Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from Ancient Greek φυτόν (phutón), meaning "plant", and πλαγκτός (planktós), meaning "drifter, wanderer, roamer", and thus, "plant drifter". Phytoplankton obtain their energy through photosynthesis, as trees and other plants do on land. This means phytoplankton must receive sunlight, so they live in the well-lit surface layers (euphotic zone) of oceans and lakes. Compared with terrestrial plants, phytoplankton are distributed over a larger surface area, experience less seasonal variation, and have markedly faster turnover rates (days versus decades). As a result, phytoplankton respond rapidly on a global scale to climate variations. Phytoplankton form the base of marine and freshwater food webs and are key players in the global carbon cycle. They account for about half of global photosynthetic activity and at least half of the oxygen production, despite amounting to only about 1% of the global plant biomass. Phytoplankton are very diverse, comprising photosynthesizing bacteria (cyanobacteria) and various unicellular protist groups (notably the diatoms). Most phytoplankton are too small to be individually seen with the unaided eye. However, when present in sufficient numbers, some varieties may appear as colored patches on the water surface due to chlorophyll in their cells and, in some species, accessory pigments (such as phycobiliproteins or xanthophylls).
Types Phytoplankton are photosynthesizing microscopic protists and bacteria that inhabit the upper sunlit layer of marine and freshwater bodies of water on Earth. Paralleling plants on land, phytoplankton undertake primary production in water, creating organic compounds from carbon dioxide dissolved in the water. Phytoplankton form the base of — and sustain — the aquatic food web, and are crucial players in the Earth's carbon cycle.
Phytoplankton are very diverse, comprising photosynthesizing bacteria (cyanobacteria) and various unicellular protist groups (notably the diatoms). Many other organism groups formerly named as phytoplankton, including coccolithophores and dinoflagellates, are now no longer included as they are not only phototrophic but can also eat other organisms. These organisms are now more correctly termed mixoplankton. This recognition has important consequences for how the functioning of the planktonic food web is viewed.
Ecology
Phytoplankton obtain energy through the process of photosynthesis and must therefore live in the well-lit surface layer (termed the euphotic zone) of an ocean, sea, lake, or other body of water. Phytoplankton account for about half of all photosynthetic activity on Earth. Their cumulative energy fixation in carbon compounds (primary production) is the basis for the vast majority of oceanic and also many freshwater food webs (chemosynthesis is a notable exception). While almost all phytoplankton species are obligate photoautotrophs, some are mixotrophic and other, non-pigmented species that are actually heterotrophic (the latter are often viewed as zooplankton). Of these, the best known are dinoflagellate genera such as Noctiluca and Dinophysis, that obtain organic carbon by ingesting other organisms or detrital material. Phytoplankton live in the photic zone of the ocean, where photosynthesis is possible. During photosynthesis, they assimilate carbon dioxide and release oxygen. If solar radiation is too high, phytoplankton may fall victim to photodegradation. Phytoplankton species feature a large variety of photosynthetic pigments, which enable them to absorb different wavelengths of the variable underwater light. This implies different species can use the wavelength of light differently efficiently. The light is not a single ecological resource but a multitude of resources depending on its spectral composition. By that it was found that changes in the spectrum of light alone can alter natural phytoplankton communities even if the same intensity is available. For growth, phytoplankton cells additionally depend on nutrients, which enter the ocean by rivers, continental weathering, and glacial ice meltwater on the poles. Phytoplankton release dissolved organic carbon (DOC) into the sea. Since phytoplankton are the basis of marine food webs, they serve as prey for zooplankton, fish larvae, and other heterotrophic organisms. They can also be degraded by bacteria or by viral lysis. Although some phytoplankton cells, such as dinoflagellates, can migrate vertically, they are still incapable of actively moving against currents, so they slowly sink and ultimately fertilize the seafloor with dead cells and detritus.
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