Protist in Encyclopedia

Protists (), are a diverse group of eukaryotic organisms that are generally microscopic. They are treated as the kingdom Protista in traditional taxonomies but there is no simple evolutionary relationship between these organisms - they are a paraphyletic grade, rather than a natural, (monophyletic) group. Consequently, this group is not often used 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 Because these groups often overlap, they have been replaced by phylogenetic-based classifications.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.

Classification

Historical classifications

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).

Modern classifications

The taxonomy of protists is still changing. Newer classifications^[1] 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)^[2] 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.^[3]

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 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.^[4] 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.^[5]

Protist

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