THE PROTOZOA

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Chapter Overview

This chapter discusses the characteristics of those protists that are commonly referred to as the protozoa. Protozoa exhibit different types of locomotion and this has been traditionally important in their classification. Using molecular methods, the protozoa have been shown to be polyphyletic. In addition to a discussion of their general features and the vast array of their niches and habitats, individual coverage of some representative protozoa is given. 

 

Chapter Objectives

After reading this chapter you should be able to:

1. describe the various habitats, types of locomotion, and specialized organelles of protozoa
2. discuss the characteristics of the seven phyla of protozoa
3. describe the reproductive strategies employed by protozoa
4. discuss the various types of nuclei that are found in protozoa
5. describe the various feeding mechanisms used by protozoa

These are the most important concepts you are learning in this chapter:

1. Protozoa are protists exhibiting heterotrophic nutrition and various types oflocomotion. They occupy a vast array of habitats and niches and have organelles similar to those found in other eucaryotic cells, and also specialized organelles.
2. Current protozoan taxonomy divides the protozoa into seven phyla: Sarcomastigophora, Labyrinthomorpha, Apicomplexa, Microspora, Ascetospora,Myxozoa, and Ciliophora. These phyla represent four major groups: flagellates, amoebae, ciliates, and sporozoa. In molecular classification schemes, the protozoa are polyphyletic eucaryotes.
3. Protozoa usually reproduce asexually by binary fission. Some have sexual cycles,involving meiosis and the fusion of gametes or gametic nuclei resulting in a diploid zygote. The zygote is often a thick-walled, resistant, and resting cell called a cyst.
4. Some protozoa undergo conjugation in which nuclei are exchanged between cells.
5. All protozoa have one or more nuclei; some have a macro- and micronucleus.
6. Various protozoa feed by holophytic, holozoic, or saprozoic means; some are predatory or parasitic.

Study Outline

1. Introduction
1. Protozoa are a polyphyletic group of organisms
2. Protozoa are unicellular, eucaryotic protists that are usually motile
3. Protozoology is the study of protozoa
2. Distribution
1. Primarily in moist habitats, including freshwater, marine, and moist terrestrial environments
2. Most are free living, but some are parasitic in plants and animals
3. Importance
1. Serve as an important link in food chains and food webs (zooplankton)
1. Food chain-series of organisms, each feeding on the preceding one
2. Food web-complex interlocking series of food chains
2. Important in the study of biochemistry and molecular biology because they use the same metabolic pathways as multicellular eucaryotes
3. Causative agents of some important diseases in humans and other animals
4. Morphology
1. Many aspects of their morphology are the same as those of cells of multicellular animals; however, protozoa have some unique features
1. Ectoplasm is the gelatinous cytoplasm just inside the plasma membrane; it provides some rigidity and shape
2. Pellicle consists of the plasma membrane and the structures immediately beneath it
3. Endoplasm is the more fluid cytoplasm in the interior of the cell
4. Some have one nucleus, some have two or more identical nuclei, and some have two distinct types of nuclei
1. The macronucleus is associated with trophic activities and regenerative processes
2. The micronucleus controls reproductive activities by sequestering genetic material for exchange during reproduction
2. Vacuoles are usually present
1. Contractile vacuoles are osmoregulatory
2. Phagocytic vacuoles are sites of food digestion
3. Secretory vacuoles usually contain enzymes for specific functions, such as excystation
3. Some protozoa are anaerobic (e.g., Trichonympha lives in the gut of termites)
1. Most anaerobic protozoa do not have mitochondria or cytochromes, and have an incomplete TCA cycle
2. Some anaerobic protozoa contain hydrogenosomes-small membrane-delimited organelles containing a unique electron transfer system that uses protons as terminal electron acceptors to form molecular hydrogen
5. Nutrition
1. In holozoic nutrition, nutrients are acquired by phagocytosis; some ciliates have a specialized structure, called a cytosome, for phagocytosis
2. In saprozoic nutrition, nutrients are acquired by pinocytosis, diffusion, or carrier-mediated transport (facilitated diffusion or active transport)
6. Encystment and Excystment
1. Encystation is the development of a resting stage structure called a cyst
1. The cyst is a dormant form that has a wall and greatly reduced metabolic activity
2. Functions of cysts
1. Protect against adverse changes in the environment
2. Function as sites for nuclear reorganization and cell division
3. Serve as a means of transfer from one host to another for parasitic species
2. Excystation is the escape of vegetative forms, called trophozoites, from the cyst; it is usually triggered by a return to a favorable environment (e.g., such as entry into a new host for parasitic species)
7. Locomotory Organelles
1. A few protozoa are nonmotile
2. Most use one of three major types of locomotory organelles
1. Pseudopodia-cytoplasmic extensions
2. Cilia-filamentous extensions (short)
3. Flagella-filamentous extensions (long)
8. Reproduction
1. The most common method of asexual reproduction is binary fission, which involves mitosis followed by cytokinesis
2. The most common type of sexual reproduction is conjugation, an exchange of gametic nuclei between paired protozoa of complementary mating types
9. Classification
1. The most accepted scheme classifies protozoa as a subkingdom of protists, containing seven phyla; classification is based primarily on types of nuclei, mode of reproduction, and mechanism of locomotion
2. Recently, other schemes have been suggested
1. Cavalier-Smith has proposed elevating the protozoa to the status of a kingdom with 18 phyla
2. Molecular classification schemes suggest that the protozoa do not exist as an evolutionary taxon, but rather that the protozoa are polyphyletic
10. Representative Types
1. Phylum Sarcomastigophora
1. This phylum includes protists with a single type of nucleus and flagella or pseudopodia; they reproduce asexually and sexually
2. Subphylum Mastigophora contains both phytoflagellates (chloroplast-bearing flagellates) and zooflagellates; zooflagellates have the following characteristics:
1. Do not have chlorophyll; are holozoic or saprozoic
2. Asexual reproduction occurs by longitudinal binary fission; sexual reproduction is known for a few species, and encystment is common
3. One group, the kinetoplastids, has mitochondrial DNA in a special region called the kinetoplast
4. Some are free living; some are endosymbiotic (e.g., Trichonympha species in the intestines of termites
5. Many are important human parasites (e.g., Trichomonas vaginalis, Giardia lamblia, and Trypanosoma brucei)
3. Subphylum Sarcodina-contains amoeboid organisms
1. Found in aquatic and terrestrial habitats, where they take up nutrients by phagocytosis and pinocytosis
2. Reproduction is usually by simple asexual binary fission; some form cysts
3. Some have a loose-fitting shell called a test (e.g., foraminiferans and radiolarians, which are primarily marine amoebae; a few occur in fresh or brackish water)
4. Some are endosymbionts and can be either commensals or parasites; some are free-living, disease-causing amoebae
2. Phylum Labyrinthomorpha
1. Protists with spindle-shaped or spherical, nonamoeboid, vegetative cells; some move by gliding motion on mucous tracks
2. Most members are marine organisms and are either saprozoic or parasitic on algae
3. Phylum Apicomplexa
1. Often called sporozoans because they have a spore-forming stage in their life cycle; lack locomotory organelles, except the male gametes and the zygotes (ookinetes); are either intra- or intercellular parasites having a characteristic structure called the apical complex
2. Apical complex-a unique arrangement of fibrils, tubules, vacuoles, and other organelles at one end of the cell
1. One or two polar rings at the apical end
2. Conoid-spirally arranged fibers adjacent to the polar rings
3. Subpellicular microtubules radiate from the polar rings and probably serve as support elements
4. Rhoptries extend to the plasma membrane and secrete their contents at the cell surface (probably aids in host cell penetration)
5. Micropores take in nutrients
3. Have complex life cycles involving two different hosts (usually mammal and often a mosquito)
1. Life cycle has both asexual and sexual phases and is characterized by an alternation of haploid and diploid generations
2. At some point in the life cycle, they undergo schizogony, a rapid series of mitotic events producing a large number of small infective organisms through the formation of uninuclear buds
3. Sexual reproduction involves the formation of a thick-walled oocyst after fertilization; meiosis within this structure then produces haploid infective spores
4. This group includes some very important pathogens
1. Plasmodium-malaria
2. Cryptosporidium-cryptosporidiosis
3. Toxoplasma-toxoplasmosis
4. Eimeria-coccidiosis
4. Phylum Microspora
1. Obligately intracellular parasites lacking mitochondria and transmitted by a resistant spore
2. Several economically important pathogens of insects
3. There has been increased interest in their use as biological pest control
4. Recently, five genera have been implicated in human diseases in immunosuppressed patients (e.g., AIDS patients)
5. Phylum Acetospora-parasitic protists with spores that lack polar caps or polar filaments, parasitic in mollusks
6. Phylum Myxozoa-parasitic protists with resistant spores having one to six coiled polar filaments; parasitic on freshwater and marine fish; can cause a major economic problem in cultured salmon
7. Phylum Ciliophora
1. The largest of the seven phyla; these organisms are distinguished by the use of cilia as locomotory organelles
1. Cilia arranged in longitudinal rows or spirals
2. Oblique stroke of cilia causes ciliates to rotate as they swim
3. Can move forward or backward
2. Numerous interesting morphological characteristics are observed: slipper-shaped cells, stalked cells, tentacles, and threadlike darts called toxicysts
3. Feeding behavior
1. Food is captured by action of cilia around the buccal cavity; food enters the cytostome and passes to phagocytic vacuoles that fuse with lysosomes, where digestion occurs
2. After digestion the vacuoles fuse with a special region of the pellicle, called the cytoproct, which empties the cell's waste material to the outside
4. Most have two types of nuclei
1. Micronucleus-diploid; functions in mitosis and meiosis
2. Macronucleus-polyploid for some genes; maintains routine cellular functions
5. Asexual reproduction is by transverse binary fission; sexual reproduction usually is by conjugation
6. Most are free-living; some are harmless commensals; others are disease-causing parasites