) are a heterogeneous group of organisms, comprising those eukaryotes that are not animals, plants, or fungi. They are usually treated as the kingdomProtista or Protoctista. The protists are a paraphyletic grade, rather than a natural (monophyletic) group, and do not have much in common besides a relatively simple organization (unicellular, or multicellular without highly specialized tissues). Some call it the "left-overs" from the other eukaryotic kingdoms.
Protists were traditionally (for the last 150 years) subdivided into several groups based on similarities to the higher kingdoms: the animal-like protozoa, the plant-like algae, and the fungus-like slime moulds and water moulds. These groups often overlap, and have been replaced by phylogenetic classifications. However, they are still useful as informal groups for describing the morphology and ecology of protists.
At one time, bacteria were also considered protists, under the three-kingdom system of Animalia (corresponding closely to the modern kingdom), Plantae (which included Fungi as well as plants), and Protista (everything else). See kingdom (biology). However, most recent texts treat bacteria separately.
Protozoa, the Animal-like Protists
Protozoa are mostly single-celled, motile protists that feed by phagocytosis, though there are numerous exceptions. They are usually only 0.01-0.5 mm in size, generally too small to be seen without a microscope. Protozoa are ubiquitous throughout aqueous environments and the soil, commonly surviving dry periods as cysts or spores, and include several important parasites. Based on locomotion, protozoa are grouped into:{|cellpadding="5"| Flagellates
Algae gather light energy through photosynthesis. They include many single-celled creatures that are also considered protozoa, such as Euglena, which have acquired chloroplasts through secondary endosymbiosis. Others are non-motile, and some (called seaweeds) are truly multicellular, including members of the following groups:
The green and red algae, along with a small group called the glaucophytes, appear to be close relatives of other plants, and so some authors treat them as Plantae despite their simple organization. Most other types of algae, however, developed separately. They include the haptophytes, cryptomonads, dinoflagellates, euglenids, and chlorarachniophytes, all of which have also been considered protozoans.
Note some protozoa host endosymbiotic algae, as in Paramecium bursaria or radiolarians, that provide them with energy but are not integrated into the cell.
Fungus-like protists
Various organisms with a protist-level organization were originally treated as fungi, because they produce sporangia. These include chytrids, slime moulds, water moulds, and Labyrinthulomycetes. Of these, the chytrids are now known to be related to other fungi and are usually classified with them. The others are now placed among the heterokonts (which have cellulose rather than chitin walls) and the Amoebozoa (which do not have cell walls).
The classification of protists is still changing. Newer classifications attempt to present monophyletic groups based on ultrastructure, biochemistry, and genetics. Because the protists as a whole are paraphyletic, such systems often split up or abandon the kingdom, instead treating the protist groups as separate lines of eukaryotes. The recent scheme by Adl et al. (2005) is an example which does not bother with ranks (phylum, class, etc.).
Some of the main groups of protists that are now recognized, which may be treated as phyla, are listed in the taxobox at right. Some smaller groups are listed under the traditional categories, linked to above. For more discussion of relationships between different protists, see eukaryote or the articles referenced below.
* Cavalier-Smith, T. (2003) "Protist phylogeny and the high-level classification of Protozoa". European Journal of Protistology39:338-348. * Sina M. Adl, Alastair G. B. Simpson, Mark A. Farmer, Robert A. Andersen, O. Roger Anderson, John R. Barta, Samuel S. Bowser, Guy Brugerolle, Robert A. Fensome, Suzanne Fredericq, Timothy Y. James, Sergei Karpov, Paul Kugrens John Krug, Christopher E. Lane, Louise A. Lewis, Jean Lodge, Denis H. Lynn, David G. Mann, Richard M. Mccourt, Leonel Mendoza, Øjvind Moestrup, Sharon E. Mozley-Standridge, Thomas A. Nerad, Carol A. Shearer, Alexey V. Smirnov, Frederick W. Spiegel and Max F. J. R. Taylor. (2005) "The New Higher Level Classification of Eukaryotes with Emphasis on the Taxonomy of Protists". Journal of Eukaryotic Microbiology 52:(5) 399-451.
