23
The Protists
CHAPTER OVERVIEW
This chapter discusses the characteristics of the diverse, polyphyletic group of organisms commonly known as
the protists, which includes the algae and the protozoans. They range from single cells to multicellular
organisms over 75 meters in length. They are found in oceans and freshwater environments and are the major
producers of oxygen and organic material. In addition to a discussion of their general features and the vast
array of their niches and habitats, individual coverage of some representative protists is given.
CHAPTER OBJECTIVES
After reading this chapter you should be able to:
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discuss the various habitats, types of locomotion, and specialized organelles of protists
discuss the various morphological characteristics of protists
discuss the taxonomic relationships of this diverse, polyphyletic group of organisms
discuss asexual and sexual reproduction of protists
discuss the various types of nuclei found in protists
describe the various feeding mechanisms used by protozoans
place the protists within current classification schemes
CHAPTER OUTLINE
I.
II.
Introduction
A. Protists are a polyphyletic collection of organisms. Most are unicellular and lack the level of tissue
organization found in higher organisms
B. The protists include protozoans (chemoorganotrophic protists) and algae (photosynthetic protists);
protozoology is the study of protozoans; phycology is the study of algae; protistology is the general
study of protists
C. Older classification schemes separated protozoans based on locomotion; the current system of
classification uses morphological, biochemical, and phylogenetic analyses
D. Found in moist environments, mainly free living (planktonic) in freshwater or marine environments;
in terrestrial systems associated with decaying organic matter
E. Every group of protists contains species that live in association with other organisms such as in
lichens (symbiosis of algae and fungi)
F. Photosynthetic protists are aerobes that perform plant-like oxygenic photosynthesis; some are
photoheterotrophic
G. Chemoheterotrophic protists are holozoic (phagocytize solid foods such as bacteria) or saprozoic
(take up soluble nutrients); Some protists are mixotrophic, having flexible metabolisms (e.g.,
simultaneous photosynthesis and holozoic feeding)
Supergroup Excavata
A. Most primitive and deeply branching eukaryotes; most possess a suspension-feeding groove
(cytostome) with a flagellum to create feeding currents
B. Fornicata
1. Have flagella but lack mitochondria, although some may have mitosomes of similar function
and history; reproduce asexually by binary fission
2. Most are harmless symbionts with some free-living forms in polluted waters
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3.
C.
D.
III.
A.
B.
C.
D.
E.
F.
Can be parasites in fish, pathogens in turkeys, and Giardia in humans causes diarrhea and is a
public health concern
Parabasilia
1. Flagellated, but without a distinct cytostome; use phagocytosis for feeding; most are
endosymbionts of animals
2. Trichonymphida
a. Obligate mutualist in the digestive tract of wood-eating insects (termites)
b. Secrete cellulase to aid animal digestion
c. Usually asexual but sometimes reproduce sexually in response to host hormone called
ecdysone
3. Trichomonadida
a. Anaerobic, do not contain mitochondria, but have hydrogenosomes, and reproduce
asexually
b. Symbionts of digestive, reproductive, and respiratory tracts of vertebrates
i.
Tritrichomonas foetus can cause spontaneous abortion in cattle
ii.
Dientamoeba fragilis causes diarrhea in humans
iii.
Trichomonas vaginalis can cause disease in human genitourinary tract
Euglenozoa
1. Commonly found in freshwater, but can be marine
2. Some are photoautotrophic, while the majority are mainly saprotrophic chemoorganotrophs
3. Euglena is the representative photoautotroph; contains chlorophylls a and b and carotenoids;
primary storage is in paramylon (polysaccharide of -1,3 linked glucose); has giant eye spot
(stigma) for orientation to light, two flagella, and a large contractile vacuole
4. Some Euglenozoa are pathogens, particularly the trypanosomes
a. Leishmania cause systemic and skin/mucous afflictions (leishmaniasis)
b. Trypanosoma cruzi cause Chaga's disease in Central and South America characterized by
peripheral nervous system dysfunction
c. T. gamiense and T. rhodesiense cause African sleeping sickness
d. Pathogenic trypanosomes rapidly change the glycoprotein coating of their cell walls
(antigenic variation), which helps them evade the host's immune system
Supergroup Amoebozoa
Amoeboid motility involves pseudopodia (cell extensions) that are rounded (lobopodia), long and
narrow (filopodia), or form a mesh (reticulopodia)
Amoebae can lack a cell wall (naked amoebae) or be covered (testate amoebae)
Asexual reproduction is by binary fission, although some form cysts
Tubulinea
1. Widely found in moist environments; Amoeba proteus is commonly used in laboratories
2. Some are endosymbionts, commensals, or parasites of animals
3. Some harbor intracellular symbionts including algae, bacteria, and viruses
Entamoebida
1. Contain mitosomes like Excavata, but no mitochondria or hydrogenosomes
2. Entamoeba histolytica causes amoebic dysentary, a leading cause of parasitic death
3. Cysts ingested from feces-contaminated food or water pass through stomach and multiply in
intestines, producing digestive enzymes that damage gut epithelial cells; can spread to other
areas of the body
Eumycetozoa
1. Slime molds have complex morphology, life cycle, and behavior
2. Myxogastria are acellular slime molds that live on rotting organic matter, where multinucleate
masses of protoplasm (plasmodium) move in an amoeboid fashion, feeding by endocytosis;
under adverse conditions, fruiting bodies form with stalks and spores; spores germinate to
produce haploid amoeboflagellates and eventually zygotes and new plasmodium
3. Dictyostelium discoideum is the classic cellular slime mold
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a.
IV.
A.
B.
C.
V.
A.
B.
Masses called pseudoplasmodium retain their cell walls; under adverse conditions cells
aggregate into motile multicellular slugs that form fruiting bodies with spores that can
germinate into amoeboid cells
b. Cells can differentiate into what resemble primitive immune cells (sentinel cells) that
patrol for pathogens
c. Sexual reproduction involves special spores called macrocysts that arise via conjugation
Supergroup Rhizaria
Amoeboid cells with filopodia; some filopodia called axopodia are supported by microtubules that
protrude from an axoplast and are used for feeding
Radiolaria
1. Most Radiolaria have internal skeletons of siliceous material or strontium sulfate; some have
siliceous exoskeletons, while others lack skeletons
2. Feed by endocytosis via entrapment in mucous; some have algal endosymbionts
3. Reproduction can be sexual with release of many biciliated isogametic cells or asexual by
binary or multiple fission or budding
Foraminifera
1. Amoeboid with reticulopedia and form calcerous shells; can have algal symbionts
2. Complex life cycle that may alternate between asexual and sexual phases; the sexual phase
includes flagellated gametes and haploid gamonts; autogamy is known
3. Foraminifera are found in marine and estuarine waters and most are benthic; some may be
capable of denitrification; their calcerous shells accumulate on the sea floor creating chalk,
limestone, and marble layers hundreds of meters deep
Supergroup Chromalveolata
A diverse group that includes autotrophic, mixotrophic, and heterotrophic protists, deriving their
chloroplasts from archaeplastids
Alveolata—a large group of protists that includes dinoflagellates, apicomplexans, and ciliates
1. Dinoflagellates
a. Dinoflagellata—large group of common marine plankton; involved in coral formation;
the causative agents of toxic red tides
b. Many are photosynthetic but not autotrophic; most are saprotrophic, but some use
endocytosis
c. Two flagella cause the cells to whirl; thecate cells are armored with cellulose plates
d. Most are free-living, but some are endosymbionts that release motile cells called
zooxanthellae
2. Ciliates
a. Ciliophora are large group (12,000 species) of chemoorganotrophs that inhabit benthic
and planktonic aquatic systems and moist soils; Paramecium and Stentor are well-known
members
b. Employ many cilia for movement and feeding; the action of cilia is coordinated; some
have tentacles or expel poison darts (toxicysts) to capture prey
c. Most have macro- and micronuclei; the micronucleus contains the diploid chromosomes
for sexual reproduction; the macronucleus is polyploid and produces mRNA to maintain
cellular metabolism and function
d. Some ciliates reproduce asexually by binary fusion, while many can reproduce sexually
by conjugation; during sexual reproduction, the macronuclei degrade and gametic nuclei
are produced from the micronucleus in a complex process
e. Some ciliates are symbiotic and may be parasites; cause ick in fish
3. Apicomplexans
a. Acomplexa are intra- and intercellular parasites of animals with a unique array of
cytoskeleton, vacuoles, and other organelles at one end of the cell called the apical
complex designed to penetrate host cells; some have motile gametes or zygotes
b. Apical complex includes apicoplasts, endosymbiotic organelles for the synthesis of lipids
and heme
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c.
C.
D.
VI.
A.
B.
Complex life cycle has sexual and asexual stages that sometimes occur in different hosts;
the motile haploid infective stage is called the sporozoite; in the host, gamonts are
generated, which fuse to form zygotes that become spores; these undergo meiosis to
generate more haploid sporozoites
d. Apicomplexans are important infectious agents
i.
Eimera—cecal coccidiosis in chickens
ii.
Cryptosporidia—intestinal infection
iii.
Toxoplasma—toxoplasmosis from cat feces
iv.
Theilaria—tick-borne blood diseases in cattle
v.
Plasmodium—malaria, a major killer worldwide
Stramenophila (stramenopiles)
1. Large, diverse group that includes photosynthetic diatoms, brown and golden algae, brown
seaweeds and kelp, and chemoorganotrophic öomcytes and labyrinthulids; all possess
heterokont flagella (straw hair flagella) in a life cycle phase
2. Diatoms
a. Photoautotrophs with chlorophylls a and c1/c2 and fucoxanthin; major carbohydrate is
chrysolaminarin (polysaccharide of -1,3-linked glucose)
b. Diatoms form silica shells (frustules) made of two overlapping halves; frustule
morphology is diverse and used for identification; can be unicellular, colonial, or
filamentous; lack flagella
c. Asexual reproduction occurs until frustule becomes too small and then sexual
reproduction occurs where the zygote develops into an auxospore that divides and forms
a new wall to produce vegetative cells with frustules
d. Found in aquatic systems; major contributors to carbon fixation in the ocean and vital to
global carbon cycles
3. Öomycetes
a. Once considered fungi but have cellulose and -glucan cell walls instead of chitin
b. Form large egg cell during sexual reproduction that is fertilized by a sperm cell or
antheridium, generating zoospores with heterokont flagellation
c. Important members include saprophytes on decaying organic matter, fish parasites,
downy mildews on plants, and Phytophthora, the causative agent of the Irish potato
blight of the 1840s
4. Labyrinthulids
a. Once considered fungi, they form heterokont flagellated spores
b. Move along an external ectoplasmic network of actinlike filaments
c. Mainly feed by osmotrophy, releasing enzymes to degrade organic matter
Haptophyta—planktonic photosynthetic protists that are encrusted in calcite scales and influence
global carbon cycles
Supergroup Archaeplastida
Includes all organisms with a photosynthetic plastid arising from endosymbiosis with an ancient
cyanobacterium; the group includes higher plants and protists
Chloroplastida
1. Commonly called green algae, these phototrophs are found in aquatic and soil systems; many
cell types including unicellular, filamentous, and colonial
2. Have chlorophylls a and b and carotenoids; store carbohydrates as starch
3. Asexual reproduction is through zoospores and sexual reproduction involves four flagellated
zygotes and resting phase with meiosis, producing haploid organisms
4. Chlamydomonas is highly studied; has two flagella, a haploid nucleus, a giant chloroplast,
contractile vacuoles, and a stigma (eyespot)
5. Protheca is common in soil and can cause severe subcutaneous and systemic blood infections
(protothecosis) in animals and humans
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TERMS AND DEFINITIONS
Place the letter of each term in the space next to the definition or description that best matches it.
____ 1.
____ 2.
____ 3.
____ 4.
____ 5.
____ 6.
____ 7.
____ 8.
____ 9.
____ 10.
____ 11.
____ 12.
____ 13.
____ 14.
____ 15.
____ 16.
____ 17.
____ 18.
____ 19.
____ 20.
____ 21.
____ 22.
Term that describes algae that are suspended in the
aqueous environment
A disease caused by a green alga; the disease usually
starts as a small subcutaneous lesion and spreads until
it covers large areas of the body
Asexual spores that are flagellated and, therefore,
motile
Vegetative cells that are the site of egg formation
A structure that aids in phototactic responses
Intricately patterned coverings on the plasma
membrane of some algae
The distinctive two-piece wall (valve) of silica found
on diatoms
The larger of the two pieces of the diatom valve
The smaller of the two pieces of the diatom valve
Symbiotic dinoflagellates found in marine organisms
The vegetative form of a protozoan
The dormant form of a protozoan
The single site of phagocytosis in some ciliated
protozoans
Cytoplasmic extensions used for locomotion and food
capture
Nuclear mitotic division followed by division of the
cytoplasm
A rapid series of mitotic events producing many small,
infective organisms through the formation of
uninuclear buds
The process whereby there is an exchange of gametes
between paired protozoans of complementary mating
types
The loose-fitting shell around some amoeboid
organisms
An arrangement of fibrils, microtubules, vacuoles, and
other organelles at one end of the cell
A region of the pellicle of ciliates where phagocytic
vacuoles empty their contents after food digestion has
taken place
Specialized region found in some zooflagellates where
their mitochondrial DNA is stored
A plastid in apicomplexans that is not involved in
photosynthesis
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
m.
n.
o.
p.
q.
r.
s.
t.
u.
v.
apical complex
apicoplast
binary fission
conjugation
cyst
cytoproct
cytostome
epitheca
frustule
hypotheca
kinetoplast
oogonia
planktonic
protothecosis
pseudopodia
scales
schizogony
stigma (eyespot)
test
trophozoite
zoospores
zooxanthellae
FILL IN THE BLANK
1.
2.
3.
The term
was originally used to define simple “aquatic plants.” The study of these organisms
is called ____________.
Many of the algae have a structure called an eyespot, or ____________, that aids the organism in
phototactic responses.
Brown algae are multicellular and can be very large. The largest are called
.
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4.
The Stramenopiles includes golden-brown algae, yellow-green algae, and
. The latter have a
two-piece cell wall of silica, called a
, that accumulates at the bottom of aquatic
environments. This material is used in detergents, polishes, paint removers, and many other products.
5. The
, though photosynthetic, are most closely related to ciliated protozoans. Algal
blooms of these organisms, called
, can be very destructive.
6. The Euglenozoa lack cell walls but have a flexible proteinaceous structure inside the plasma membrane.
This structure is called a
. The Euglenozoa that are pathogenic to humans fall in class called
__________ and includes two systemic diseases of global importance, _____________, and
__________.
7. In some species of protozoans, the cytoplasm immediately under the plasma membrane is gelatinous and
is termed ____________. The plasma membrane and structures immediately beneath it are referred to as
the ____________. The more fluid portion of the cytoplasm in the interior of the cell is referred to as the
____________.
8. Two types of heterotrophic nutrition are observed in protozoans. In
nutrition, nutrients are
acquired by phagocytosis. In ciliated organisms, phagocytosis occurs at a single location called the
____________. In
nutrition, nutrients are acquired by pinocytosis or other forms of direct
transport.
9. The
slime molds feed as a multinucleate mass of protoplasm called a
. When
food or water is in short supply, they form a fruiting body that produces and releases spores. When the
conditions are again favorable, the spores germinate to release either nonflagellated amoeboid
____________ or flagellated ____________ cells that feed and are haploid. The
slime
molds feed as individual amoeboid cells called
. When their food supply is exhausted, the
cells swarm together to form a sluglike
, which eventually forms a fruiting body called a
(
).
10. The
, or water molds, even though phylogenetically distinct, resemble fungi in appearance
because they consist of finely branched filaments called ____________.
MULTIPLE CHOICE
For each of the questions below select the one best answer.
1.
2.
3.
Which of the following is NOT a
mechanism used by algae for asexual
reproduction?
a. fragmentation
b. spore formation
c. binary fission
d. All of the above are mechanisms
used by algae for asexual
reproduction.
Diatom shells have been used for which of
the following?
a. abrasives in detergents and polishes
b. filtering agents
c. soundproofing and insulating
material
d. All of the above are correct.
Which of the following is NOT a reason
for studying protozoans?
a. They are important links in many
food chains and food webs.
b. They are good models for studying
eukaryotic metabolism.
c.
4.
5.
6.
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They cause important diseases in humans
and other animals.
d. All of the above are reasons for studying
protozoa.
Which of the following is a method of
locomotion used by protozoans?
a. flagella
b. cilia
c. pseudopodia
d. All of the above are correct.
Which of the following is NOT a flagellated
parasite to humans?
a. Giardia lamblia
b. Entamoeba histolytica
c. Trichomonas vaginalis
d. Trypanosoma cruzi
Protozoans that have spindle-shaped or
spherical nonamoeboid vegetative cells, and that
move within a network of mucous tracks by a
typical gliding motion, belong to which of the
following phyla?
a. Apicomplexa
b. Ascetospora
7.
c. Labyrinthomorpha
d. Myxozoa
Which of the following is NOT true about
the apicomplexans?
a. The motile infective stage is called
the sporozoite.
b. Their complex life cycles involve an
alternation of diploid and haploid
generations.
c.
8.
The male and female gamonts fuse to form
the zygote.
d. All of the above are true about the phylum
Apicomplexa.
Which of the following diseases is NOT caused
by members of the apicomplexans?
a. malaria
b. toxoplasmosis
c. giardiasis
d. coccidiosis
TRUE/FALSE
____
____
____
____
1.
2.
3.
4.
____ 5.
____ 6.
____ 7.
____ 8.
____ 9.
____10.
____11.
____12.
____13.
____14.
____15.
There are no diseases of humans caused by photosynthetic protists.
Algae are not known to be endosymbionts, parasites, or members of mutualistic relationships.
Euglena uses a contractile vacuole and an anterior reservoir for osmotic regulation.
While most photosynthetic protists are photoautotrophic, some chemoheterotrophs have been
identified.
The photosynthetic protists are polyphyletic, meaning that they are associated with multiple lineages
with independent origins and do not, therefore, represent a single evolutionary branch of
development.
Photosynthetic protists can be unicellular, colonial, filamentous, membranous, or tubular.
Photosynthetic protists have well-developed vascular systems and complex reproductive structures.
The distribution of protists in nature is limited to marine habitats.
In conjugation, the micronuclei divide by meiosis to form the haploid gamete nuclei.
In conjugation, the parental macronuclei disappear and the daughter cells develop new macronuclei
after the diploid zygote has formed.
In both classical and more recent molecular classification schemes, some protozoans and some algae
belong to the same taxa.
All protozoans exhibit motility as adult organisms.
Protozoans have been shown to be polyphyletic and therefore not an evolutionary distinct taxon.
In many respects, the morphology and physiology of protozoans resembles those of multicellular
animals.
Protozoans do not exhibit sexual reproduction.
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PROTOZOAL TAXONOMY
Super Group
Evolutionary
Lineage
Representative
Genera
Fornicata
Excavata
Parabasilia
Euglenozoa
Tubulinea
Amoebozoa
Entamoebida
Eumycetozoa
Radiolaria
Rhizaria
Foraminifera
Alveolata
Chromalveolata
Stramenopila
Haptophyta
Archaeplastida
Chloroplastida
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Important Characteristics
(structural, physiological, ecological)
CRITICAL THINKING
1.
The cyanobacteria were once included in the algae. Why were they once included? Why aren’t they now
included?
2.
Many protozoans use contractile vacuoles as osmoregulatory organelles. How do these organelles
function to maintain osmotic balance? Why are they found in freshwater protozoans but not in marine
protozoans?
3
Many algae and all the protozoans are referred to as protists. Compare and contrast algal protists and
protozoans. Do recent molecular studies support their being grouped together in a single taxon? Explain
your answer.
ANSWER KEY
Terms and Definitions
1. m, 2. n, 3. u, 4. l, 5. r, 6. p, 7. i, 8. h, 9. j, 10. v, 11. t, 12. e, 13. g, 14. o, 15. c, 16. q, 17. d, 18. s, 19. a, 20. f,
21. k, 22. b
Fill in the Blank
1. algae; phycology 2. stigma 3. kelps 4. diatoms; frustule 5. dinoflagellates; red tides 6. pellicle;
trypanosomes; Chaga's disease; African sleeping sickness 7. ectoplasm; pellicle; endoplasm 8. holozoic;
cytostome; saprozoic 9. acellular (plasmodial); plasmodium; myxamoebae; swarm; cellular; myxamoebae;
pseudoplasmodium; sorus; sorocarp 10. oomycetes; hyphae
Multiple Choice
1. d, 2. d, 3. d, 4. d, 5. b, 6. c, 7. c, 8. c
True/False
1. F, 2. F, 3. T, 4. T, 5. T, 6. T, 7. F, 8. F, 9. T, 10. T, 11. T, 12. F, 13. T, 14. T, 15. F
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