A protist ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic group encompassing the entire eukaryote tree of life, from which land plants, animals, and fungi evolved. They are primarily single-celled, exhibiting a wide range of forms such as amoebae, ciliates, thick-walled microalgae and, more commonly, flagellates. Several transitions to multicellularity have occurred among protists, from colonies with alternating cell types to giant slime molds, fungus-like organisms, and seaweeds with differentiated tissues.
Protists were historically regarded as a separate taxonomic kingdom known as Protista or Protoctista, or were lumped together as part of the traditional plant and animal kingdoms as algae and protozoa, respectively. With the advent of molecular phylogenetics and electron microscopy studies, some protists were shown to be more closely related to animals or plants than to other protists, and algae were found to be intermixed with protozoa. The classification suffered major revisions, as seemingly unrelated forms were found to be evolutionarily related, and vice versa.
In modern classifications, protists are spread across several large clades known as supergroups, many of them containing disparate forms. For example, the Archaeplastida includes mostly phototrophs like red and green algae, from which land plants evolved. Opisthokonta groups fungi, animals, and their single-celled relatives. Amoebozoa and Rhizaria harbor the majority of amoeboid organisms, such as testate amoebae, foraminifers and radiolarians. Stramenopiles and Alveolata are diverse groups of flagellates, many of which have evolved into major parasites (e.g., oomycetes, apicomplexans) or phototrophs (diatoms, brown algae, dinoflagellates). The earliest diverging groups, collectively known as Excavata (e.g., euglenids, metamonads), are flagellates that represent the ancestral traits of the last eukaryotic common ancestor (LECA). Despite the comparatively low number of described species, protists compose the majority of eukaryotic diversity as indicated by environmental DNA studies. Most protists are yet undescribed.
Protists encompass almost all of the biological traits seen in eukaryotes, and many exhibit unique adaptations. These include a range of nutritional modes through specialized feeding structures (phagotrophy, osmotrophy, myzocytosis) or chloroplasts (phototrophy), often mixing both as mixotrophy. Cellular respiration also varies due to modifications of their mitochondria. Almost all protists have a complex cytoskeleton composed of relatively conserved structures across evolution, namely a flagellar apparatus with basal bodies from which microtubules emerge and support the remaining cellular structures. Many protists have unique organelles that serve other functions, such as contractile vacuoles for homeostasis, or eyespots for light perception. Protist cells tend to host symbionts such as bacteria and archaea, usually to support their metabolism and nutrition. Although traditionally presumed to be asexual, protists are capable of sexual reproduction, and can exhibit diverse and complex life cycles with different generations and life stages.
Protists are abundantly present in all ecosystems, including extreme habitats, as important components of the biogeochemical cycles and trophic webs. As producers, they are responsible for a large portion of global primary production and carbon fixation. As consumers and decomposers, they regulate fungal and bacterial populations, and release nutrients to other trophic levels. Some form mutualistic relationships with other protists or animals such as corals and termites. Others are important parasites. Pathogenic protists cause many well-known human and animal diseases such as malaria and toxoplasmosis, or significant plant diseases like clubroot and potato blight. Free-living protists can also negatively impact aquatic life as harmful algal blooms.
The early evolution of protists corresponds with the evolution of eukaryotes, which split from archaea around 3 billion years ago and eventually gave rise to a common ancestor (LECA) with essential traits such as mitochondria and a complex endomembrane system, some time during the Paleo- or Mesoproterozoic eras. In the gap between these two events, fossils are often interpreted as stem-group eukaryotes, with intermediate traits. Following the appearance of LECA, its descendants (crown-group eukaryotes) experienced a rapid diversification in the span of 300 million years that originated the modern supergroups. Still, their abundance in the fossil record remained low until the Neoproterozoic, when the first fossils of opisthokonts, amoebae, and multicellular algae appear. Throughout the Phanerozoic, protists evolved into the forms that dominate ecosystems today, leaving an extensive fossil record of primarily siliceous and calcareous shells.
Definition
Protists are defined as all eukaryotes that are not animals, land plants or fungi, the three traditional "higher" kingdoms of eukaryotes. Because of this definition by exclusion, protists compose a paraphyletic group from which the ancestors of those three kingdoms evolved. As such, there is no unique trait that unifies all protists yet excludes non-protists. Still, together they exhibit a remarkable diversity of life cycles, trophic levels, modes of locomotion, and cellular structures that dwarfs those seen in "higher" eukaryotes. A less popular view is that protists are defined as exclusively single-celled eukaryotes, but this disregards the various transitions to multicellularity among protists.
The distinction between protists and other kingdoms was blurry before genetic analysis. Organisms that are unquestionably known as protists include a wide range of photosynthetic species, known as "algae", including various types of macroalgae that have a similar complexity to plants. Other protists have a fungus-like nutrition or appearance, such as the oomycetes. The remaining heterotrophic protists are often called "protozoa". Some minuscule animals (the myxozoans) and the "lower" fungi (namely the aphelids, rozellids and microsporidians, collectively the opisthosporidians) were traditionally classified as protists, and some algae (particularly red and green algae) were lumped with plants.
According to the current consensus, the label 'protist' specifically excludes animals, embryophytes (land plants)—meaning that all eukaryotic algae fall under this label—and all fungi. Opisthosporidians are considered part of a larger fungal kingdom, although they are studied by protistologists and mycologists alike.
Morphology
Protists exist in diverse shapes and sizes. While most are single-celled, others have evolved various forms of multicellularity, ranging from simple colonies of cells to large, complex organisms like giant kelp and slime molds. Single-celled protists are traditionally grouped by their shape and how they move, into four broad categories.
Flagellates are the most common protists, and very likely the most abundant eukaryotes on Earth. They move using one or more whip-like structures called flagella. Most are heterotrophic (known as zooflagellates), feeding on bacteria or other organisms, ranging from filter feeders like choanoflagellates to active predators like provorans. Many are photo- or mixotrophic (known as phytoflagellates) and are studied as algae, like the dinoflagellates.
Amoebae are known for their often flexible shape and ability to form extensions of the cytoplasm known as pseudopodia. These extensions come in various forms, such as lobose (blunt, rounded, as in Amoeba), filose (thin, tapering, as in cercozoans), or reticulose (branching networks, as in foraminifers). Some, called axopodia, take the shape of radiating projections supported by microtubules, characteristic of heliozoa and radiolaria. Some amoebae can grow to sizes visible to the naked eye, reaching up to 20 cm. Amoeboflagellates can produce both pseudopodia and flagella within the same life cycle. Ciliates have larger cells with two types of nuclei and rows of small flagella, known as cilia. They are often at the top of the microbial food web. Although ciliates compose a single lineage, some protists have evolved unusual large ciliate-like cells, such as the opalines. Certain parasitic protists traditionally described as sporozoa are immobile in their adult stage and reproduce through thick-walled spores, such as haplosporidians and apicomplexans. This term also included fungi and animals that have evolved a similar lifestyle—microsporidians and myxozoans, respectively. Other than their lifestyle, they have very little in common, and have evolved in distantly related groups.
Other single-celled algae exist in forms beyond the motile flagellates. Some are non-motile and encased in hard cell walls (coccoid, like diatoms) or embedded in a mucilage matrix (capsalean, like glaucophytes); others are amoeboid, like the reticulose chlorarachniophytes.
Multicellularity has evolved numerous times to various degrees among protists, resulting in organisms built either by cells aggregating together (aggregative) or by cells dividing without separating (clonal). For example:
Fungus-like protists, traditionally studied by mycologists, are osmotrophic and produce macroscopic fruiting bodies for dispersal (sorocarps if aggregative, sporocarps if clonal). They may have hyphae-like bodies, or they may be slime molds, composed of individual amoebae until they form fruiting bodies. Some can reach lengths of several meters. Multicellular algae range from simple filaments and coenocytes to the highly complex brown algae, which have fully differentiated tissues (parenchymatous) resembling those of plants, or the red algae, with only partial differentiation (pseudoparenchymatous). In green algae alone multicellularity is thought to have evolved over 20 separate times, with some groups like the stoneworts developing specialized reproductive organs.
Other multicellular protists include amoebae that fuse into large networks, and colonial heliozoa and ciliates with new features not seen in solitary cells. The ciliate Haplozoon is interpreted to have animal-like embryonic development and cell type differentiation. Choanoflagellates, the closest living relatives of animals, include alternating cell types that are interpreted as early stages of animal multicellularity.
Classification
Brief history
Starting in the 17th and 18th centuries, after the discovery of microscopic life by Antonie van Leeuwenhoek, the classification of single-celled protists was largely based on observations under light microscopy. Protists were incorporated into the traditional dichotomy that defined all life as either plant or animal: non-motile algae were considered part of the plant kingdom, and all other protists joined the animal kingdom. They were popularly known as "infusion animals" or infusoria, together with bacteria and small invertebrates. Otto Friedrich Müller was the first to introduce microbial protists to the Linnean system of binomial nomenclature.