14 protists | Text for Biology at Roxbury Community College (2023)

ONEprotistsis any eukaryotic organism (one with cells that contain a nucleus) that is not an animal, plant, or fungus. Although protists likely 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. Thus, some protists may be more closely related to animals, plants, or fungi than to other protists; however, like algae, invertebrates, or protozoa, grouping is used for convenience.

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Figure 14.1:A sample of protists:red algae (Chondrus crispus); brown algae (giant algae); ciliate (Frontonia); golden algae (Dinobryon); Foraminifera (Radiolaria); parasitic flagellate (Giardia muris); pathogenic amoeba (Acanthamoeba); Amebozoan mucilaginous mold (Fuligo septica)

In ancient times, both animal and plant lineages were recognized. The Swedish botanist gave them the taxonomic category of Kingdom.Carl Linneo🇧🇷 Although he included fungi with plants with some reservations, he later realized that they are quite distinct and warrant a separate kingdom, the composition of which was not entirely clear until the 1980s. you met. In 1818, German biologist Georg A. Goldfuss coined the word protozoa to refer to organisms such as ciliates, and this group was expanded to include all single-celled eukaryotes, which were given their own kingdom, Protista, byErnst Haeckelem 1866.

Eukaryotes are now composed of four kingdoms:

  • kingdom protista
  • vegetal kingdom
  • mushroom kingdom
  • animal Kingdom

Protists were understood to be "primitive forms" and therefore an evolutionary grade, united by their primitive unicellular nature. The revelation of deep divisions in the tree of life did not really begin until DNA sequencing, leading to a system of domains instead of kingdoms like the top level proposed by Carl Woese, uniting all eukaryotic kingdoms under the eukaryotic domain. At the same time, work on the protist tree intensified and is actively continuing to this day. Several alternative classifications have been presented, although there is no consensus in the field.

A revised classification in 2012 recognizes five supergroups of eukaryotes, as shown in Table14.1.

Table 14.1:A recent (2012) classification of protists.
supergroup nameOrganisms
Archaeplastida (or Primoplantae)Land plants, green algae, red algae and glaucophytes
SAR supergroupStramenopiles (brown algae, diatoms, etc.), Alveolata and Rhizaria (Foraminifera, Radiolaria and various other amoeboid protozoa)
Excavatavarious flagellated protozoa
amoebaMost loboseamoeboid and slimy molds
collegeAnimals, fungi, choanoflagellates, etc.

The classification of a separate kingdom for animals and plants was first proposed by John Hogg in 1860 as the kingdom Protista; in 1866 Ernst Haeckel also proposed a third kingdom Protista as "the kingdom of primitive forms". Originally they also included prokaryotes, but over time they would be moved to a fourth kingdom, Monera. In the popular five-kingdom scheme proposed by Robert Whittaker in 1969, protists were defined as eukaryotes "organisms that are unicellular or colonial-unicellular and that do not form tissues", and the fifth kingdom Fungi was established. In Lynn Margulis's five-kingdom system, the term protist is reserved for microscopic organisms, while the more inclusive kingdom Protista (or protists) included certain large multicellular eukaryotes such as seaweed, red algae, and slime fungi. Others use the term protist interchangeably with Margulis protist to encompass unicellular and multicellular eukaryotes, including those that form specialized tissues but do not fit into any of the other traditional kingdoms.

Aside from their relatively simple levels of organization, protists don't necessarily have much in common. When used, the term "protists" is now understood to mean a paraphyletic assemblage of taxa (biological groups) similar in appearance but diverse; These taxa do not have a unique common ancestor other than being composed of eukaryotes, and have different life cycles, trophic levels, modes of locomotion, and cellular structures. Examples of protists include: amoebae (including nuclei and foraminifera); choanaflagellates; ciliates; diatoms; dinoflagellates; Giardia; Plasmodium (which causes malaria); oomycetes (including Phytophthora, the cause of the Great Irish Famine); 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 taxa Protista or Protista, as both terms refer to a paraphyletic group that spans the entire eukaryotic tree of life. In cladistic classification, the contents of Protista are distributed mainly among several supergroups: examples include the SAR supergroup (of stramenopiles or heterokonts, alveolates and Rhizaria); Archaeplastida (or Plantae sensu lato); Excavata (which are mainly unicellular flagellates); and Opisthokonta (which commonly includes unicellular flagellates, but also animals and fungi). Therefore, “Protista”, “Protista” and “Protozoa” are considered obsolete. However, the term "protist" continues to be used informally as a general term for eukaryotic organisms not found in other traditional kingdoms. For example, the word "protist pathogen" can be used to mean any disease-causing organism that is not a plant, animal, fungus, prokaryote, virus, or subvirus.

The term protist was first used by Ernst Haeckel in 1866. Traditionally, protists have been subdivided into several groups based on similarities to "higher" kingdoms, such as:

  • Protozoa: Single-celled "animal-like" (heterotrophic and sometimes parasitic) organisms that are subdivided based on characteristics such as motility, such as the Flagellata (flagellate), Ciliophora (ciliate), amoeba (phagocytic), and the sporozoans (sporozoans). spores).
  • Protophyta: Plant-like (autotrophic) organisms that are composed primarily of single-celled algae. Dinoflagellates, diatoms, and Euglena-like flagellates are photosynthetic protists.
  • Molds: "Mold" usually refers to fungus; but slime molds and water molds are fungus-like protists (saprophytes), although some are pathogenic.

Some protists, sometimes called ambirenal protists, have been considered both protozoans and algae or fungi (e.g., slime fungi and flagellate algae). Conflicts like these, for example the dual classification of Euglenids and Dinobryons, which are mixotrophs, is an example of why the kingdom Protista was adopted.

These traditional subdivisions, based mainly on superficial similarities, have been replaced by classifications based on phylogenetics (evolutionary relationships between organisms). Molecular analyzes in modern taxonomy have been used to redistribute ancient members of this group into diverse and sometimes distantly related phyla. For example, aquatic fungi are now considered closely related to photosynthetic organisms such as brown algae and diatoms, slime fungi are grouped primarily under the Amoebozoa, and the Amoebozoa themselves include only a subset of the "Amoeba" group and a significant number of the former genera" Amoeboid" 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.

Today's systematists do not treat Protista as a formal taxon, but the term "protista" is still commonly used for convenience in two ways. The most popular contemporary definition is phylogenetic, which identifies a paraphyletic group: a protist is any eukaryote that is not an animal, plant (terrestrial) or fungus (true); this definition excludes many unicellular groups, such as Microsporidia (fungi), many Chytridiomycetes (fungi) and yeasts (fungi), and also a non-unicellular group included in Protista in the past, the Myxozoa (animals).

The taxonomy of protists is still changing. More recent classifications attempt to present monophyletic groups based on morphological (especially ultrastructural), biochemical (chemotaxonomy) and DNA sequence (molecular research) information. Because protists as a whole are paraphyletic, new systems often split or leave the kingdom, treating groups of protists as separate lines of eukaryotes.

Table 14.2:Nutritional types in protist metabolism
nutritional typePower supplycarbon sourceexamples
photoautotrophicSun lightOrganic compounds or carbon fixationMostalgas
chemoheterotrophsorganic compoundsorganic compoundsApicomplexa, trypanosomes or amoebae


protozoa(also protozoa, plural protozoa) is an informal term for a group of single-celled, free-living, or parasitic eukaryotes that feed on organic matter, such as other microorganisms or organic tissues and detritus. Historically, protozoa have been considered "single-celled animals" because they often have animal-like behaviors, such as motility and predation, and lack a cell wall, as found in plants and many algae. Although the traditional practice of grouping protozoa with animals is no longer considered valid, the term continues to be used loosely to describe unicellular protists (i.e., eukaryotes that are not animals, plants, or fungi) that feed on heterotrophy.

In some systems of biological classification, protozoa remain a high-ranking taxonomic group. When Georg Goldfuss first introduced them in 1818, protozoa were a class within animals, and their etymology is literally "first animals". In later classification schemes, it has been elevated to a variety of higher ranks, including phylum, subkingdom, and kingdom, and has sometimes been subsumed within Protista or Protista. With the advent of techniques such as molecular phylogenetics, it was understood that protozoa did not represent a natural group; but although it is not an accepted taxon in cladistic analyses, some systematics continue to use it as a formal taxon.

In a series of classifications proposed by Thomas Cavalier-Smith and his collaborators since 1981, protozoa are classified as a kingdom. The seven kingdoms scheme presented by Ruggiero et al. in 2015, he places eight phyla in Kingdom Protozoa: Euglenozoa, Amoebozoa, Metamonada, Choanozoa sensu Cavalier-Smith, Loukozoa, Percolozoa, Microsporidia and Sulcozoa. Notably, this kingdom excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, dinoflagellates, foraminifera, and parasitic apicomplexans, all of which are classified in Kingdom Chromista. Kingdom Protozoa, as defined in this scheme, does not form a natural group or clade, but rather a paraphyletic group or evolutionary grade, within which members of Fungi, Animalia, and Chromista are believed to have evolved.

The word “protozoan” (singular protozoan or protozoan) was coined in 1818 by the zoologist Georg August Goldfuss, as the Greek equivalent of the German Urthiere, meaning “primitive or original animals” (ur- 'proto-' + Thier 'animal'). . Goldfuss created protozoa as a class containing what he believed to be the simplest animals. Originally, the group included not only single-celled microorganisms, but also some "lower" multicellular animals, such as rotifers, corals, sponges, jellyfish, bryozoans, and polychaete worms. The term protozoan is formed by the Greek words πρῶτος (prôtos), which means “first”, and ζῶα (zôa), plural of ζῶον (zôon), which means “animal”. Some researchers have discouraged the use of protozoa as a formal taxon, mainly because the term implies kinship with animals (metazoans) and promotes an arbitrary separation of "animal-like" organisms from "plant-like" organisms.

In 1848, as a result of advances in cell theory pioneered by Theodor Schwann and Matthias Schleiden, the anatomist and zoologist C. T. von Siebold proposed that the bodies of protozoa, such as ciliates and amoebae, consisted of individual cells, similar to those from which tissues are formed. of plants and animals were built. Von Siebold redefined protozoa to include only single-celled forms, excluding all metazoans (animals). At the same time, he raised the group to the level of a phylum containing two major classes of microorganisms: Infusoria (mainly ciliates and flagellated algae) and Rhizopoda (amoeboid organisms). The definition of protozoa as a phylum or sub-kingdom formed by "unicellular animals" was adopted by the zoologist Otto Bütschli, celebrated in his centenary as the "architect of protozoology", and the term became more widespread.

As a phylum under Animalia, protozoa were firmly entrenched in the old "two kingdoms" classification of life, under which all living things were classified as either animals or plants. As long as this scheme remained dominant, Protozoa were understood as animals and studied in Zoology departments, while photosynthetic microorganisms and microscopic fungi, called Protophyta, were assigned to Plant Departments and studied in Botany departments.

Criticism of this system began in the second half of the 19th century, when it was noticed that many organisms met the criteria for inclusion between plants and animals. For example, Euglena and Dinobryon algae have chloroplasts for photosynthesis, but they can also feed on organic matter and are motile. In 1860, John Hogg argued against the use of "protozoa", claiming that "Naturalists are divided in opinion, and probably some will remain so, whether many of these organisms, or living beings, animals or plants". As an alternative, he proposed a new kingdom called Primigenum, composed of protozoa and single-celled algae (protophyta), which he combined under the name "Protoctista". In Hoggs' conception, the animal and vegetable kingdoms resembled two great "pyramids" that merged at their bases in the Kingdom Primigenum.

Six years later, Ernst Haeckel also proposed a third kingdom of life, which he called Protista. At first Haeckel included some multicellular organisms in this kingdom, but in later work he restricted protists to unicellular organisms, or single colonies whose individual cells do not differentiate into different tissue types.

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Despite these proposals, protozoa emerged as the preferred taxonomic location for heterotrophic microorganisms such as amoebae and ciliates, and remained so for over a century. However, as the 20th century progressed, the old "two kingdoms" system began to weaken, with the growing realization that fungi did not belong to plants and that most single-celled protozoa were no more related to animals than to fungi. . They went to the plants. By mid-century, some biologists, such as Herbert Copeland, Robert H. Whittaker, and Lynn Margulis, advocated the revival of Haeckel's Protista or Hogg's Protista as a kingdom-level eukaryotic group, along with Plants, Animals, and Fungi. A variety of multi-kingdom systems have been proposed, and kingdoms Protista and Protista have become well established in biology textbooks and curricula.

While many taxonomists abandoned protozoa as a higher-level group, Thomas Cavalier-Smith retained them as a kingdom in the various classifications he proposed. As of 2015, Cavalier-Smith's protozoa exclude several major groups of organisms traditionally placed among protozoa, including ciliates, dinoflagellates, and foraminifera (all members of the SAR supergroup). In its present form, his kingdom Protozoa is a paraphyletic group that includes a common ancestor and most of its descendants, but excludes two major branching clades within it: the animals and the fungi.

As protozoa, as traditionally defined, can no longer be considered "primitive animals", the terms "protists", "Protista" or "Protista" are sometimes preferred. In 2005, members of the Society of Protozoologists voted to change its name to the International Society of Protistologists. Free-living protozoa are common and often abundant in fresh, brackish, and salt water, as well as other moist environments such as soils and mosses. 🇧🇷 Some species thrive in extreme environments such as hot springs and hypersaline lakes and ponds. All protozoans require a moist habitat; however, some can survive for long periods of time in dry environments, forming resting cysts that allow them to remain dormant until conditions improve.

Parasitic and symbiotic protozoa live on or in other organisms, including vertebrates and invertebrates, as well as plants and other single-celled organisms. Some are harmless or beneficial to their host organisms; others can be important causes of diseases such as babesia, malaria and toxoplasmosis.

The association between protozoan symbionts and their host organisms can be mutually beneficial. Flagellated protozoa such as Trichonympha and Pyrsonympha inhabit the guts of termites, where they allow their host insects to digest the wood, helping to break down complex sugars into smaller, more easily digestible molecules. A wide variety of protozoa live as commensals in the rumen of ruminant animals such as cattle and sheep. These include flagellates such as Trichomonas and ciliated protozoa such as Isotricha and Entodinium. The ciliate subclass Astomatia is composed entirely of mouthless symbionts adapted to live in the guts of annelid worms.

All protozoa are heterotrophs and obtain nutrients from other organisms, either by eating them whole or by consuming their organic remains and waste. Some protozoa ingest food by phagocytosis, devouring organic particles with pseudopodia (as amoebas do) or ingesting food through a mouth-shaped opening called a cytostome. Others ingest food by osmotrophy, absorbing dissolved nutrients through their cell membranes.

Parasitic protozoa use a wide variety of feeding strategies and some may change feeding methods at different stages of their life cycle. For example, the malaria parasite Plasmodium feeds by pinocytosis during its immature trophozoite life stage (ring stage), but develops a dedicated feeding organelle (cytostome) as it matures within a host's red blood cells.

Protozoa can also live as mixotrophs, supplementing a heterotrophic diet with some form of autotrophy. Some protozoans form close associations with symbiotic photosynthetic algae, which live and grow within larger cell membranes and provide nutrients to the host. Others practice kleptoplasty, stealing prey's chloroplasts and keeping them inside their own cell bodies while continuing to produce nutrients through photosynthesis. The ciliate Mesodinium rubrum retains functional plastids from the cryptophyte algae it feeds on, using them for autotrophy. These, in turn, can move on to dinoflagellates of the genus Dinophysis, which feed on Mesodinium rubrum but keep the plastids enslaved. Within Dinophysis, these plastids can continue to function for months.

Organisms traditionally classified as protozoa are abundant in aqueous environments and soils, occupying a variety of trophic levels. The group includes flagellates (which move with the help of whip-like structures called flagella), ciliates (which move using hair-like structures called cilia), and amoebae (which move using foot-like structures called pseudopodia). Some protozoa are sessile and do not move.

Unlike plants, fungi, and most types of algae, protozoans generally do not have a rigid cell wall, but are usually enclosed in elastic membrane structures that allow for cell movement. In some protozoans, such as ciliates and euglenozoans, the cell is supported by a composite membranous envelope called a "pelicle". The pellicle gives the cell shape, especially during locomotion. The films of protozoan organisms range from flexible and elastic to quite rigid. In ciliates and Apicomplexa, the pellicle is supported by densely packed vesicles called alveoli. In euglenids, it is formed by strips of proteins arranged in a spiral along the body. Familiar examples of pellicled protists are the euglenoids and the ciliate Paramecium. In some protozoans, the pellicle harbors epibiotic bacteria that adhere to the surface via their fimbriae (attachment hairs).

Some protozoa have two-phase life cycles, alternating between proliferative stages (eg, trophozoites) and dormant cysts. Like cysts, protozoa can survive in harsh conditions, such as exposure to extreme temperatures or harmful chemicals, or long periods without access to nutrients, water, or oxygen for periods of time. Being a cyst allows parasitic species to survive outside a host and allows their transmission from one host to another. When protozoa are in the form of trophozoites (from the Greek tropho = to nourish), they actively feed. Conversion from a trophozoite to a cyst form is known as encystment, while the process of transforming back into a trophozoite is known as de-encystment.

Protozoa reproduce asexually by binary fission or multiple fission. Many protozoan species also exchange genetic material by sexual means (usually by conjugation), but this is usually unrelated to the reproductive process and does not immediately result in a population increase.

Although meiotic sex is widespread among living eukaryotes, until recently it was unclear whether or not eukaryotes were sexual early in their evolution. Due to recent advances in gene detection and other techniques, evidence of some form of meiotic sex has been found in an increasing number of ancient lineage protozoans that diverged early in eukaryotic evolution. (See eukaryotic reproduction). Therefore, such findings suggest that meiotic sex arose early in eukaryotic evolution. Examples of protozoan meiotic sexuality are described in the articles Amoebozoa, Giardia lamblia, Leishmania, Plasmodium falciparum biology, Paramecium, Toxoplasma gondii, Trichomonas vaginalis and Trypanosoma brucei.

Historically, protozoans were classified as "single-celled animals" as opposed to protophytes, single-celled photosynthetic organisms (algae) considered to be primitive plants. Both groups commonly received the rank of phylum, under the kingdom Protista. In older classification systems, the phylum Protozoa was commonly divided into several subgroups, reflecting means of locomotion. Classification schemes differed, but for much of the 20th century, major protozoan groups included:

  • Flagellates or Mastigophora (motile cells equipped with whip-like organelles for locomotion, e.g. Giardia lamblia)
  • Amoebas or Sarcodina (cells that move by extending pseudopodia or lamellipodia, e.g. Entamoeba histolytica)
  • Sporozoa, or Sporozoa (spore-producing parasitic cells whose adult form lacks motility organs, e.g. Plasmodium knowlesi)
    • Apicomplexa (now in Alveolata)
    • Microsporidia (now in fungi)
    • Ascetosporea (now in Rhizaria)
    • Myxosporidia (now in Cnidaria)
  • Ciliates, or Ciliophora (cells equipped with a large number of short hair-like organs of locomotion, e.g. Balantidium coli)

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Figure 14.3:composite imageof various Apicomplexan parasites showing Babesia microti in red blood cells (top left), Toxoplasma gondii (top right), a septate eugregarine (bottom left), Lankesteria cystodytae (bottom center) and Plasmodium falciparum in red blood cells (bottom right) .

With the advent of molecular phylogenetics and tools that allowed researchers to directly compare the DNA of different organisms, it became clear that, of the major protozoan subgroups, only the ciliates (Ciliophora) formed a natural group or monophyletic clade (i.e., a distinct lineage of organisms that share a common ancestor). The other classes or subphyla of protozoans were all polyphyletic groups made up of organisms that, despite similarities in appearance or way of life, were not necessarily closely related to one another. In the eukaryotic classification system currently endorsed by the International Society of Protistologists, members of the ancient phylum Protozoa were distributed among a variety of supergroups.

Several pathogenic protozoa are human parasites and cause diseases such as malaria (Plasmodium), amoebiasis, giardiasis, toxoplasmosis, cryptosporidiosis, trichomoniasis, Chagas disease, leishmaniasis, African trypanosomiasis (sleeping sickness), Acanthamoeba keratitis, and primary amoebic meningoencephalitis (naegleriasis) . 🇧🇷 🇧🇷

The protozoan Ophryocystis elektroscirrha is a parasite of butterfly larvae, which is transmitted from the female to the caterpillar. Severely infected individuals are weak, unable to spread their wings or hatch, and have a shorter lifespan, but parasite levels vary in populations. The infection creates a culling effect, making infected migrating animals less likely to complete the migration. This results in populations with lower parasite loads at the end of the migration. This is not the case with commercial or laboratory breeding, where after a few generations, all individuals can become infected.


In the oldest classifications, flagellate protozoa were grouped in Flagellata (= Mastigophora), sometimes divided into Phytoflagellata (= Phytomastigina, mainly autotrophic) and Zooflagellata (= Zoomastigina, heterotrophic). They are sometimes grouped with Sarcodina (ameboid) in the Sarcomastigophora group.

Excavata is an important supergroup of eukaryotic unicellular organisms that are classified based on their flagellar structures and are considered the most basal flagellate lineage. It contains a variety of free-living and symbiotic forms, and also includes some important human parasites, including Giardia and Trichomonas. With the exception of Euglenozoa, all are non-photosynthetic.

Euglena is a genus of unicellular flagellated eukaryotes. It is the best known and studied member of the class Euglenoidea, a diverse group containing about 54 genera and at least 800 species. Euglena species are found in both fresh and salt water. They tend to be abundant in calm inland waters, where they can bloom in sufficient numbers to color the surface of ponds and ditches green (E. viridis) or red (E. sanguinea).

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Figure 14.4: Euglena.

The species Euglena gracilis has been widely used in the laboratory as a model organism.

Most Euglena species have photosynthetic chloroplasts within the cell body, allowing them to feed by autotrophy, like plants. However, they can also feed heterotrophically, like animals. Since Euglena has characteristics of both animals and plants, early taxonomists, working within Linnaeus's two-kingdom biological classification system, found it difficult to classify them. It was the question of where to place these "unclassifiable" creatures that led Ernst Haeckel to add a third living kingdom (a fourth kingdom in its entirety) to Linnaeus' Animale, Vegetabile (and Lapideum): Kingdom Protista.

When feeding as a heterotroph, Euglena absorb nutrients by osmotrophy and can survive without light on a diet of organic matter such as meat extract, peptone, acetate, ethanol or carbohydrates. When there is enough sunlight to feed by phototrophy, it uses chloroplasts containing the pigments chlorophyll a and chlorophyll b to produce sugars by photosynthesis. Euglena's chloroplasts are surrounded by three membranes, whereas those of plants and green algae (between which early taxonomists used to place Euglena) have only two membranes. This fact was taken as morphological evidence that Euglena's chloroplasts evolved from a eukaryotic green alga. Thus, the similarities between Euglena and plants would not have arisen from kinship, but from a secondary endosymbiosis. Molecular phylogenetic analysis provided support for this hypothesis, and it is now generally accepted.

Euglena's chloroplasts contain pyrenoids, used in the synthesis of paramylon, an energy-storage form of starch that allows Euglena to survive periods of light deprivation. The presence of pyrenoids is used as an identifying characteristic of the genus, separating it from other euglenoids, such as Lepocinclis and Phacus.

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Euglena have two flagella rooted in basal bodies located in a small reservoir at the front of the cell. Normally, one flagellum is very short and does not protrude outside the cell, while the other is long enough to be seen under light microscopy. In some species, such as Euglena mutabilis, both flagella are "non-emergent" (completely confined within the cell reservoir) and, consequently, cannot be seen under the light microscope. In species that have a long, emergent flagellum, it can be used to help the organism swim. The surface of the flagellum is covered with about 30,000 extremely thin filaments called mastigonemes.

Like other euglenoids, Euglena has a red eyelet, an organelle composed of carotenoid pigment granules. The red spot itself is not considered photosensitive. Instead, it filters sunlight that falls on a light-sensing structure at the base of the flagellum (a bump, known as the paraflagellar body), allowing only certain wavelengths of light to reach it. As the cell rotates relative to the light source, the eyelet partially blocks the source, allowing Euglena to find the light and move toward it (a process known as phototaxis).

Euglena does not have a cell wall. Instead, it has a pellicle formed by a layer of protein supported by a substructure of microtubules, arranged in ribbons that spiral around the cell. The action of these film strips sliding over each other, known as metabolism, gives Euglena its exceptional flexibility and contractility. The mechanism of this euglenoid movement is not understood, but its molecular basis may be similar to that of amoeboid movement.

In conditions of low humidity, or when food is scarce, Euglena forms a protective wall around itself and lies dormant like a resting cyst until the environment.

Euglena reproduces asexually through binary fission, a form of cell division. Reproduction begins with mitosis of the cell nucleus, followed by division of the cell itself. Euglena divides longitudinally, starting at the front end of the cell, with duplication of the flagellar processes, throat and stigma. Currently, a slit is formed at the front, and a V-shaped fork gradually moves to the rear, until the two halves are completely separated.


Ciliates are a group of protozoa characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are generally shorter and present in much greater numbers, with a wavy pattern different from that of flagella. found in all members of the group (although the peculiar Suctoria only has them for part of its life cycle) and are used in various ways for swimming, crawling, clinging, feeding, and feeling.

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Figure 14.5:composite imageof various ciliates: Lacrymaria olor, Paramecium bursaria, Coleps, Dileptus, Stentor coeruleus.

Ciliates are an important group of protists, common almost everywhere there is water: in lakes, ponds, oceans, rivers and soils. About 4,500 unique free-living species have been described, and the potential number of extant species is estimated to be between 27,000 and 40,000. This number includes many ectosymbiotic and endosymbiotic species, as well as some obligate and opportunistic parasites. Ciliated species vary in size from as little as 10 µm in some colpods to 4 mm in length in some geleids and include some of the most morphologically complex protozoa.

Unlike most other eukaryotes, ciliates have two different types of nuclei: a tiny diploid micronucleus (the "generative nucleus", which carries the cell's germ line) and a large polyploid macronucleus (the "vegetative nucleus". general, expressing the phenotype of the organism). The latter is generated from the micronucleus by genome amplification and hard editing. The micronucleus passes its genetic material to the offspring, but does not express its genes. The macronucleus provides the small nuclear RNA for vegetative growth.

The division of the macronucleus occurs by amitosis and the segregation of the chromosomes occurs by a process whose mechanism is unknown. This process is not perfect, and after about 200 generations, the cell shows signs of aging. Periodically, macronuclei must be regenerated from micronuclei. In most cases, this occurs during conjugation. Here two cells line up, the micronuclei undergo meiosis, some of the haploid daughters switch and then fuse to form new micronuclei and macronuclei.

Food vacuoles are formed by phagocytosis and generally follow a specific path through the cell as their contents are digested and broken down by lysosomes, so the substances contained in the vacuole are small enough to diffuse across the food vacuole membrane to inside the cell. Whatever remains in the food vacuole by the time it reaches the cytoproct (anal pore) is discharged by exocytosis. Most ciliates also have one or more prominent contractile vacuoles, which collect water and expel it from the cell to maintain osmotic pressure or, in some function, maintain ionic balance. In some genera, such as Paramecium, they have a distinct star shape, each point being a collecting tube.

Most ciliates are heterotrophs, feeding on smaller organisms such as bacteria and algae and debris drawn into the oral sulcus (mouth) by modified oral cilia. This usually includes a series of membranes to the left of the mouth and a paroral membrane to the right, both arising from polykinetids, groups of many cilia along with associated structures. Cilia move food through the pore of the mouth and into the throat, forming food vacuoles.

However, feeding techniques vary considerably. Some ciliates do not have a mouth and feed by absorption (osmotrophy), while others are predators and feed on other protozoa and mainly on other ciliates. Some ciliates parasitize animals, although only one species, Balantidium coli, is known to cause disease in humans.

Ciliates reproduce asexually, by various types of fission. During fission, the micronucleus undergoes mitosis and the macronucleus elongates and undergoes amitosis (except among Karyorelictean ciliates, whose macronuclei do not divide). The cell then divides in two, and each new cell gets a copy of the micronucleus and the macronucleus.

Typically, the cell divides transversely, with the front half of the ciliate (the proter) forming one new organism and the rear half (the opiste) forming another. However, other types of fission occur in some ciliate groups. These include budding (the appearance of tiny ciliated offspring, or "swarms", from a mature parent's body); strobilization (multiple divisions along the cell body, producing a chain of new organisms); and palinthia (multiple fissions, usually within a cyst).

Fission can occur spontaneously as part of the vegetative cell cycle. Alternatively, it may occur as a result of self-fertilization (autogamy) or it may follow conjugation, a sexual phenomenon in which ciliates of compatible mating types exchange genetic material. Although conjugation is sometimes described as a form of reproduction, it is not directly related to reproductive processes and does not directly result in increasing the number of individual ciliates or their progeny.

Ciliate conjugation is a sexual phenomenon that results in genetic recombination and nuclear reorganization within the cell. During conjugation, two ciliates of a compatible mating type form a bridge between their cytoplasms. The micronuclei undergo meiosis, the macronuclei disappear, and the haploid micronuclei are exchanged across the bridge. In some ciliates (peritric, conotrich, and some suctorians), the conjugate cells are permanently fused and one conjugate is absorbed by the other. However, in most ciliated groups, the cells separate after conjugation and both form new macronuclei from their micronuclei. Conjugation and autogamy are always followed by fission.

In many ciliates, such as Paramecium, conjugation partners (gamonts) are similar or indistinguishable in size and shape. This is known as "isogamous" conjugation. In some groups, pairs are different in size and shape. This is known as “anisogamous” conjugation. In sessile peritricians, for example, one sexual partner (the microconjugant) is small and mobile, while the other (macroconjugant) is large and sessile.

In Paramecium caudatum, the stages of conjugation are as follows (see diagram on the right):

  1. Compatible mating strains meet and partially fuse
  2. The micronuclei undergo meiosis, producing four haploid micronuclei per cell.
  3. Three of these micronuclei disintegrate. The fourth undergoes mitosis.
  4. The two cells exchange a micronucleus.
  5. The cells are then separated.
  6. The micronuclei of each cell fuse, forming a diploid micronucleus.
  7. Mitosis occurs three times, giving rise to eight micronuclei.
  8. Four of the new micronuclei transform into macronuclei and the old macronucleus decays.
  9. Binary fission occurs twice, producing four identical daughter cells.

Ciliates contain two types of nuclei: the somatic "macronucleus" and the germline "micronucleus." Only the DNA in the micronucleus is transmitted during sexual reproduction (conjugation). On the other hand, only the DNA in the macronucleus is actively expressed and results in the phenotype of the organism. Macronuclear DNA is derived from micronuclear DNA through incredibly extensive DNA amplification and rearrangement.

The macronucleus starts out as a copy of the micronucleus. Micronuclear chromosomes are broken into many smaller pieces and amplified to make many copies. The resulting macronuclear chromosomes usually contain a single gene. In Tetrahymena, the micronucleus has 10 chromosomes (five per haploid genome), while the macronucleus has more than 20,000 chromosomes.

Furthermore, micronuclear genes are interrupted by numerous "internal deleted sequences" (IES). During macronucleus development, the IES are removed and the remaining gene segments, the macronuclear targeting sequences (MDS), are spliced ​​into the working gene. Tetrahymena has about 6000 IES and about 15% of the micronuclear DNA is removed during this process. The process is guided by tiny epigenetic markers of RNA and chromatin.

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In spirotric ciliates (such as Oxytricha), the process is even more complex due to "gene coding": the MDS in the micronucleus usually has a different order and orientation than the macronuclear gene; therefore, in addition to deletion, DNA reversion and translocation are required to "untangle". This process is guided by long RNAs derived from the parental macronucleus. More than 95% of micronuclear DNA is removed during macronuclear development of the spirotric.


Amoebozoa is an important taxonomic group containing about 2,400 described species of amoeboid protists, often having finger-like pseudopods and tubular mitochondrial cristae. In most classification schemes, Amoebozoa is classified as a phylum within kingdom Protista or kingdom Protozoa. In the classification favored by the International Society of Protistologists, there remains an unclassified "supergroup" within the Eukaryota. Molecular genetic analysis supports Amoebozoa as a monophyletic clade. Most phylogenetic trees identify it as the sister group to Opisthokonta, another large clade containing fungi and animals, as well as about 300 species of unicellular protists. Amoebozoa and Opisthokonta are sometimes grouped together into a higher-level taxon, variously referred to as Unikonta, Amorphea, or Opimoda.

Amoebozoa includes many of the better known amoeboid organisms such as Chaos, Entamoeba, Pelomyxa and the genus Amoeba itself. Amoebozoa species may be peeled (tested) or naked, and the cells may have flagella. Free-living species are common in fresh and salt water, as well as soil, moss, and leaf litter. Some live as parasites or symbionts of other organisms, and some are known to cause disease in humans and other organisms.

While most amoebozoan species are single-celled, the group also includes several varieties of slime fungi, which have a macroscopic multicellular life stage during which individual amoeboid cells aggregate to produce spores.

Amebozoans vary greatly in size. Some are only 10–20 µm in diameter, while others are among the largest protozoans. The well-known species Amoeba proteus, which can reach 800 µm in length, is often studied in schools and laboratories as a representative cell or model organism, in part because of its convenient size. Multinucleated amoebae, such as Chaos and Pelomyxa, can be several millimeters long, and some multicellular amoebozoans, such as the slimy “dog vomit” fungus Fuligo septica, can cover an area of ​​several square meters.

Amoebozoa are a large and diverse group, but certain characteristics are common to many of its members. The amebozoan cell is typically divided into a central granular mass, called the endoplasm, and a transparent outer layer, called the ectoplasm. During locomotion, endoplasm flows forward and ectoplasm flows backward along the outside of the cell. On the move, many amoebozoans have a clearly defined front and back and may assume a "monopodial" form, with the entire cell functioning as a single pseudopod. Large pseudopodia can produce numerous clear projections called subpseudopodia (or determinate pseudopodia), which extend to a certain length and then retract, either for the purposes of locomotion or food ingestion. A cell can also form multiple indeterminate pseudopods, through which all cell contents flow in the direction of locomotion. These are more or less tubular and are mostly filled with granular endoplasm. The cell mass flows into a main pseudopodia and the others eventually retract unless the organism changes direction.

While most amoebozoans are "naked", like the relatives Amoeba and Chaos, or covered in a loose layer of tiny scales, like Cochliopodium and Korotnevella, members of the order Arcellinida form rigid shells, or tests, equipped with a single opening through from which pseudopodia arise. Evidence of arcelinids can be secreted from organic materials, as in Arcella, or constructed from collected and cemented particles, as in Difflugia.

In all amoebozoans, the main mode of nutrition is phagocytosis, in which the cell surrounds potential food particles with its pseudopodia, sealing them in vacuoles within which they can be digested and absorbed. Some amebozoans have a posterior bulb called the uroid, which can serve to accumulate debris, periodically separating from the rest of the cell. [citation needed] When food is scarce, most species can form cysts, which can be airborne to new environments. [citation needed] In slime molds, these structures are called spores and are formed into stem structures called fruiting bodies or sporangia.

Most Amoebozoa lack flagella and generally do not form microtubule-supported structures except during mitosis. However, flagella do occur among the Archamoebae, and many slime fungi produce biflagellate gametes. The flagellum is usually anchored by a cone of microtubules, suggesting a close relationship with opisthoconts. However, among the Archamoebae, which are adapted to anoxic or microaerophilic habitats, mitochondria have been lost.

Amebiasis, also known as amebiasis or entamoebiasis, is an infection caused by any of the amebozoans of the Entamoeba group. Symptoms are more common with Entamoeba histolytica infection. Amebiasis can present without symptoms, mild or severe. Symptoms may include abdominal pain, mild diarrhea, bloody diarrhea, or severe colitis with tissue death and perforation. This last complication can cause peritonitis. Affected individuals may develop anemia due to blood loss.

Invasion of the intestinal lining causes bloody amoebic diarrhea or amoebic colitis. If the parasite reaches the bloodstream, it can spread throughout the body, most often ending up in the liver, where it causes amoebic liver abscesses. Liver abscesses can occur without prior diarrhea. Entamoeba cysts can survive up to a month in soil or up to 45 minutes under fingernails. It is important to differentiate between amoebiasis and bacterial colitis. The preferred diagnostic method is examination of stool under a microscope, but this requires a qualified microscopist and may be unreliable when infection is excluded. However, this method may not be able to separate between specific types. Increased white blood cell count is present in severe cases but not in mild cases. The most accurate test is for antibodies in the blood, but it can still be positive after treatment.

Prevention of amoebiasis consists of separating food and water from the faeces and by appropriate sanitary measures. There is no vaccine. There are two treatment options depending on the location of the infection. Tissue amebiasis is treated with metronidazole, tinidazole, nitazoxanide, dehydroemetine or chloroquine, while luminal infection is treated with diloxanide furoate or iodoquinoline. For treatment to be effective against all stages of amoeba, a combination of drugs may be needed. Infections without symptoms do not require treatment, but infected people can transmit the parasite to others and treatment may be considered. Treatment of Entamoeba infections other than E. histolytica is not required.

Amoebiasis is present all over the world. Approximately 480 million people are infected with what appears to be E. histolytica, killing between 40,000 and 110,000 people each year. Most infections are now attributed to E. dispar. E. dispar is more common in certain areas and symptomatic cases may be fewer than previously reported. The first case of amoebiasis was documented in 1875 and in 1891 the disease was described in detail, giving rise to the terms amoebic dysentery and amoebic liver abscess. More evidence from the Philippines in 1913 found that by ingesting E. histolytica cysts, volunteers developed the disease. At least one non-disease-causing species of Entamoeba (Entamoeba coli) has been known since 1897, but the WHO first formally recognized E. histolytica as two species in 1997, although it was first proposed in 1925. In addition to the now recognized E. dispar, evidence shows that there are at least two other species of Entamoeba that look the same in humans: E. moshkovskii and Entamoeba bangladeshi. The reason these species were not differentiated until recently is because of reliance on appearance.

14.5It started

The Archaeplastida (or kingdom Plantae sensu lato) are an important group of autotrophic eukaryotes, comprising red algae (Rhodophyta), green algae, and land plants, along with a small group of single-celled freshwater algae called glaucophytes. Archaeplastida have chloroplasts that are surrounded by two membranes, suggesting that they were acquired directly from endosymbiotic cyanobacteria. All other groups, except the amoeboid Paulinella chromatophora, have chloroplasts enclosed by three or four membranes, suggesting that they were secondarily acquired from red or green algae. Unlike red and green algae, glaucophytes have never been involved in secondary endosymbiosis events.

Archaeplastida cells normally lack centrioles and have mitochondria with flat cristae. They usually have a cell wall that contains cellulose and food is stored in the form of starch. However, these features are also shared with other eukaryotes. The main evidence that Archaeplastida form a monophyletic group comes from genetic studies, which indicate that their plastids probably had a single origin. This evidence is disputed. Based on the evidence to date, it is not possible to confirm or refute alternative evolutionary scenarios to a single primary endosymbiosis. Photosynthetic organisms with plastids of different origin (such as brown algae) do not belong to Archaeplastida.

Archaeplastidans fall into two main evolutionary lines. Red algae are pigmented with chlorophyll a and phycobiliproteins, like most cyanobacteria, and accumulate starch outside the chloroplasts. Green algae and land plants, known collectively as Viridiplantae (Latin for "green plants") or Chloroplastida, are pigmented with chlorophylls a and b but lack phycobiliproteins and starch accumulates in chloroplasts. Glaucophytes have pigments typical of cyanobacteria and are unusual in retaining a cell wall within their plastids (called cyanelles).

It startedis an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping, which includes species from several different clades. The organisms included range from single-celled microalgae such as Chlorella and diatoms to multicellular forms such as giant kelp, a large brown algae that can grow up to 50m in length. Most are aquatic and autotrophic, lacking many of the different types of cells and tissues, such as stomata, xylem, and phloem, found in land plants. The largest and most complex marine algae are called algae, while the most complex freshwater forms are the Charophyta, a division of green algae that includes, for example, Spirogyra and stoneworts.

No definition of algae is generally accepted. One definition is that algae "have chlorophyll as their main photosynthetic pigment and lack a sterile covering of cells around their reproductive cells". Although cyanobacteria are often called "blue-green algae", most authorities exclude all prokaryotes from the definition of algae.

Algae constitute a polyphyletic group, as they do not include a common ancestor, and although their plastids appear to have the same origin, from cyanobacteria, they were acquired in different ways. Green algae are examples of algae that have primary chloroplasts derived from endosymbiotic cyanobacteria. Diatoms and brown algae are examples of algae with secondary chloroplasts derived from an endosymbiotic red alga.

Algae exhibit a wide range of reproductive strategies, from simple asexual cell division to complex forms of sexual reproduction.

Algae lack the various structures that characterize land plants, such as the leaves (leaf-like structures) of bryophytes, the rhizoids of non-vascular plants, and the roots, leaves, and other organs found in tracheophytes (plants). vascular). Most are phototrophs, although some are mixotrophs, obtaining energy both from photosynthesis and from the absorption of organic carbon, either by osmotrophy, myzotrophy or phagotrophy. Some unicellular species of green algae, many golden algae, euglenids, dinoflagellates, and other algae have become heterotrophic (also called colorless or apochlorotic algae), sometimes parasitic, completely dependent on external energy sources, and have limited or no photosynthetic apparatus. Some other heterotrophic organisms, such as apicomplexans, also derive from cells whose ancestors possessed plastids, but are not traditionally considered algae. Algae have a photosynthetic machinery ultimately derived from cyanobacteria that produce oxygen as a by-product of photosynthesis, unlike other photosynthetic bacteria such as purple and green sulfur bacteria. Fossilized filamentous algae from the Vindhya Basin date from 1.6 to 1.7 billion years ago.

The first land plants probably evolved from shallow freshwater charophyte algae, much like Chara, nearly 500 million years ago. These probably had an isomorphic alternation of generations and were probably filamentous. Fossils of isolated land plant spores suggest that land plants may have existed 475 million years ago.

Most of the simpler algae are unicellular or amoeboid flagellates, but colonial and immobile forms evolved independently among several of the groups.

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In three strains of algae, even higher levels of organization were achieved, with complete tissue differentiation. They are brown algae, some of which can reach 50m in length (kelps), red algae and green algae. The most complex forms are found among the charophyte algae (see Charales and Charophyta), in a lineage that eventually led to higher land plants. The defining innovation of these algae-free plants is the presence of female reproductive organs with layers of protective cells that protect the zygote and developing embryo. Therefore, land plants are called embryophytes.

Rhodophyta, Chlorophyta and Heterokontophyta, the three main divisions of algae, have life cycles that show considerable variation and complexity. In general, there is an asexual phase where the kelp cells are diploid, a sexual phase where the cells are haploid, followed by the fusion of male and female gametes. Asexual reproduction allows for efficient population increases, but less variation is possible. Commonly, in the sexual reproduction of unicellular and colonial algae, two specialized, sexually compatible haploid gametes make physical contact and fuse to form a zygote. To ensure successful mating, the development and release of gametes are highly synchronized and regulated; pheromones may play a key role in these processes. Meiosis has been shown to occur in many different species of algae.

For example, Fucus is a genus of brown algae found in the intertidal zones of rocky coasts almost everywhere in the world. Fucus species are recorded almost all over the world. They are dominant off the coasts of the British Isles, the northeast coast of North America and California. These algae have a relatively simple life cycle, producing a single type of thallus that grows to a maximum size of 2 m. The fertile cavities, the conceptacles, which contain the reproductive cells, are immersed in receptacles near the ends of the branches. After meiosis, the oogonia and antheridia, the female and male reproductive organs, produce eggs and sperm, respectively, which are released into the sea where fertilization takes place. The resulting zygote develops directly into the diploid plant. This is in contrast to the flowering plant life cycle, where eggs and sperm are produced by a haploid, albeit greatly reduced, multicellular generation, and the eggs are fertilized within the mother plant's ova and then released as seeds.

Algae are prominent in bodies of water, common in terrestrial environments, and found in unusual environments such as snow and ice. Seaweed grows primarily in shallow marine waters below 100 m (330 ft) depth; however, some such as Navicula pennata have been recorded at depths up to 360 m (1,180 ft). A type of algae, Ancylonema nordenskioeldii, has been found in Greenland in areas known as the "Dark Zone", causing an increase in the rate of melting of the ice sheet. The same algae was found in the Italian Alps after pink ice appeared on parts of the Presena glacier.

The various types of algae play an important role in aquatic ecology. The microscopic forms that live suspended in the water column (phytoplankton) provide the food base for most marine food chains. At very high densities (algal blooms), these algae can discolor the water and overwhelm, poison, or suffocate other forms of life.

14.6slime molds

Slime mold or slime mold is an informal name given to several types of unrelated eukaryotic organisms that can live freely as single cells but can aggregate to form multicellular reproductive structures. Slime molds were formerly classified as fungi, but are no longer considered part of that kingdom. Although they do not form a single monophyletic clade, they are grouped within the paraphyletic group called kingdom Protista.

More than 900 species of slime fungi are found worldwide. Their common name refers to the part of the life cycle of some of these organisms where they can appear as gelatinous "sludge". This is mostly seen with Myxogastria, which are the only macroscopic slime fungi. Most slime molds measure less than a few centimeters, but some species can reach sizes of up to several square meters and masses of up to 20 kilograms.

Many slimy fungi, particularly "cellular" slime molds, do not spend most of their time in this state. When food is plentiful, these slime molds exist as single-celled organisms. When food is scarce, many of these single-celled organisms come together and start moving as one body. In this state, they are sensitive to airborne chemicals and can detect food sources. They can easily change the shape and function of parts, and can form stems that produce fruiting bodies, releasing countless spores, light enough to be carried by the wind or hitchhiked by passing animals.

They feed on microorganisms that live on any type of dead plant material. They contribute to the decomposition of dead vegetation and feed on bacteria, yeasts and fungi. For this reason, slime molds are usually found in soil, lawns and forest floors, usually on deciduous logs. In tropical areas they are also common in inflorescences and fruits, and in aerial situations (for example, in the canopy of trees). In urban areas, they are found in mulch or leaf mold in rain gutters and also grow in air conditioners, especially when the drain is clogged.

Slime fungi begin life as amoeba-like cells. These single-celled amoebae are commonly haploid and feed on bacteria. These amoebas can mate if they find the right mating type and form zygotes that later become plasmodia. These contain many nuclei with no cell membranes between them and can grow to meters in size. Fuligo septica is often seen as a slimy yellow web in and over rotten logs. Amoebas and plasmodia encompass microorganisms. Plasmodium grows on an interconnected network of protoplasmic filaments.

14 protists | Text for Biology at Roxbury Community College (10)

Figure 14.10:Septic soot, a mucilaginous moldFuligo septica is a species of plasmodial slime mold and a member of the class Myxomycetes. It is commonly known as scrambled egg slime or tan flowers due to its peculiar appearance of yellowish color and bile. Also known as dog vomit mold, it is common with a worldwide distribution, often found on bark in urban areas after heavy rain or excessive watering. Its spores are produced in aerial sporangia and are spread by the wind.

Within each protoplasmic strand, cytoplasmic contents flow rapidly. If you watch a thread closely for about 50 seconds, you can see the cytoplasm slow down, stop, and change direction. The flow of protoplasm within a plasmodial filament can reach speeds of up to 1.35 mm per second, which is the fastest rate recorded for any microorganism. Plasmodium migration is achieved when more protoplasm flows into the main areas and protoplasm is taken from the posterior areas. When the food supply decreases, Plasmodium migrates to the surface of its substrate and transforms into rigid fruiting bodies. Fruiting bodies or sporangia are commonly seen. Superficially, they look like fungi or molds, but they are not related to true fungi. These sporangia will release spores which will turn into amoebas to start the life cycle all over again.

14 protists | Text for Biology at Roxbury Community College (11)

Figure 14.11:Dictyostelium fruiting bodyDictyostelium is a genus of unicellular and multicellular eukaryotic phagotrophic bacteriophages. Although they are protists and not fungi at all, they are traditionally known as "slime molds". They are present in most terrestrial ecosystems as a normal and often abundant component of soil microflora and play an important role in maintaining the balance of bacterial populations in soils.

14.6.1reproduction of protists

Some protists reproduce sexually using gametes, while others reproduce asexually.

Some species, for examplePlasmodium falciparumthey have extremely complex life cycles involving multiple forms of the organism, some of which reproduce sexually and some of which reproduce asexually. However, it is unclear how often sexual reproduction causes genetic exchange between different Plasmodium strains in nature, and most populations of parasitic protists may be clonal lineages that rarely exchange genes with other members of their species.

Eukaryotes emerged in evolution more than 1.5 billion years ago. The first eukaryotes were probably protists. Although sexual reproduction is widespread among extant eukaryotes, until recently it seemed unlikely that sex could be a primary and fundamental feature of eukaryotes. The main reason for this view was that sex seemed to be lacking in certain pathogenic protists whose ancestors diverged early from the eukaryotic family tree. However, it is now known that several of these protists are capable of, or have recently been capable of, meiosis and therefore sexual reproduction. For example, the common intestinal parasite Giardia lamblia was once thought to descend from a lineage of protists that predated the onset of meiosis and sex. However, G. lamblia was recently discovered to possess a core set of genes that function in meiosis and are widely present among sexual eukaryotes. These results suggest that G. lamblia is capable of meiosis and therefore of sexual reproduction. Furthermore, direct evidence of meiotic recombination, indicative of sex, was also found in G. lamblia.

Pathogenic parasitic protists of the genus Leishmania have been shown to be capable of a sexual cycle in the invertebrate vector, similar to the meiosis that occurs in trypanosomes.

Protists generally reproduce asexually under favorable environmental conditions, but tend to reproduce sexually under stressful conditions such as starvation or heatstroke. Oxidative stress, which is associated with the production of reactive oxygen species that cause DNA damage, also appears to be an important factor in inducing sex in protists.

Some commonly encountered protist pathogens such asToxoplasma gondiiare capable of infecting and asexually reproducing in a wide variety of animals -acting as a secondary or intermediate host- but they can reproduce sexually only in the primary or definitive host (for example: felids such as domestic cats in this case).

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Free-living protists occupy almost all environments that contain liquid water. Many protists, such as algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of plankton. Protists constitute a large part of the biomass in marine and terrestrial environments.

Other protists include pathogenic species such as the kinetoplastid Trypanosoma brucei, which causes sleeping sickness, and Plasmodium apicomplex species, which causes malaria.

Some protists are important parasites of animals (for example, five species of parasites in the 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 other protists. Protist pathogens share many metabolic pathways with their eukaryotic hosts. This makes the development of therapeutic targets extremely difficult: a drug that harms a protist parasite is likely to also harm its animal or plant host. A deeper understanding of protist biology may allow these diseases to be treated more efficiently. For example, the apicoplast (a non-photosynthetic chloroplast but essential for carrying out important functions beyond photosynthesis) present in apicomplexans provides an attractive target for treating diseases caused by dangerous pathogens such as Plasmodium.


What are the 4 main protists? ›

The Protozoa is often divided into 4 phyla : Amoebalike protists, flagellates, ciliates, and spore-forming protists.

How many protist classes are there? ›

Classification of Protista

They can be broadly classified into three categories: Protozoan protists. Photosynthetic protists or Protist algae. Slime moulds.

What are some common protists? ›

Examples of protists include algae, amoebas, euglena, plasmodium, and slime molds. Protists that are capable of photosynthesis include various types of algae, diatoms, dinoflagellates, and euglena. These organisms are often unicellular but can form colonies.

What are the 3 main types of protists? ›

Based on their nutrition, protists can be classified into three broad categories:
  • Autotrophs. These protists are generally non-motile and can photosynthesize their food just like plants. ...
  • Heterotrophs. These protists are grouped based on the type or lack of locomotive parts. ...
  • Mixotrophs.

What is a protist and name 3 examples? ›

Protist (biology definition): Any of a group of eukaryotic organisms belonging to the Kingdom Protista. Protists include: (1) protozoa, the animal-like protists, (2) algae, the plant-like protists, and (3) slime molds and water molds, the fungus-like protists.

What are the 7 plant-like protists? ›

Examples of Plant-like Protists. The 7 major groups of algae are red algae, green algae, brown algae, fire algae, golden-brown algae, yellow-green algae, and euglenids.

What are two common protists? ›

Protozoans, such as photosynthetic euglenoids, free-living dinoflagellates, amoeboids (e.g., foraminiferans), radiolarians, and volvox, are other common protists.

What are protists in biology? ›

The Protista is a large complex grouping of mostly unicellular eukaryotic organisms. They are morphologically diverse and can be found in most terrestrial, aquatic, and marine habitats as free-living forms and as parasites of other protists, of fungi, and of plants and animals.

Where are 4 places protists are found? ›

Protist Habitats

They are found mainly in damp soil, marshes, puddles, lakes, and the ocean. Some protists are free-living organisms. Others are involved in symbiotic relationships. They live in or on other organisms, including humans.

What are the 6 protist supergroups? ›

Abstract. Nearly all of eukaryotic diversity has been classified into 6 suprakingdom-level groups (supergroups) based on molecular and morphological/cell-biological evidence; these are Opisthokonta, Amoebozoa, Archaeplastida, Rhizaria, Chromalveolata, and Excavata.

What are the 6 plant like protists? ›

Ans-: Depending on the type of pigment possessed, plan-like protists are classified under six phyla: Green algae, Brown algae, Red algae, Diatoms, Dinoflagellates, and Euglenoids. Plant-like protists are eukaryotic, photosynthetic, unicellular, or multicellular, possess pyrenoids, gametangia, and a flagellum.

What are 5 animal like protists? ›

Animal-like protists are commonly called protozoa (singular, protozoan).
Classification of Protozoa.
Type of ProtozoaHow It MovesExample (Genus)
Sporozoandoes not move (as adult)Plasmodium
5 Mar 2021

What are 3 protists that are animal like? ›

Animal-like protists include the flagellates, ciliates, and the sporozoans.

What plants are protists? ›

  • Plant-like protists are called algae. They include single-celled diatoms and multicellular seaweed.
  • Like plants, algae contain chlorophyll and make food by photosynthesis.
  • Types of algae include red and green algae, euglenids, and dinoflagellates.
5 Mar 2021

What are 5 facts about protists? ›

Interesting Facts about Protists
  • Many protists act as pathogens to humans. ...
  • The disease malaria is caused by the protist Plasmodium falciparum.
  • If an amoeba is cut in half, the half with the nucleus will survive, while the other half will die.
  • The word "pseudopod" comes from Greek words meaning "false feet."

Are protists bacteria? ›

A protist is a one- or many-celled organism that lives in moist or wet surroundings. Unlike bacteria, protists' cells are eukaryotic. These organisms have a membrane-bound nucleus and other membrane-bound structures in their cytoplasm.

Where are protists found? ›

The Protista is a large complex grouping of mostly unicellular eukaryotic organisms. They are morphologically diverse and can be found in most terrestrial, aquatic, and marine habitats as free-living forms and as parasites of other protists, of fungi, and of plants and animals.

Is Mushroom a protist? ›

Protists consist of all the organisms that are not plants, nor animals nor fungi. Hence, mushrooms do not belong under protists.

What is the most important protist? ›

Protists are a taxonomically inhomogeneous group of mostly unicellular eukaryotic microorganisms. Dinoflagellates (superphylum Alveolata) are protists and are the most important source of natural products (see Chapter 2.09).

What were the first protists? ›

The protists are thought to have arisen from bacteria, with symbiotic associations being involved in some way. Some researchers have hypothesized that the first protists were of a nonpigmented heterotrophic form. From within the vast array of protists, there must have arisen the early eukaryotes.

What are the 5 main groups of protozoans? ›

Protozoa are unicellular, heterotrophic, eukaryotic organisms comprising four organization types: amebae, flagellates, ciliates, and parasitic sporozoans.

What are 2 protists? ›

Examples of protists include various unicellular red algae, such as Cyanidioschyzon merolae; unicelluar green algae, such as Chlamydomonas reinhardtii; and marine diatoms, such as Thalassiosira pseudonana.

What are animal like protists like? ›

Animal-like Protists

Like animals, they can move, and they are heterotrophs. That means they eat things outside of themselves instead of producing their own food. Animal-like protists are very small, measuring only about 0.01–0.5mm. Animal-like protists include the flagellates, ciliates, and the sporozoans.

What are the 6 plant-like protists? ›

Ans-: Depending on the type of pigment possessed, plan-like protists are classified under six phyla: Green algae, Brown algae, Red algae, Diatoms, Dinoflagellates, and Euglenoids. Plant-like protists are eukaryotic, photosynthetic, unicellular, or multicellular, possess pyrenoids, gametangia, and a flagellum.

What are 4 examples of protozoa? ›

For our purposes, there are only 4 groups of protozoa that will be covered here: these groups are separated by motility and cell structure.
  • Amebas (representative: Ameba proteus)
  • Flagellates (representative: Trypanosoma, Euglena)
  • Ciliates (representative: Paramecium)
  • Apicomplexa (representative: Plasmodium)
1 Aug 2021

What are 3 examples of protozoa? ›

Commonly known protozoans include representative dinoflagellates, amoebas, paramecia, and the malaria-causing Plasmodium.

What are the 4 classes of protozoa? ›

Protozoa can be divided into four phyla based on their locomotion: Mastigophora, Sarcodina, Ciliophora, and Sporozoa.

What are 5 protists that cause disease? ›

Some severe diseases of humans caused by protists (primarily blood parasites) are Malaria, Trypanosomiasis (African sleeping sickness), Leishmaniasis, Toxoplasmosis, and Amoebic dysentery.

How do protists eat? ›

Some make their own food, but most take in or absorb food. Most protists move with the help of flagella, pseudopods, or cilia. Some protists, like the one-celled amoeba and paramecium, feed on other organisms. Others, such as the one-celled euglena or the many-celled algae, make their food by photosynthesis.

Where is protists found? ›

The Protista is a large complex grouping of mostly unicellular eukaryotic organisms. They are morphologically diverse and can be found in most terrestrial, aquatic, and marine habitats as free-living forms and as parasites of other protists, of fungi, and of plants and animals.

How do protists work? ›

Protists function at several levels of the ecological food web: as primary producers, as direct food sources, and as decomposers. In addition, many protists are parasites of plants and animals that can cause deadly human diseases or destroy valuable crops.


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