A protist (/) is any eukaryotic organism (one with cells containing a nucleus) that is not an animal, plant or fungus. The protists do not form a natural group, or clade, since they exclude certain eukaryotes; but, like algae or invertebrates, they are often grouped together for convenience. In some systems of biological classification, such as the popular five-kingdom scheme proposed by Robert Whittaker in 1969, the protists make up a kingdom called Protista, composed of "organisms which are unicellular or unicellular-colonial and which form no tissues".[A]
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. In the classification system of Lynn Margulis, the term protist is reserved for microscopic organisms, while the more inclusive term Protoctista is applied to a biological kingdom that includes certain large multicellular eukaryotes, such as kelp, red algae and slime molds. Others use the term protist more broadly, to encompass both microbial eukaryotes and macroscopic organisms that do not fit into the other traditional kingdoms.
In cladistic systems (classifications based on common ancestry), there are no equivalents to the taxa Protista or Protoctista, both terms referring to a paraphyletic group that spans the entire eukaryotic tree of life. In cladistic classification, the contents of Protista are distributed among various supergroups (SAR, such as protozoa and some algae, Archaeplastida, such as land plants and some algae, Excavata, which are a group of unicellular organisms, and Opisthokonta, such as animals and fungi, etc.). "Protista", ''Protoctista'' and "Protozoa" are considered obsolete. However, the term "protist" continues to be used informally as a catch-all term for unicellular eukaryotic microorganisms. For example, the word "protist pathogen" may be used to denote any disease-causing microbe that is not bacteria, virus, viroid, prion, or metazoa.
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:
- These unicellular "animal-like" (heterotrophic, and sometimes parasitic) organisms 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.
- These "plant-like" (autotrophic) organisms are composed mostly of unicellular algae.
- Slime molds and water molds are "fungus-like" (saprophytic) organisms.
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 "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.