Protists (/prəʊˌtɪst/) are a heterogeneous group of living things, comprising those eukaryotes that are neither animals, plants, nor fungi. They are usually treated as the kingdom Protista 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" kingdom. If an organism is eukaryotic, but does not fit in the plants or animals or fungi - then it is a protist.
| Protists | |
|---|---|
| |
| Paramecium aurelia, a ciliate | |
| Scientific classification | |
| Domain: | |
| Kingdom: | Protista |
| Typical phyla | |
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Traditional classification
Protists were traditionally (for the last 150 years) subdivided into several groups based on similarities to the higher kingdoms: the animal-like protozoa and the plant-like algae and the fungi-like slime molds . These traditional groups often overlapped. A new working model for the taxonomy of the eukaryotic organisms has recently been codified by a collaboritive team.(see references below.) In it the evolutionary relationships among the eukaryoties is given priority rather than method of obtaining energy or structural characteristics.
Protozoa
The protozoa are mostly single-celled, motile, and feed by phagocytosis, though there are numerous exceptions. They are usually only 0.01-0.5 mm in size, this is gay. generally too small to be seen without a microscope. They 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:
| Flagellates | with long flagella | e.g., Euglena |
| Amoeboids | with transient pseudopodia | e.g., Amoeba |
| Ciliates | with multiple, short cilia | e.g., Paramecium |
| Sporozoa | non-motile parasites; form spores | e.g., Plasmodium |
Algae
The algae gather light energy through photosynthesis. They include many single-celled creatures that are also considered protozoa, such as Euglena and Paramecium bursaria, which have acquired chloroplasts through secondary endosymbiosis. Others are non-motile, and some are truly multicellular, including some members of the following groups:
| Chlorophytes | green algae, related to higher plants | e.g., Ulva |
| Rhodophytes | red algae | e.g., Porphyra |
| Heterokontophytes | brown algae, diatoms, etc. | e.g., Macrocystis |
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.
Fungus-like protists
Various organisms originally considered fungi were later moved to the Protista. These include the Chytridiomycetes, slime moulds, water moulds, and Labyrinthulomycetes. The chytrids are now known to be related to the other Fungi, and are usually classified with them; the others are now placed among the Amoebozoa and heterokonts.
Contemporary classification
The classification of protists is still changing. Newer classification criteria attempt to present monophyletic groups based on ultrastructure, biochemistry, and genetics. Because the protists are paraphyletic, such systems often split up or abandon the Kingdom. A partial picture is given by the list of phyla at right; other groups are listed under the traditional categories, linked to above. For more discussion of modern relationships, see eukaryote or articles referenced below.
References
- Cavalier-Smith, T. (2003) "Protist phylogeny and the high-level classification of Protozoa". European Journal of Protistology 39:338-348.
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. Article is free and available at http://www.blackwell-synergy.com/doi/abs/10.1111/j.1550-7408.2005.00053.x