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27 The Protozoa

 

CHAPTER OVERVIEW

This chapter discusses the characteristics of those protists that are commonly referred to as the protozoa. These protists exhibit a variety of locomotion mechanisms and demonstrate the great adaptive potential of unicellular eucaryotic organisms. 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 examples is given.

 

CHAPTER OBJECTIVES

After reading this chapter you should be able to:

! describe the various habitats, types of locomotory ability, and the specialized organelles of protozoa

! discuss the characteristics of the seven phyla in which protozoa are classified

! explain the reproductive strategies employed by protozoa

! discuss the various arrangements of nuclei that are found in protozoa

! describe the various feeding mechanisms used by protozoa

 

CHAPTER OUTLINE

I. Introduction

A. Protozoa are unicellular, eucaryotic protists that are usually motile

B. Protozoologists are scientists that study protozoa

C. Protozoology is the study of protozoa

D. Protozoa have diverse, non related, polyphyletic origins

II. DistributionCprimarily in moist habitats

A. Found in freshwater, marine, and moist terrestrial environments

B. Most are free living, but some are parasitic in plants and animals

III. Importance

A. Serve as an important link in food chains and food webs (zooplankton)

B. Important in the study of molecular biology because they use the same metabolic pathways as multicellular eucaryotes

C. Causative agents of some important diseases in humans and other animals

IV. Morphology

A. Ectoplasm is the gelatinous cytoplasm just inside the plasma membrane; it provides some rigidity and shape through the formation of a pellicle

B. Endoplasm is the more fluid cytoplasm in the interior of the cell

C. The pellicle consists of the plasma membrane and the structures immediately beneath it

D. The macronucleus is associated with trophic activities and regenerative processes

E. The micronucleus controls reproductive activities by sequestering genetic material for exchange during reproduction

F. Vacuoles

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

G. Some protozoa are anaerobic (e.g., Trichonympha lives in the gut of termites)

1. They have no mitochondria or cytochromes, and they have an incomplete TCA cycle

2. They contain hydrogenosomesCsmall membrane-delimited organelles containing a unique electron transfer system that uses protons as terminal electron acceptors to form molecular hydrogen

V. NutritionCchemoheterotrophic

A. In holozoic nutrition, nutrients are acquired by phagocytosis

B. Some ciliates have a specialized structure, called a cytosome, for phagocytosis

C. In saprozoic nutrition, nutrients are acquired by pinocytosis, diffusion, or carrier-mediated transport (facilitated diffusion or active transport)

VI. Encystment and Excystment

A. Encystation is the development of a resting stage structure called a cyst

1. The cyst is a dormant form marked by the presence of a wall and by the reduction of metabolic activity to a very low level

2. Functions of cysts

a. Protect against adverse changes in the environment

b. Function as sites for nuclear reorganization and cell division

c. Serve as a means of transfer from one host to another for parasitic species

B. Excystation is the escape of vegetative forms, called trophozoites, from the cyst; it is usually triggered by a return to favorable environment, such as entry into a new host for parasitic species

VII. Locomotory OrganellesCa few protozoa are nonmotile; most use one of three major types of locomotory organelles

A. PseudopodiaCcytoplasmic extensions

B. CiliaCfilamentous extensions (short)

C. FlagellaCfilamentous extensions (long)

VIII. Reproduction

A. The most common method of asexual reproduction is binary fission, which involves mitosis followed by cytokinesis

B. The most common type of sexual reproduction is conjugation, an exchange of gametic nuclei between paired protozoa of complementary mating types

IX. Classification

A. Protozoa are currently classified as a subkingdom of protists although Cavalier-Smith has proposed elevating the protozoa to the status of a kingdom

B. There are seven phyla within this subkingdom

C. Classification is based primarily on types of nuclei, mode of reproduction, and mechanism of locomotion

D. More recently, molecular classification schemes suggest that the protozoa do not exist as an evolutionary taxon, but rather that the protozoa are polyphyletic and are found at all evolutionary levels

X. Representative Types

A. Sarcomastigophora

1. Single type of nucleus

2. Flagella (subphylum Mastigophora)Czooflagellates

a. Some are free living

b. Some cause parasitic disease

(1) Trichomonas vaginalis causes a sexually transmitted disease

(2) Giardia lamblia causes a gastrointestinal disorder

(3) Trypanosoma brucei and other trypanosomes (also called hemoflagellates) are important blood pathogens

c. Kinetoplastids are a small group of mastigophorans whose mitochondrial DNA is in a special region called the kinetoplast

3. Pseudopodia (subphylum Sarcodina)

a. Amoeboid organisms

b. Some form cysts

c. Reproduction is usually by simple asexual binary fission

d. Some have a loose-fitting shell (test)

e. Foraminiferans and radiolarians are primarily marine amoebae with a few occurring in fresh or brackish water

f. There are some symbionts (parasites)

(1) Endamoeba blattae is common in the intestines of insects

(2) Entamoeba histolytica causes a parasitic disease in humans

B. Labyrinthomorpha

1. They are spindle-shaped or spherical, nonamoeboid, vegetative cells

2. Most members are marine organisms, and are either saprozoic or parasitic on algae

3. Move by gliding motion on mucous tracks

C. ApicomplexaCsporozoans

1. Lack locomotor organelles, except the male gametes and the zygotes (ookinetes)

2. Apical complexCa unique arrangement of fibrils, tubules, vacuoles, and other organelles at one end of the cell

a. Consist of one or two polar rings at the apical end

b. The conoidCspirally arranged fibers adjacent to the polar rings

c. Subpellicular microtubules radiate from the polar rings and probably serve as support elements

d. Rhoptries extend to the plasma membrane and secrete their contents at the cell surface (probably aids in host cell penetration)

e. Micropores take in nutrients

3. Complex life cycle involves two different hosts (usually mammal and mosquito)

4. Undergo an alternation of haploid and diploid generations

5. Undergo schizogony, a rapid series of mitotic events producing a large number of small infective organisms through the formation of uninuclear buds

6. Sexual reproduction involves the formation of a thick-walled oocyst after fertilization; meiosis within this structure then produces haploid infective spores

7. This group incudes some very important parasitic disease causers

a. PlasmodiumCmalaria

b. CryptosporidiumCcryptosporidiosis

b. ToxoplasmaCtoxoplasmosis

c. EimeriaCcoccidiosis

D. Microspora

1. Obligate intracellular parasites

2. Abundant and pathogenic in insects

3. Increased interest in their use as biological pest control

4. Recently, five genera have been implicated in human diseases in immunosuppressed patients and in AIDS patients

E. AcetosporaCspores lack polar caps or polar filaments, parasitic in mollusks

F. MyxozoaCresistant spore with one to six coiled polar filaments; parasitic on freshwater and marine fish; can cause a major economic problem in cultured salmon

G. Ciliophora

1. Use cilia as locomotory organelles (longitudinal rows or spirals)

2. Can move forward or backward by control of ciliary movement

3. Food processing is somewhat complex

a. Food enters the cytostome; it is brought in by the action of the surrounding cilia

b. The food passes to phagocytic vacuoles that fuse with lysosomes, where digestion occurs

c. 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

a. MicronucleusCdiploid; functions in mitosis and meiosis

b. MacronucleusCpolyploid for some genes; maintains routine cellular functions

5. Asexual reproduction is by transverse binary fission

6. Sexual reproduction usually is by conjugation

7. Some ciliates are harmless commensals, while others are disease-causing parasites

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