Aquatic ecosystems comprise the largest portion of the biosphere and include both freshwater and marine ecosystems. The sources of organic matter in these systems can be both internal (autochthonous) and external (allochthonous). In general, the autochthonous material has higher available N concentration and is structurally easier to decompose than the allochthonous plant residues. Decomposition in aquatic ecosystems follows similar patterns as in terrestrial environments (i.e., it involves leaching, fragmentation, and chemical alteration), though with some major differences due to the aquatic environment. A major form of organic matter in aquatic ecosystems is the particulate organic matter (POM). POM can come from both autochthonous and allochthonous sources. The allochthonous sources include terrestrial leaves and small twigs, which are usually colonized by fungi and fragmented by shredders, leading to the formation of POM. Autochthonous POM is derived from the fragmentation of dead organisms and other organic material. POM is partly ingested, digested, and mineralized by organisms and microorganisms before settling on the bottom. The remaining organic matter that reaches the bottom is further broken down by bacteria both through aerobic and anaerobic processes. Another important component of organic matter in aquatic ecosystems is the dissolved organic matter (DOM). Major sources of DOM in the water column are (1) exudates excreted by macroalgae, phytoplankton, and zooplankton and (2) auto-lysis - the remains of phytoplankton and zooplankton. DOM is taken up by bacteria and converted into bacterial biomass without undergoing any breakdown into inorganic compounds. This bacterial biomass is later consumed by the zooplankton, which in turn, excretes nutrients in the form of exudates, contributing to a significant portion of the suspended material in the water column. Bacteria then take these exudates (even at very low concentration) to obtain both carbon and nutrients, and a new cycle starts. Thus, in contrast to terrestrial ecosystems, bacteria in aquatic systems act as converters rather than as decomposers, whereas phytoplankton and zooplankton play major roles in the release of available nutrients.
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