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A protist (/ ˈ p r oʊ t ɪ s t / PROH-tist) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus.Protists do not form a natural group, or clade, but are a polyphyletic grouping of several independent clades that evolved from the last eukaryotic common ancestor.
Protistology is a scientific discipline devoted to the study of protists, a highly diverse group of eukaryotic organisms. All eukaryotes apart from animals, plants and fungi are considered protists. [1]
Marine protists are defined by their habitat as protists that live in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. Life originated as marine single-celled prokaryotes (bacteria and archaea) and later evolved into more complex eukaryotes .
Although protist flagella have a diversity of forms and functions, [11] two large families, flagellates and ciliates, can be distinguished by the shape and beating pattern of their flagella. [ 2 ] In the phylogenetic tree on the right, aquatic organisms (living in marine, brackish, or freshwater environments) have their branches drawn in blue ...
Protists are distributed across all major groups of eukaryotes, including those that contain multicellular algae, green plants, animals, and fungi. If photosynthetic and fungal protists are distinguished from protozoa, they appear as shown in the phylogenetic tree of eukaryotic groups.
Ciliates are an important group of protists, common almost anywhere there is water—in lakes, ponds, oceans, rivers, and soils, including anoxic and oxygen-depleted habitats. [2] About 4,500 unique free-living species have been described, and the potential number of extant species is estimated at 27,000–40,000. [ 3 ]
Microalgae are important components of the marine protists, as well as the marine phytoplankton. They are very diverse. It has been estimated there are 200,000–800,000 species of which about 50,000 species have been described. [64] Depending on the species, their sizes range from a few micrometers (μm) to a few hundred micrometers.
Systematists today do not treat Protista as a formal taxon, but the term "protist" is still commonly used for convenience in two ways. [22] The most popular contemporary definition is a phylogenetic one, that identifies a paraphyletic group: [23] a protist is any eukaryote that is not an animal, (land) plant, or (true) fungus; this definition [24] excludes many unicellular groups, like the ...