For the journal, see Protist (journal).
|ProtistTemporal range: Paleoproterozoic – Present
|Cladistically included but traditionally excluded taxa|
The study of protists is termed protistology.
The classification of a 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".
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.
Others 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.
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); choanaflagellates; ciliates; diatoms; dinoflagellates; Giardia; Plasmodium (which causes malaria); oomycetes (including Phytophthora, the cause of the Great Famine of Ireland); and slime molds.
These examples are unicellular, although oomycetes can form filaments, and slime molds can aggregate.
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 tree of life.
In cladistic classification, the contents of Protista are mostly distributed among various supergroups: examples include the SAR supergroup (of stramenopiles or heterokonts, alveolates, and Rhizaria); Archaeplastida (or Plantae sensu lato); Excavata (which is mostly unicellular flagellates); and Opisthokonta (which commonly includes unicellular flagellates, but also animals and fungi).
"Protista", "Protoctista", and "Protozoa" are therefore considered obsolete.
However, the term "protist" continues to be used informally as a catch-all term for eukayotic organisms that aren't within other traditional kingdoms.
See also: Eukaryote § Five supergroups
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: 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.
- Protophyta: These "plant-like" (autotrophic) organisms are composed mostly of unicellular algae. The dinoflagelates, diatoms and Euglena-like flagellates are photosynthetic protists.
- Molds: "Mold" 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.
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.
See also: Kingdom (biology) § Summary
Further information: and
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. MüllerC. and G. EhrenbergFélix Dujardin.
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.
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).
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, or (true) fungus; this definition excludes many unicellular groups, like the Microsporidia (fungi), many Chytridiomycetes (fungi), and yeasts (fungi), and also a non-unicellular group included in Protista in the past, the Myxozoa (animal).
Some systematists judge paraphyletic taxa acceptable, and use Protista in this sense as a formal taxon (as found in some secondary textbooks, for pedagogical purpose).
The other definition describes protists primarily by functional or biological criteria: protists are essentially those eukaryotes that are never multicellular, that either exist as independent cells, or if they occur in colonies, do not show differentiation into tissues (but vegetative cell differentiation may occur restricted to sexual reproduction, alternate vegetative morphology, and quiescent or resistant stages, such as cysts); this definition excludes many brown, multicellular red and green algae, which may have tissues.
The taxonomy of protists is still changing.
However, there are sometimes discordances between molecular and morphological investigations; these can be categorized as two types: (i) one morphology, multiple lineages (e.g. morphological convergence, cryptic species) and (ii) one lineage, multiple morphologies (e.g. phenotypic plasticity, multiple life-cycle stages).
Because the protists as a whole are paraphyletic, new 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) does not recognize formal ranks (phylum, class, etc.) and instead treats groups as clades of phylogenetically related organisms.
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, upper right.
Many are thought to be monophyletic, though there is still uncertainty.
Nutrition can vary according to the type of protist.
Some are mixotrophic.
Some protists that do not have / lost chloroplasts/mitochondria have entered into endosymbiontic relationship with other bacteria/algae to replace the missing functionality.
|Nutritional type||Source of energy||Source of carbon||Examples|
|Photoautotrophs||Sunlight||Organic compounds or carbon fixation||Most algae|
|Chemoheterotrophs||Organic compounds||Organic compounds||Apicomplexa, Trypanosomes or Amoebae|
For most important cellular structures and functions of animal and plants, it can be found a heritage among protists.
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.
Eukaryotes emerged in evolution more than 1.5 billion years ago.
The earliest eukaryotes were likely protists.
Although sexual reproduction is widespread among extant eukaryotes, it seemed unlikely until recently, that sex could be a primordial and fundamental characteristic of eukaryotes.
A principal reason for this view was that sex appeared to be lacking in certain pathogenic protists whose ancestors branched off early from the eukaryotic family tree.
However, several of these protists are now known to be capable of, or to recently have had the capability for, meiosis and hence sexual reproduction.
For example, the common intestinal parasite Giardia lamblia was once considered to be a descendant of a protist lineage that predated the emergence of meiosis and sex.
However, G. lamblia was recently found to have a core set of genes that function in meiosis and that are widely present among sexual eukaryotes.
These results suggested that G. lamblia is capable of meiosis and thus sexual reproduction.
Furthermore, direct evidence for meiotic recombination, indicative of sex, was also found in G. lamblia.
Trichomonas vaginalis, a parasitic protist, is not known to undergo meiosis, but when Malik et al.
tested for 29 genes that function in meiosis, they found 27 to be present, including 8 of 9 genes specific to meiosis in model eukaryotes.
These findings suggest that T. vaginalis may be capable of meiosis.
Since 21 of the 29 meiotic genes were also present in G. lamblia, it appears that most of these meiotic genes were likely present in a common ancestor of T. vaginalis and G. lamblia.
These two species are descendants of protist lineages that are highly divergent among eukaryotes, leading Malik et al.
to suggest that these meiotic genes were likely present in a common ancestor of all eukaryotes.
Based on a phylogenetic analysis, Dacks and Roger proposed that facultative sex was present in the common ancestor of all eukaryotes.
This view was further supported by a study of amoebae by Lahr et al.
Amoeba have generally been regarded as asexual protists.
However, these authors describe evidence that most amoeboid lineages are anciently sexual, and that the majority of asexual groups likely arose recently and independently.
Early researchers (e.g., Calkins) have interpreted phenomena related to chromidia (chromatin granules free in the cytoplasm) in amoeboid organisms as sexual reproduction.
Protists generally reproduce asexually under favorable environmental conditions, but tend to reproduce sexually under stressful conditions, such as starvation or heat shock.
Oxidative stress, which is associated with the production of reactive oxygen species leading to DNA damage, also appears to be an important factor in the induction of sex in protists.
Some commonly found Protist pathogens such as Toxoplasma gondii are capable of infecting and undergoing asexual reproduction in a wide variety of animals – which act as secondary or intermediate host – but can undergo sexual reproduction only in the primary or definitive host (for example: felids such as domestic cats in this case).
Free-living Protists occupy almost any environment that contains liquid water.
Protists make up a large portion of the biomass in both marine and terrestrial environments.
Parasitism: role as pathogens
Some protists are significant parasites of animals (e.g.; five species of the parasitic genus Plasmodium cause malaria in humans and many others cause similar diseases in other vertebrates), plants (the oomycete Phytophthora infestans causes late blight in potatoes) or even of other protists.
Protist pathogens share many metabolic pathways with their eukaryotic hosts.
A more thorough understanding of protist biology may allow these diseases to be treated more efficiently.
For example, the apicoplast (a nonphotosynthetic chloroplast but essential to carry out important functions other than photosynthesis) present in apicomplexans provides an attractive target for treating diseases caused by dangerous pathogens such as plasmodium.
Recent papers have proposed the use of viruses to treat infections caused by protozoa.
This turns the flies into a vector that can spread the pathogenic protist between red fire ant colonies.
Many protists have neither hard parts nor resistant spores, and their fossils are extremely rare or unknown.
Others are relatively common in the fossil record, as the diatoms, golden algae, haptophytes (coccoliths), silicoflagellates, tintinnids (ciliates), dinoflagellates, green algae, red algae, heliozoans, radiolarians, foraminiferans, ebriids and testate amoebae (euglyphids, arcellaceans).
Some are even used as paleoecological indicators to reconstruct ancient environments.
More probable eukaryote fossils begin to appear at about 1.8 billion years ago, the acritarchs, spherical fossils of likely algal protists.
Another possible representative of early fossil eukaryotes are the Gabonionta.
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Protist.