of‬ ‭investigation, but an important one. Viruses make up 94% of living particles in plankton, but they‬ ‭are small in size so they only make up 5% of the water column. It turned out that viruses are 10‬ ‭times more abundant than bacterioplankton, which until recently were considered the most‬ ‭abundant. Their concentration varies from 10 6 kilome to 10 7 kilome /ml in surface ocean waters‬ ‭to 10 8 kilome/ ml in coastal waters.‬ ‭viruses are capable of infecting any organism, but bacteriophages are the most abundant. The‬ ‭action of viruses is felt in the regulation of bacterial and phytoplankton biomass: cell lysis‬ ‭phenomena occur whose effects are particularly evident in phytoplankton, especially after a‬ ‭bloom, i.e. an explosion of phytoplankton abundance, when there is often a sudden and rapid‬ ‭decline caused by viral lysis. To demonstrate the importance of the virus in this context, both field‬ ‭surveys and experimental approaches on laboratory cultures were used. It has also been seen‬ ‭that a reduced availability of phosphates, but not that of nitrates alone, seems to reduce the viral‬ ‭effects of phytoplankton: just think of the composition of viruses, based on nucleic acids for the‬ ‭formation of which phosphate is essential. Another consequence also arises from cellular lysis:‬ ‭through lysis the viruses cause a continuous release of Dom into the sea.‬