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protist

[proh-tist]

Any member of a kingdom (Protista) of diverse eukaryotes, including algae, protozoans, and lower fungi (see fungus). Most are single-celled organisms, though the algae tend to be multicellular. Many can move, mainly by using flagella (see flagellum), cilia (see cilium), or footlike extensions (pseudopodia). The kingdom was developed to accommodate intermediate organisms that, even though they possessed some plant or animal characteristics, did not exhibit the specialized features indicative of those groups. Some protists are considered the ancestors of multicellular plants, animals, and fungi. The term was first suggested in 1866 by Ernst Haeckel. With the development of advanced biochemical, genetic, and imaging techniques, previously established relationships have come under scrutiny, and it is now thought that some groups are less closely related to one another than once believed. As a result, the classification of protists, while convenient, is no longer entirely satisfactory.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Protist

Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista but this group is no longer recognized in modern taxonomy. The protists do not have much in common besides a relatively simple organization -- either they are unicellular, or they are multicellular without specialized tissues. This simple cellular organization distinguishes the protists from other eukaryotes, such as fungi, animals and plants.

The term protista was first used 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 Because these groups often overlap, they have been replaced by phylogenetic-based classifications.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.

Protists live in almost any environment that contains liquid water. Many protists, such as the algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of the plankton. Other protists, such as the Kinetoplastids and Apicomplexa are responsible for a range of serious human diseases, such as malaria and sleeping sickness.

Classification

Historical classifications

The first division of the protists from other organisms came in the 1820's, when the German biologist Georg A. Goldfuss introduced the word protozoa to refer to organisms such as ciliates and corals. This group was expanded in 1845 to include all "unicellular animals", such as Foraminifera and amoebae. The formal taxonomic category Protoctista was first proposed in the early 1860's John Hogg, who argued that the protists should include what he saw as primitive unicellular forms of both plants and animals. He defined the Protoctista as a "fourth kingdom of nature", in addition to the then-traditional kingdoms of plants, animals and minerals. The kingdom of minerals was later removed from taxonomy by Ernst Haeckel, leaving plants, animals, and the protists as a “kingdom of primitive forms”.

Herbert Copeland resurrected Hogg's label almost a century later, 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 diatoms, green algae and fungi. This classification was the basis for Whittaker's later definition of Fungi, Animalia, Plantae and Protista as the four kingdoms of life. The kingdom Protista was later modified to separate prokaryotes into the separate kingdom of Monera, leaving the protists as a group of eukaryotic microorganisms. These five kingdoms remained the accepted classification until the development of molecular phylogenetics in the late 20th century, when it became apparent that neither protists or monera were single groups of related organisms (they were not monophyletic groups).

Modern classifications

Currently, the term protist is used to refer to unicellular eukaryotes that either exist as independent cells, or if they occur in colonies, do not show differentiation into tissues. The term protozoa is used to refer to heterotrophic species of protists that do not form filaments. These terms are not used in current taxonomy, and are retained only as convenient ways to refer to these organisms.

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 formal ranks (phylum, class, etc.) and instead lists organisms in hierarchical lists. This is intended to make the classification more stable in the long term and easier to update.

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.

Types of protists

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:

Flagellates	with long flagella	e.g., Euglena
Amoeboids	with transient pseudopodia	e.g., Amoeba
Ciliates	with multiple, short cilia	e.g., Paramecium
Sporozoa	non-mobile parasites; some can form spores	e.g., Toxoplasma

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:

Chlorophytes	green algae, are 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.

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).

Metabolism

Protists obtain nutrients and digest nutrients in a complex acquirement and assimilation system. Many protists also feed on bacteria, these organisms engulf food and digest it internally. They extend their cell wall and cell membrane around the food material to form a food vacuole. This is then taken into the cell 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.

Nutritional types in protist metabolism
Nutritional type	Source of energy	Source of carbon	Examples
Phototrophs	Sunlight	Organic compounds or carbon fixation	Algae, Dinoflagellates or Euglena
Organotrophs	Organic compounds	Organic compounds or carbon fixation	Apicomplexa, Trypanosomes or Amoebae

Reproduction

Some protists reproduce sexually, while others reproduce asexually.

Some species, for example Plasmodium falciparum, have extremely complex life cycles that involve multiple forms of the organism, some of which reproduce sexually and others asexually. However, it is unclear how frequently sexual reproduction causes genetic exchange between different strains of Plasmodium in nature and most populations of parasitic protists may be clonal lines that rarely exchange genes with other members of their species.