Biological Sciences
What is a Protist
A protist is a diverse group of eukaryotic microorganisms that are not plants, animals, or fungi. They can be unicellular, colonial, or multicellular and are found in various aquatic and moist environments. Protists play important roles in ecosystems as producers, consumers, and decomposers, and some are known to cause diseases in humans and other organisms.
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9 Key excerpts on "What is a Protist"
eBook - PDFCell Physiology Sourcebook
A Molecular Approach
- Nicholas Sperelakis(Author)
- 2001(Publication Date)
- Academic Press(Publisher)
Figure 1 gives an infor-mal indication of current thinking about relationships among protistan and other eukaryotic groups, as indicated by molecular evidence, particularly rRNA homology (Sogin, 1989, 1991) as well as ultrastructural information. Molecular and other evidence suggests that many protist lines may have originated over a billion years ago. Thus, the genetic divergence (in the sense of divergence in nucleic acid sequences) among the various protists is at least com-parable to that separating the animal, plant, and fungal king-doms. Although they are certainly phylogenetically diverse, the protists do have a certain physiological unity, based on common problems confronted by unicellular organisms. While, in general, they are typical eukaryotic cells and follow the principles described elsewhere in this book, a number of evolutionary solutions have appeared here that are not present in the cells of multicellular organisms. An example is the light antenna (eyespot) present in many photoautotrophic flagellates. This structure, which ap-pears to have evolved independently in several protist groups, is not found in the multicellular animals and plants. A feature of protists, just starting to be appreciated, is the apparent ease with which many cells capture and use physiological units of other cells. A prime example is the se-questration and use of prey organelles such as chloroplasts by phagotrophic protists. Indeed, the incorporation of sym-bionts is a pervasive feature of protistan physiology, occur-ring in most if not all groups. There are a variety of endo-symbiotic phenomena, ranging from the temporary uptake, endocytosis, and use of foreign cells or organelles, to estab-lished symbioses in which the endosymbiont has become a required part of the host cell. A dramatic example of this was the appearance of an intracellular bacterial infection in a laboratory culture of Amoeba.
eBook - PDF- Cecie Starr, Christine Evers, Lisa Starr, , Cecie Starr, Cecie Starr, Christine Evers, Lisa Starr(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
TAKE-HOME MESSAGE 14.6 ● ● The protists are a diverse collection of eukaryotic lineages, some of which are only distantly related to one another. ● ● Most protists live as single cells, but there are colonial and multicelled species. ● ● Protists include both producers and consumers. They live in freshwater, seas, and damp places on land. ● ● Some protists also live inside other eukaryotes, including humans. Protist diseases are spread by insect bites, by water contaminated with cysts, and by sexual contact. ● ● Green algae are the closest protist relatives of land plants, and choanoflagellates are the closest protist relatives of animals. Figure 14.30 Plasmodial slime mold on a log. This multinucleated mass (the plasmodium) streams along at a rate of about a millimeter an hour, engulfing any food it encounters. As the plasmodium travels, it lays down a trail of slime. If it later happens across its own trail, it will move off in a different direction. In this way, the slime mold “remembers” where it has been and avoids revisiting areas where it has already depleted its food supply. Edward S. Ross A. Structure of a solitary choanoflagellate. flagellum actin-reinforced filaments of collar Figure 14.31 Choanoflagellates, the modern protist group most closely related to animals. (B) Courtesy of Damian Zanette B. A colonial choanoflagellate. Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 286 UNIT 3 EVOLUTION AND DIVERSITY 14.7 Viruses LEARNING OBJECTIVES ●●● ● Explain how viruses replicate.
eBook - PDFMicrobiology
Principles and Explorations
- Jacquelyn G. Black, Laura J. Black(Authors)
- 2018(Publication Date)
- Wiley(Publisher)
Although most protists are microscopic, they vary in diameter from 5 μm to 5 mm. The Importance of Protists Protists have captured the fancy of biologists since Leeuwenhoek made his first microscopes. In fact, most of the “animalcules” he observed were protists. Like Leeuwenhoek, many people find protists inherently inter- esting, and biologists have learned much about life pro- cesses from protists. Protists also are important to humans for other rea- sons. For instance, they are a key part of food chains. Autotrophic protists capture energy from sunlight. Some heterotrophic protists ingest autotrophs and other het- erotrophs. Others decompose, or digest, dead organic matter, which then can be recycled to living organisms. Protists also serve as food for higher-level consumers. Ultimately, some energy originally captured by protists reaches humans. For example, energy from the sun is transferred to protists, protists are eaten by clams, and the clams are eaten by humans. Protists can be economically beneficial or detrimen- tal. Certain protists have tests, or shells, of calcium carbon- ate. Carbonate shells deposited in great numbers by such protists that lived in ancient oceans formed the white cliffs of Dover, England, and the limestone used in building the pyramids of Egypt. Because different test-forming pro- tists gained prominence during different geological eras, the identification of the protists in rock layers helps deter- mine the age of the rocks. Certain test-forming protists tend to occur in rock layers near petroleum deposits, so geologists looking for oil are pleased to find them. Some which the intracellular rickettsial bacterium, Wolbachia, infects various filiarial worms, such as the ones that cause elephantiasis, power blindness, and heartworm. If you give the host doxycycline, an antibiotic, which kills the Wolbachia, the worm loses the ability to reproduce and eventually dies.
- Jason Birkett, John Lester(Authors)
- 2018(Publication Date)
- CRC Press(Publisher)
Chapter 3The eucaryotic protists
3.1 Introduction
In Chapter 2 it was stated that the bacteria are the most important group from the public health or environmental engineering standpoint. The microbial world also includes many eucaryotic organisms, however, which fall into three main groups: the algae, fungi and protozoa. These organisms will all be considered together in this chapter. This is not because they share similar characteristics, since the group is very diverse, but because they constitute all of the true micro-organisms with a eucaryotic cell organization.3.2 Algae
The organisms which constitute this group are essentially plant-like. It includes both macroscopic as well as microscopic forms. The name of the group is derived from the Latin word for sea-wrack, macroscopic algal forms frequently washed onto beaches. Most algae are aquatic organisms and they may inhabit fresh or saline waters. They are usually free-living; however, some aquatic forms have adopted symbiotic relationships with marine invertebrate animals (e.g. corals and sponges). The terrestrial species normally grow in soil or on the bark of trees, but some have established symbiotic relationships with fungi to form lichens. These two-membered natural associations form slowly growing colonies in many inhospitable environments and in particular rock surfaces.Some 70% of the Earth’s surface is covered by water and it is probable that the algae fix more carbon dioxide than all the land plants combined. This may not be so readily apparent if one thinks only of the macroscopic algae which are confined to the shore line or shallow water. However, the overwhelming majority of algae are microscopic, unicellular, floating forms which constitute the phytoplankton. Their density is low and they therefore rarely impart any colour to the oceans. However, the enormous volume of the oceans which they occupy makes them the most abundant of all photosynthetic organisms.
eBook - PDF- Mark Carlson Williams(Author)
- 1977(Publication Date)
- Yale University Press(Publisher)
Electron microscopic, genetic, and biochemical studies since the 1960s have revealed a diversity of structure and life-styles in the microcosm of protoctists that was unanticipated from the perspective of the macroscopic world. The best-known protoctists are called algae (or eukaryotic algae, since the blue-greens are prokaryotes) and protozoa. The presence of plastids in very closely related organisms converts a protozoan to an alga. The more that is learned about the protoctists, the more these terms become obsolete. They should be used only informally, to refer to nutritional modes (algal: photosynthetic; protozoological: heterotrophic) rather than to evolution and systematics. In addition to protozoa and algae the protoctists include groups that have traditionally been placed with the fungi (water molds, chytrids, slime molds) and with the plants (seaweeds). Distinctionnramong the protoctist groups are made on the basis of morphology and life-cycle stages, presence or absence of organellar systems (mitochondria, undulipodia), mor-phology of organellar systems, especially the kinetosomal-microtubule ar-rangement at the base of the undulipodia, which, by definition, are kinetids (Moestrup, 1982). Mitosis and meiotic sexuality may be absent. In the sexual species the nature of the sexual stages is crucial for the differentiation of the groups. The preceding brief description of protists is meant to accompany the diagrams on the following pages and is by way of an introduction to the protoctist kingdom of living organisms. Only recently has comprehen-sive information about protoctists been available in one source, Handbook of Protoctista (Margulis et al., 1990), comparable to those for bacteria: Ber-geys Manual (Buchanan and Gibbons, 1974) and more recently The Pro-karyotes (Starr et al., 1983). We summarize here the most recently recog-nized list of monophyletic higher taxa in full recognition that with further
eBook - PDF- Samantha Fowler, Rebecca Roush, James Wise(Authors)
- 2016(Publication Date)
- Openstax(Publisher)
13 | DIVERSITY OF MICROBES, FUNGI, AND PROTISTS Figure 13.1 Living things are very diverse, from simple, single-celled bacteria to complex, multicellular organisms. (credit "ringworm": modification of work by Dr. Lucille K. Georg, CDC; credit "Trypanosomes": modification of work by Dr. Myron G. Schultz, CDC; credit “tree mold”: modification of work by Janice Haney Carr, Robert Simmons, CDC; credit "coral fungus": modification of work by Cory Zanker; credit "bacterium": modification of work by Dr. David Cox, CDC; credit "cup fungus": modification of work by "icelight"/Flickr; credit "MRSA": modification of work by Janice Haney Carr, CDC; credit "moldy grapefruit": modification of work by Joseph Smilanick) Chapter Outline 13.1: Prokaryotic Diversity 13.2: Eukaryotic Origins 13.3: Protists 13.4: Fungi Introduction Until the late twentieth century, scientists most commonly grouped living things into five kingdoms—animals, plants, fungi, protists, and bacteria—based on several criteria, such as absence or presence of a nucleus and other membrane-bound organelles, absence or presence of cell walls, multicellularity, and mode of nutrition. In the late twentieth century, the pioneering work of Carl Woese and others compared nucleotide sequences of small-subunit ribosomal RNA (SSU rRNA), which resulted in a dramatically different way to group organisms on Earth. Based on differences in the structure of cell membranes and in rRNA, Woese and his colleagues proposed that all life on Earth evolved along three lineages, called domains. The three domains are called Bacteria, Archaea, and Eukarya. Chapter 13 | Diversity of Microbes, Fungi, and Protists 291 Two of the three domains—Bacteria and Archaea—are prokaryotic, meaning that they lack both a nucleus and true membrane-bound organelles. However, they are now considered, on the basis of membrane structure and rRNA, to be as different from each other as they are from the third domain, the Eukarya.
eBook - PDFBiology
Concepts and Applications
- Cecie Starr, Christine Evers, Lisa Starr, , Cecie Starr, Christine Evers, Lisa Starr(Authors)
- 2017(Publication Date)
- Cengage Learning EMEA(Publisher)
Most protists are single cells, but some are colonial organisms or multicellular organisms. Some single-celled protists have a pellicle that helps them maintain their shape. Others have a secreted cell wall or shell. Single-celled freshwater protists use a contractile vacuole to expel excess water. Some protists are het- erotrophs and others have chloroplasts that evolved from cyano- bacteria by primary endosymbiosis. Still others have chloroplasts that evolved from an alga by secondary endosymbiosis. Section 20.3 Members of the eukaryotic supergroup Excavata are single-celled protists with flagella and no cell wall. Diplomonads and parabasalids are anaerobic. Both groups include species that infect humans. Trypanosomes are parasites with a single mitochondrion. Some cause deadly disease in humans. Euglenoids are free-living in lakes and ponds. Some have chloroplasts that evolved from a green alga. Sections 20.4, 20.5 The SAR super- group unites three lineages of protists. Stramenopiles include two photosynthetic groups. The silica-shelled diatoms are mostly single cells that are part of the phytoplankton, whereas brown algae range from microscopic strands to giant kelps. Water molds, a third stramenopile lineage, are decomposers and parasites that grow as a mesh of absorptive filaments. The second SAR lineage, the alveolates, has tiny sacs (alveoli) beneath the plasma membrane. Dinoflagellates are whirling aquatic cells. They include autotrophs and heterotrophs. Most are free-living, but some live in corals. Some dinoflagellates can emit light (are bioluminescent). Ciliates use cilia to move and to feed. Most are aquatic predators, but some are parasites. All apicomplexans are parasitic alveolates that live in animal cells. The disease malaria is caused by the apicomplexan parasite Plas- modium and transmitted by mosquitoes. The parasite lives in human liver and blood cells. Untreated malaria is deadly, but drugs can prevent and cure it.
eBook - PDF- Gene E. Likens(Author)
- 2010(Publication Date)
- Academic Press(Publisher)
In aquatic systems, it is likely that heterotrophic protists contribute to the diet of at least some life stage of most zooplankton, many fish, molluscs, and aquatic insects that also feed on photosynthetic protists. Nutrient Cycling The transfer of nutrients through different chemical states and ecological compartments (biotic and abi-otic) is tightly linked to microorganisms – especially in aquatic systems where protists dominate primary production. Availability of dissolved phosphorus and/ or nitrogen limits primary production, and recycling is most important where allochthonous input of N and P is low. Heterotrophic protists can be efficient nutri-ent remineralizers, depending on the nutrient content of their prey, and contribute to phytoplankton nutrient demand in the epilimnion of stratified lakes when measurable dissolved nutrients are low. Recycling often leads to high nutrient retention in benthic sys-tems within the photic zone of lakes and streams. In the benthos, phototrophic and heterotrophic micro-organisms, both prokaryotic and eukaryotic, tend to live in close proximity and often within boundary layers that can lead to localized nutrient limitation and a subsequent importance of recycling. Summary Protists represent a ubiquitous, though taxonomically ill-defined, group of generally microscopic eukaryotes that include amoeboid, flagellated, and ciliated taxa. They contribute substantially to primary production, food web interactions, and nutrient recycling in aquatic ecosystems. All trophic levels, including primary pro-ducers, have protistan members. Heterotrophs feed as bacterivores, herbivores, carnivores, histophages, and parasites, or as omnivores. Several protistan groups have members that combine photosynthesis and heterotrophy (mixotrophy). Protists are the major pre-dators of bacteria and are prey of benthic and plank-tonic metazoans.
eBook - PDFBiogeography of Microscopic Organisms
Is Everything Small Everywhere?
- Diego Fontaneto(Author)
- 2011(Publication Date)
- Cambridge University Press(Publisher)
However scientifically unfashionable it may become, it is important that we continue to dis- cover and describe new protist taxa. There are not many reliable data about which free-living protists are globally rare, or highly specialised. Studying these would help to resolve to what extent morphology, abundance, population size and eco- logical generalism are drivers of global dispersal of protists. The overview in this chapter strongly reinforces the necessity of looking at each group of organisms individually and with fresh eyes. It is the differences between them, where these can be shown to be robust and then investigated in a hypothesis-driven frame- work, that could prove to be the most biologically interesting and informative sources of biogeographic insight. References Amato, A., Kooistra, W.H., Ghiron, J.H. et al. (2007). Reproductive isolation among sympatric cryptic species in marine diatoms. Protist 158, 193–207. Baas Becking, L.G.M. (1934). Geobiologie of inleiding tot de milieukunde. The Hague: Van Stockum and Zoon. BIOGEOGRAPHY OF MICROSCOPIC ORGANISMS 104 Barth, D., Krenek, S., Fokin, S.I., Berendonk, T.U. (2006). Intraspecific genetic variation in Paramecium revealed by mitochondrial cytochrome c oxidase I sequences. Journal of Eukaryotic Microbiology 53, 20–25. Bass, D., Cavalier-Smith, T. (2004). Phylum- specific environmental DNA analysis reveals remarkably high global biodiversity of Cercozoa (Protozoa). International Journal of Systematic and Evolutionary Microbiology 54, 2393–2404. Bass, D., Richards, T.A., Matthai, L., Marsh, V., Cavalier-Smith, T. (2007). DNA evidence for global dispersal and probable endemicity of protozoa. BMC Evolutionary Biology 7 , 162. Bass, D., Howe, A.T., Mylnikov, A.P. et al. (2009a). Phylogeny and classification of Cercomonadida: Cercomonas, Eocercomonas, Paracercomonas, and Cavernomonas gen. n. Protist 160, 483–521. Bass, D., Chao, E.E., Nikolaev, S.
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