How can microscopic protists and fungi be characterized

One plant pathogen is Phytophthora infestans , the causative agent of late blight of potatoes, such as occurred in the nineteenth century Irish potato famine. Rhizaria are a supergroup of protists, typically amoebas, that are characterized by the presence of needle-like pseudopodia.

The Rhizaria supergroup includes many of the amoebas, most of which have threadlike or needle-like pseudopodia. Pseudopodia function to trap and engulf food particles and to direct movement in rhizarian protists.

These pseudopods project outward from anywhere on the cell surface and can anchor to a substrate. The protist then transports its cytoplasm into the pseudopod, thereby moving the entire cell.

This type of motion, called cytoplasmic streaming, is used by several diverse groups of protists as a means of locomotion or as a method to distribute nutrients and oxygen. Ammonia tepida : Ammonia tepida, a Rhizaria species viewed here using phase contrast light microscopy, exhibits many threadlike pseudopodia. Foraminiferans, or forams, are unicellular heterotrophic protists, ranging from approximately 20 micrometers to several centimeters in length; they occasionally resemble tiny snails.

As a group, the forams exhibit porous shells, called tests, that are built from various organic materials and typically hardened with calcium carbonate. The tests may house photosynthetic algae, which the forams can harvest for nutrition.

Foram pseudopodia extend through the pores and allow the forams to move, feed, and gather additional building materials. Foraminiferans are also useful as indicators of pollution and changes in global weather patterns. The life-cycle involves an alternation between haploid and diploid phases. The haploid phase initially has a single nucleus, and divides to produce gametes with two flagella.

The diploid phase is multinucleate, and after meiosis fragments to produce new organisms. The benthic forms has multiple rounds of asexual reproduction between sexual generations. Forams : These shells from foraminifera sank to the sea floor. A second subtype of Rhizaria, the radiolarians, exhibit intricate exteriors of glassy silica with radial or bilateral symmetry.

Radiolarians display needle-like pseudopods that are supported by microtubules which radiate outward from the cell bodies of these protists and function to catch food particles. The shells of dead radiolarians sink to the ocean floor, where they may accumulate in meter-thick depths.

Preserved, sedimented radiolarians are very common in the fossil record. Radiolarian shell : This fossilized radiolarian shell was imaged using a scanning electron microscope. Amoebozoa are a type of protist that is characterized by the presence of pseudopodia which they use for locomotion and feeding. Protists are eukaryotic organisms that are classified as unicellular, colonial, or multicellular organisms that do not have specialized tissues.

This identifying property sets protists apart from other organisms within the Eukarya domain. The amoebozoans are classified as protists with pseudopodia which are used in locomotion and feeding.

Amoebozoans live in marine environments, fresh water, or in soil. In addition to the defining pseudopodia, they also lack a shell and do not have a fixed body. The pseudopodia which are characteristically exhibited include extensions which can be tube-like or flat lobes, rather than the hair-like pseudopodia of rhizarian amoeba. Rhizarian amoeba are amoeboids with filose, reticulose, or microtubule-supported pseudopods and include the groups: Cercozoa, Foraminifera, and Radiolaria and are classified as bikonts.

The Amoebozoa include several groups of unicellular amoeba-like organisms that are free-living or parasites that are classified as unikonts. The best known and most well-studied member of this group is the slime mold. Additional members include the Archamoebae, Tubulinea, and Flabellinea. Pseudopodia structures : Amoebae with tubular and lobe-shaped pseudopodia, such as the ones seen under this microscope, would be morphologically classified as amoebozoans.

A subset of the amoebozoans, the slime molds, has several morphological similarities to fungi that are thought to be the result of convergent evolution. For instance, during times of stress, some slime molds develop into spore -generating fruiting bodies, similar to fungi. The slime molds are categorized on the basis of their life cycles into plasmodial or cellular types. Plasmodial slime molds are composed of large, multinucleate cells that move along surfaces like an amorphous blob of slime during their feeding stage.

Food particles are lifted and engulfed into the slime mold as it glides along. Upon maturation, the plasmodium takes on a net-like appearance with the ability to form fruiting bodies, or sporangia, during times of stress.

Haploid spores are produced by meiosis within the sporangia. These spores can be disseminated through the air or water to potentially land in more favorable environments. If this occurs, the spores germinate to form ameboid or flagellate haploid cells that can combine with each other and produce a diploid zygotic slime mold to complete the life cycle.

Badhamia utricularis : Badhamia utricularis: an example of a plasmodial slime mold with the ability to form a fruiting body. The cellular slime molds function as independent amoeboid cells when nutrients are abundant.

When food is depleted, cellular slime molds pile onto each other into a mass of cells that behaves as a single unit called a slug. Some cells in the slug contribute to a 2—3-millimeter stalk, drying up and dying in the process. Cells atop the stalk form an asexual fruiting body that contains haploid spores.

As with plasmodial slime molds, the spores are disseminated and can germinate if they land in a moist environment. One representative genus of the cellular slime molds is Dictyostelium, which commonly exists in the damp soil of forests.

Plasmodial slime mold: Physarum polycephalum : Physarum polycephalum is an example of a cellular slime mold. The Archamoebae are a group of Amoebozoa distinguished by the absence of mitochondria. They include genera that are internal parasites or commensals of animals Entamoeba and Endolimax. A few species are human pathogens, causing diseases such as amoebic dysentery.

The other genera of archamoebae live in freshwater habitats and are unusual among amoebae in possessing flagella. Most have a single nucleus and flagellum, but the giant amoeba, Pelomyxa , has many of each. The Tubulinea are a major grouping of Amoebozoa, including most of the larger and more familiar amoebae like Amoeba , Arcella , and Difflugia.

During locomotion, most Tubulinea have a roughly cylindrical form or produce numerous cylindrical pseudopods. Each cylinder advances by a single central stream of cytoplasm, granular in appearance, and has no subpseudopodia.

This distinguishes them from other amoeboid groups, although in some members this is not the normal type of locomotion. Privacy Policy. Skip to main content. Microbial Evolution, Phylogeny, and Diversity. Search for:. Protists Early Eukaryotes Protists are eukaryotes that first appeared approximately 2 billion years ago with the rise of atmospheric oxygen levels.

Learning Objectives Discuss the origins of eukaryotes in terms of the geologic time line. Key Takeaways Key Points On a geological time line, protists are among the first organisms that evolved after prokaryotes. Key Terms cyanobacteria : photosynthetic prokaryotic microorganisms, of phylum Cyanobacteria, once known as blue-green algae aerobic : living or occurring only in the presence of oxygen endomembrane : all the membraneous components inside a eukaryotic cell, including the nuclear envelope, endoplastic reticulum, and Golgi apparatus.

Excavata includes the protists: Diplomonads, Parabasalids and Euglenozoans. Diplomonads are defined by the presence of a nonfunctional, mitochrondrial-remnant organelle called a mitosome. Parabasalids are characterized by a semi-functional mitochondria referred to as a hydrogenosome; they are comprised of parasitic protists, such as Trichomonas vaginalis. Euglenozoans can be classified as mixotrophs, heterotrophs, autotrophs, and parasites; they are defined by their use of flagella for movement.

Key Terms mitosome : an organelle found within certain unicellular eukaryotes which lack mitochondria hydrogenosome : a membrane-bound organelle found in ciliates, trichomonads, and fungi which produces molecular hydrogen and ATP kinetoplast : a disk-shaped mass of circular DNA inside a large mitochondrion, found specifically in protozoa of the class Kinetoplastea.

Giardia lamblia. The mammalian intestinal parasite Giardia lamblia, visualized here using scanning electron microscopy, is a waterborne protist that causes severe diarrhea when ingested.

Chromalveolata: Alveolates Alveolates are defined by the presence of an alveolus beneath the cell membrane and include dinoflagellates, apicomplexans and ciliates. Learning Objectives Evaluate traits associated with protists classified as alveolates which include dinoflagellates, apicomplexans, and ciliates.

Key Takeaways Key Points Alveolates are classified under the group Chromalveolata which developed as a result of a secondary endosymbiotic event.

Dinoflagellates are defined by their flagella structure which lays perpendicular and fits into the cellulose plates of the dinoflagellate, promoting a spinning motion. Apicomplexans are defined by the asymmetrical distribution of their microtubules, fibrin, and vacuoles; they include the parasitic protist Plasmodium which causes malaria.

Ciliates are defined by the presence of cilia such as the oral groove in the Paramecium , which beat synchronously to aid the organism in locomotion and obtaining nutrients. Ciliates are defined by the presence of cilia, which beat synchronously, to aid the organism in locomotion and obtaining nutrients, such as the oral groove in the Paramecium.

Key Terms osmoregulation : the homeostatic regulation of osmotic pressure in the body in order to maintain a constant water content plastid : any of various organelles found in the cells of plants and algae, often concerned with photosynthesis conjugation : the temporary fusion of organisms, especially as part of sexual reproduction.

Protists live in a wide variety of habitats, including most bodies of water, as parasites in both plants and animals, and on dead organisms. Protist life cycles range from simple to extremely elaborate. Certain parasitic protists have complicated life cycles and must infect different host species at different developmental stages to complete their life cycle. Some protists are unicellular in the haploid form and multicellular in the diploid form, which is a strategy also employed by animals.

Other protists have multicellular stages in both haploid and diploid forms, a strategy called alternation of generations that is also used by plants. The slime molds are categorized on the basis of their life cycles into plasmodial or cellular types.

Plasmodial slime molds are composed of large, multinucleate cells and move along surfaces like an amorphous blob of slime during their feeding stage. The slime mold glides along, lifting and engulfing food particles, especially bacteria.

Upon maturation, the plasmodium takes on a net-like appearance with the ability to form fruiting bodies, or sporangia, during times of stress. Meiosis produces haploid spores within the sporangia. Spores disseminate through the air or water to potentially land in more favorable environments.

If this occurs, the spores germinate to form amoeboid or flagellate haploid cells that can combine with each other and produce a diploid zygotic slime mold to complete the life cycle. Plasmodial slime mold life cycle : Haploid spores develop into amoeboid or flagellated forms, which are then fertilized to form a diploid, multinucleate mass called a plasmodium. This plasmodium is net-like and, upon maturation, forms a sporangium on top of a stalk. The sporangium forms haploid spores through meiosis, after which the spores disseminate, germinate, and begin the life cycle anew.

The brightly-colored plasmodium in the inset photo is a single-celled, multinucleate mass. The cellular slime molds function as independent amoeboid cells when nutrients are abundant. When food is depleted, cellular slime molds aggregate into a mass of cells that behaves as a single unit called a slug.

Some cells in the slug contribute to a 2—3-millimeter stalk, which dries up and dies in the process. Cells atop the stalk form an asexual fruiting body that contains haploid spores. As with plasmodial slime molds, the spores are disseminated and can germinate if they land in a moist environment. One representative genus of the cellular slime molds is Dictyostelium , which commonly exists in the damp soil of forests. Cellular slime mold life cycle : Cellular slime molds may engage in two forms of life cycles: as solitary amoebas when nutrients are abundant or as aggregated amoebas inset photo when nutrients are scarce.

In aggregate form, some individuals contribute to the formation of a stalk, on top of which sits a fruiting body full of spores that disseminate and germinate in the proper moist environment. There are over , described living species of protists. Nearly all protists exist in some type of aquatic environment, including freshwater and marine environments, damp soil, and even snow. Today's classification has shifted away from a system built on morphology to one based on genetic similarities and differences.

The result is a family tree of sorts, mapping out evolutionary relationships between various organisms. In this system there are three main branches or "domains" of life: Bacteria, Archaea both prokaryotic and Eukarya the eukaryotes.

Within the eukaryotic domain, the protists are no longer a single group. They have been redistributed amongst different branches of the family tree. According to Simpson, we now know most of the evolutionary relationships amongst protists, and these are often counterintuitive. He cited the example of dinoflagellate algae, which are more closely related to the malaria parasite than they are to diatoms another group of algae or even to land plants.

Still, there are pressing questions that remain. This point is called the "root" of the eukaryotic tree of life.

Pinpointing the root will cement the understanding of eukaryotic origins and their subsequent evolution. As author Tom Williams said in a article published in the journal Current Biology, "For the eukaryotic tree, the root position is critical for identifying the genes and traits that may have been present in the ancestral eukaryote, for tracing the evolution of these traits throughout the eukaryotic radiation, and for establishing the deep relationships among the major eukaryotic groups.

Protists are responsible for a variety of human diseases including malaria, sleeping sickness , amoebic dysentery and trichomoniasis. Malaria in humans is a devastating disease. It is caused by five species of the parasite Plasmodium , which are transmitted to humans by female Anopheles mosquitoes, according to the Centers for Disease Control and Prevention CDC.

The species Plasmodium falciparum infects red blood cells, multiplies rapidly and destroys them. Infection can also cause red blood cells to stick to the walls of small blood vessels. This creates a potentially fatal complication called cerebral malaria according to the CDC.

According to their recent malaria fact sheet , in there were an estimated , deaths due to malaria in the world, the majority of which 90 percent occurred in Africa. Certain strides have been made in reducing the rates of incidence occurrence of new cases and mortality rates in part by supplying insecticide treated mosquito nets, spraying for mosquitoes and improving diagnostics. Between and the rate of incidence fell by 37 percent globally and mortality rates fell by 60 percent globally.

The WHO has a goal of eliminating malaria in at least 35 countries by Protists also play an important role in the environment. According to a review article published on the Encyclopedia of Life Sciences eLS website, nearly 50 percent of photosynthesis on Earth is carried out by algae. Protists act as decomposers and help in recycling nutrients through ecosystems, according to a review article published in the journal ACTA Protozoologica.

In addition, protists in various aquatic environments, including the open water, waterworks and sewage disposal systems feed upon, and control bacterial populations ACTA Protozoologica, Credit: Monkey Business Images Shutterstock.

Live Science.