Protist

A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exclusion of other eukaryotes means that protists do not form a natural group, or clade. Therefore, some protists may be more closely related to animals, plants, or fungi than they are to other protists. However, like the groups ''algae'', ''invertebrates'', and '' protozoans'', the biological category ''protist'' is used for convenience. Others classify any unicellular eukaryotic microorganism as a protist. The study of protists is termed protistology.

History

The classification of a third kingdom separate from animals and plants was first proposed by John Hogg in 1860 as the kingdom Protoctista; in 1866 Ernst Haeckel also proposed a third kingdom Protista as "the kingdom of primitive forms". Originally these also included prokaryotes, but with time these were removed to a fourth kingdom Monera.

In the popular five-kingdom scheme proposed by Robert Whittaker in 1969, Protista was defined as eukaryotic "organisms which are unicellular or unicellular-colonial and which form no tissues", and the fifth kingdom Fungi was established. In the five-kingdom system of Lynn Margulis, the term ''protist'' is reserved for microscopic organisms, while the more inclusive kingdom Protoctista (or protoctists) included certain large multicellular eukaryotes, such as kelp, red algae, and slime molds. Some use the term ''protist'' interchangeably with Margulis's ''protoctist'', to encompass both single-celled and multicellular eukaryotes, including those that form specialized tissues but do not fit into any of the other traditional kingdoms.

Description

Besides their relatively simple levels of organization, protists do not necessarily have much in common. When used, the term "protists" is now considered to mean a paraphyletic assemblage of similar-appearing but diverse taxa (biological groups); these taxa do not have an exclusive common ancestor beyond being composed of eukaryotes, and have different life cycles, trophic levels, modes of locomotion, and cellular structures.

Examples of protists include:
* Amoebas (including nucleariids and Foraminifera);
* Choanoflagellates; ciliates;
* Diatoms;
* Dinoflagellates;
* '' Giardia'';
* Oomycetes (including '' Phytophthora'', the cause of the Great Famine of Ireland); and
* '' Plasmodium'' (which causes malaria);
* slime molds.
These examples are unicellular, although oomycetes can join to form filaments, and slime molds can aggregate into a tissue-like mass.

In cladistic systems (classifications based on common ancestry), there are no equivalents to the taxa Protista or Protoctista, as both terms refer to a paraphyletic group that spans the entire eukaryotic branch of the tree of life. In cladistic classification, the contents of Protista are mostly distributed among various ''supergroups'': examples include the
* Archaeplastida (or Plantae ''sensu lato'');
* Excavata (which is mostly unicellular flagellates); and
* Opisthokonta (which commonly includes unicellular flagellates, but also animals and fungi).
* SAR supergroup (of stramenopiles or heterokonts, alveolates, and Rhizaria);
"Protista", "Protoctista", and " Protozoa" are therefore considered obsolete. However, the term "protist" continues to be used informally as a catch-all term for eukaryotic organisms that are not within other traditional kingdoms. For example, the word "protist pathogen" may be used to denote any disease-causing organism that is not plant, animal, fungal, prokaryotic, viral, or subviral.

Subdivisions

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 such as:

;Protozoa: Protozoans are unicellular "animal-like" ( heterotrophic, and sometimes parasitic) organisms that are further sub-divided based on characteristics such as motility, such as the ( flagellated) Flagellata, the ( ciliated) Ciliophora, the ( phagocytic) amoeba, and the (spore-forming) Sporozoa.
;Protophyta: Protophyta are "plant-like" ( autotrophic) organisms that are composed mostly of unicellular algae. The dinoflagellates, diatoms and ''Euglena''-like flagellates are photosynthetic protists.
;Mold: Molds generally refer to fungi; but slime molds and water molds are "fungus-like" ( saprophytic) protists, although some are pathogens. Two separate types of slime molds exist, the cellular and acellular forms.

Some protists, sometimes called ambiregnal protists, have been considered to be both protozoa and algae or fungi (e.g., slime molds and flagellated algae), and names for these have been published under either or both of the '' ICN'' and the '' ICZN''. Conflicts, such as these – for example the dual-classification of Euglenids and Dinobryons, which are mixotrophic – is an example of why the kingdom Protista was adopted.

These traditional subdivisions, largely based on superficial commonalities, have been replaced by classifications based on phylogenetics (evolutionary relatedness among organisms).
Molecular analyses in modern taxonomy have been used to redistribute former members of this group into diverse and sometimes distantly related phyla. For instance, the water molds are now considered to be closely related to photosynthetic organisms such as Brown algae and Diatoms, the slime molds are grouped mainly under Amoebozoa, and the Amoebozoa itself includes only a subset of the "Amoeba" group, and significant number of erstwhile "Amoeboid" genera are distributed among Rhizarians and other Phyla.

However, the older terms are still used as informal names to describe the morphology and ecology of various protists. For example, the term '' protozoa'' is used to refer to heterotrophic species of protists that do not form filaments.

Classification

Historical classifications

Among the pioneers in the study of the protists, which were almost ignored by Linnaeus except for some genera (e.g., '' Vorticella, Chaos, Volvox, Corallina, Conferva, Ulva, Chara, Fucus''), were Leeuwenhoek, O. F. Müller, C. G. Ehrenberg and Félix Dujardin. The first groups used to classify microscopic organism were the Animalcules and the Infusoria. In 1818, the German naturalist Georg August Goldfuss introduced the word '' Protozoa'' to refer to organisms such as ciliates and corals. After the cell theory of Schwann and Schleiden (1838–39), this group was modified in 1848 by Carl von Siebold to include only animal-like unicellular organisms, such as foraminifera and amoebae. The formal taxonomic category ''Protoctista'' was first proposed in the early 1860s by 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 in 1866 by Ernst Haeckel, leaving plants, animals, and the protists (''Protista''), defined as a "kingdom of primitive forms".

In 1938, Herbert Copeland resurrected Hogg's label, arguing that Haeckel's term ''Protista'' included anucleated microbes such as bacteria, which the term "Protoctista" (literally meaning "first established beings") 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 nor monera were single groups of related organisms (they were not monophyletic groups).

Modern classifications

Systematists today do not treat Protista as a formal taxon, but the term "protist" is still commonly used for convenience in two ways. The most popular contemporary definition is a phylogenetic one, that identifies a paraphyletic group: a protist is any eukaryote that is not an animal, (land) plant