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Protist

Protists (), are a diverse group of eukaryote|eukaryotic microorganisms. Historically, protists were treated as the kingdom (biology)|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 tissue (biology)|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 Because these groups often overlap, they have been replaced by phylogenetic-based classifications.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 (biology)|morphology and ecology of protists. Protists live in almost any environment that contains liquid water. Many protists, such as the algae, are photosynthesis|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 (biologist)|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 Clade|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 monophyly|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:

Reproduction

Some protists reproduce sexual reproduction|sexually, while others reproduce asexual reproduction|asexually. Some species, for example ''Plasmodium falciparum'', have extremely complex biological life cycle|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.