AskDefine | Define protist

Dictionary Definition

protist n : free-living or colonial organisms with diverse nutritional and reproductive modes [syn: protistan]

User Contributed Dictionary



  1. Any of the eukaryotic unicellular organisms of the kingdom Protoctista; including protozoans, slime molds and some algae.


eukaryotic unicellular organisms of the kingdom Protoctista

Extensive Definition

Protists (), Greek protiston -a meaning the (most) first of all ones, are a diverse group of organisms, comprising those eukaryotes that cannot be classified in any of the other eukaryotic kingdoms as fungi, animals, or plants. They are usually treated as the kingdom Protista or Protoctista. Protoctists (or protists) are a paraphyletic grade, rather than a natural, (monophyletic) group, and so do not have much in common besides a relatively simple organization -- either they are unicellular, or they are multicellular without highly specialized tissues. The term protista was coined by Ernst Haeckel in 1866.
Protists were traditionally subdivided into several groups based on similarities to the "higher" kingdoms: the one-celled animal-like protozoa, the plant-like protophyta (mostly one-celled algae), and the fungus-like slime molds and water molds. Because these groups often overlap, they have been replaced by phylogenetic-based classifications. However, they are still useful as informal names for describing the morphology and ecology of protists.
At one time, the non-nucleated bacteria were also considered protists under the three-kingdom system of Animalia (comprising the many-celled animals or metazoans), Plantae (which then included fungi as well as green land plants), and Protista (which included everything else, except viruses). However, most current textbooks treat bacteria (and the newly-discovered archaea) as either a separate kingdom or domain.

Obtaining nutrients

Protists obtain nutrients and digest nutrients in a complex acquirement and assimilation system. Most protists also feed on bacteria. Protists acquire their food material through internal digestion. They extend their cell wall and cell membrane around the food material to form a food vacuole via endocytosis (usually phagocytosis; sometimes pinocytosis).
Nutrition in some different types of protists is variable. In flagellates, for example, filter feeding may sometimes occur where the flagella find the prey. In other multicellular protists, elements like nitrogen and oxygen are acquired by constant beating of the flagella. Protists often occur in hydrophilic conditions and thus have large amounts of oxygen within them, which is necessary for them to conduct respiration and photosynthesis to desirable levels.


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 magnification. Protozoa are grouped by method of locomotion into:

Algae, the plant-like protists

They include many single-celled organisms that are also considered protozoa, such as Euglena, which many believe 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 molds, water molds, 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 term Protoctista

During the latter 20th century, the terms Protista, protist and protistan were increasingly used by biological scientists and laymen alike. Groups devoted to protistology emerged, while protozoology seemed to fade as an intellectual construct. In more recent years, however, the terms Protoctista, protoctist and protoctistan have been championed by some scholars in microbiology and micropaleontology. For example, the 50-volume Treatise on Invertebrate Paleontology -- eager to fill in the gaps left by vertebrate paleontology -- has moved from its 1953 (and onwards) use of Protista to the 21st-century use of Protoctista. So a Protist-Protoctist debate would seem to be inevitable.
The taxonomic category Protoctista was first coined by an English biologist, John Hogg, in an article entitled On the distinctions between a plant and an animal, and on a fourth kingdom of nature (1860). In this article, Hogg argued that the term Protoctist should be used to include "both the Protophyta ... and Protozoa". Therefore, he said, there should be a "fourth kingdom of nature" in addition to the then-traditional kingdoms of plants, animals and minerals. For nearly a century, however, his ideas were eclipsed by those of Haeckel, the reputed founder of protistology. Herbert F. Copeland resurrected Hogg's label almost a century later in his article, Progress report on basic classification (1947). Arguing that "Protoctista" literally meant "first established beings", Copeland complained that Haeckel's term protista included anucleated microbes such as bacteria. Copeland's use of the term protoctista did not.
In contrast, Copeland's term included nucleated eukaryotes such as brown and red algae -- but not the green algae, which he placed with the other green plants. Copeland further elaborated on his taxonomic proposal in his 1956 book, Classification of Lower Organisms (Palo Alto, California: Pacific Books). For a more recent delineation of the protoctists, see the Handbook of Protoctista (Boston: Jones & Bartlett) by Lynn Margulis, Heather I. McKhann, and Lorraine Olendzenski (1990).

Phylogenetic classifications

The taxonomy 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 that does not bother with ranks (phylum, class, etc.).
Some of the main groups of protists, which may be treated as phyla, are listed in the taxobox at right. Many are thought to be monophyletic, though there is still uncertainty. For instance, the excavates are probably not monophyletic and the chromalveolates are probably only monophyletic if the haptophytes and cryptomonads are excluded.

See also


Other references

Marguilis, L., Corliss, J.O., Melkonian, M.,and Chapman, D.J. (Editors) 1990. Handbook of Protoctista. Jones and Bartlett , Boston. ISBN 0-86720-052-9
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