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CHAPTER
12
Characterizing and Classifying Eukaryotes
Chapter Outline
General Characteristics of Eukaryotic Organisms (pp. 345–350)
Reproduction of Eukaryotes
Classification of Eukaryotic Organisms
Protozoa (pp. 350–357)
Distribution of Protozoa
Morphology of Protozoa
Nutrition of Protozoa
Reproduction of Protozoa
Classification of Protozoa
Fungi (pp. 357–366)
The Significance of Fungi
Morphology of Fungi
Nutrition of Fungi
Reproduction of Fungi
Classification of Fungi
Lichens
Algae (pp. 367–371)
Distribution of Algae
Morphology of Algae
Reproduction of Algae
Classification of Algae
Water Molds (p. 371)
Other Eukaryotes of Microbiological Interest: Parasitic Helminths
and Vectors (pp. 372–373)
Chapter Summary
General Characteristics of Eukaryotic Organisms (pp. 345–349)
Eukaryotic microbes include a fascinating and diverse assemblage, including
species that are vital for human life and
numerous human pathogens: Among the 20 most frequent microbial causes of
death worldwide, six are eukaryotic
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Copyright © 2014 Pearson Education, Inc.
microbes. Our discussion of the general characteristics of eukaryotes begins
with a survey of the events in eukaryotic
reproduction.
Reproduction of Eukaryotes
Eukaryotic reproduction is more complex than prokaryotic reproduction. Nearly
all eukaryotes package most of their
DNA with histone proteins as chromosomes in the form of chromatin fibers
located within nuclei. Eukaryotes have a
variety of methods of asexual reproduction, including binary fission,
budding, fragmentation, spore formation, and
schizogony. Many eukaryotes reproduce sexually. Algae, fungi, and some
protozoa reproduce both sexually and
asexually.
Eukaryotic reproduction involves two types of division: nuclear division and
cytoplasmic division.
Nuclear Division
Typically, a eukaryotic nucleus has either one or two complete copies of the
chromosomal portion of a cell’s genome. A nucleus with a single copy of each
chromosome is called a haploid or 1n nucleus, and one with two sets of
chromosomes is a diploid or 2n nucleus.
A nucleus that has replicated its DNA divides via a process called mitosis,
which proceeds in four stages:
1. In prophase, the cell’s DNA condenses into chromatids, and the spindle
apparatus forms.
2. In metaphase, the chromosomes line up on a plane in the middle of the cell
and attach near their centromeres to
microtubules of the spindle.
3. In anaphase, sister chromatids separate and move to opposite poles of the
spindle to form chromosomes.
4. In telophase, nuclear envelopes form around the daughter nuclei.
Mitosis results in two nuclei with the same ploidy as the original.
Meiosis is nuclear division of diploid cells that results in four haploid
daughter nuclei. It is a necessary condition
for sexual reproduction in which nuclei from two different cells fuse to form
a single diploid nucleus. Meiosis occurs
in two stages known as meiosis I and meiosis II. The phases of meiosis I are
early prophase I, late prophase I, metaphase I, anaphase I, and telophase I.
The phases of the second stage are prophase II, metaphase II, anaphase II,
and
telophase II. Although the precise actions within the phases of meiosis I
differ from those of mitosis, overall, meiosis
can be considered back-to-back mitoses without the DNA replication of
interphase between them. Crossing over
during meiosis I produces genetic recombinations, ensuring that the
chromosomes resulting from meiosis are different from the parental
chromosomes.
Cytokinesis (Cytoplasmic Division)
Cytokinesis, the division of a cell’s cytoplasm, typically occurs
simultaneously with telophase of mitosis, though in
some algae and fungi it may be postponed or may not occur at all. In these
cases, mitosis produces multinucleate
cells called coenocytes.
Schizogony
Some protozoa, such as Plasmodium, reproduce asexually within red blood cells
and liver cells via a special type of
reproduction called schizogony, in which the cell undergoes multiple mitoses
to form a multinucleate schizont. Only
then does cytokinesis occur, simultaneously releasing numerous uninucleate
daughter cells called merozoites.
Classification of Eukaryotic Organisms
The classification of eukaryotic microbes has been difficult and has changed
frequently. Historical schemes based on
similarity in morphology and chemistry have been replaced with schemes based
on nucleotide sequences and cellular
ultrastructural features.
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Copyright © 2014 Pearson Education, Inc.
Protozoa (pp. 349–357)
Protozoa are eukaryotic, unicellular organisms that lack a cell wall.
Distribution of Protozoa
Protozoa require moist environments. They are critical members of the
plankton, free-living, drifting organisms that
form the basis of aquatic food chains. Few are pathogens.
Morphology of Protozoa
Protozoa are characterized by great morphologic diversity. Some have cilia,
whereas others have flagella or pseudopodia. Some have two nuclei, others
vary in the number and type of mitochondria they contain, and some have
contractile vacuoles that pump water from cells. All free-living aquatic and
pathogenic protozoa exist as a motile
feeding stage called a trophozoite, and many also have a hardy resting stage
called a cyst, which is characterized by
a thick capsule and a low metabolic rate.
Nutrition of Protozoa
Most protozoa are chemoheterotrophic, obtaining nutrients by phagocytizing
bacteria, decaying organic matter, other
protozoa, or host tissues. A few protozoa absorb nutrients from the
surrounding water. Some protozoa are photoautotrophic, and historically were
classified with the algae.
Reproduction of Protozoa
Most protozoa reproduce asexually only, by binary fission or schizogony; a
few protozoa also undergo sexual
reproduction. Sexual reproduction for some involves formation of gametocytes
(gametes) that fuse with one another
to form a diploid cell called a zygote Others reproduce sexually by a complex
process of reciprocal exchange of
genetic material known as conjugation.
Classification of Protozoa
Taxonomists continue to revise and refine the classification of protozoa
based on 18S rRNA nucleotide sequencing
and features made visible by electron microscopy. One such scheme classifies
protozoa into six taxa: parabasalids,
diplomonads, euglenozoa, alveolates, rhizaria, and amoebozoa.
Parabasala
Parabasala species lack mitochondria; each has a single nucleus and a
parabasal body, which is a Golgi-like structure. A well-known opportunistic
pathogenic parabasalid is Trichomonas, which can proliferate to cause severe
inflammation of the human vagina.
Diplomonadida
Diplomonadida lack mitochondria, Golgi bodies, and peroxisomes. Diplomonads
have two equal-sized nuclei and
multiple flagella. An example is Giardia, a diarrhea-causing pathogen.
Euglenozoa
The group of euglenozoa called euglenoids consists of photoautotrophic,
unicellular
microbes with chloroplasts and mitochondria with disk-shaped cristae.
Euglenoids store food as a unique polysaccharide called paramylon instead of
as a starch. They are motile by means of flagella and euglenoid movement.
They
have a semi-rigid, proteinaceous, helical pellicle that underlies the
cytoplasmic membrane and helps maintain shape.
The group of euglenozoa called kinetoplastids have a single large
mitochondrion that contains a unique region of
DNA called a kinetoplast. Some, such as Trypanosoma and Leishmania, are
pathogenic.
Copyright © 2014 Pearson Education, Inc.
CHAPTER 12
Characterizing and Classifying Eukaryotes
73
Alveolates
Alveolates have cavities called alveoli beneath their cell surfaces. They
include ciliates, apicomplexans, and dinoflagellates.
Ciliates have cilia and two nuclei. All are chemoheterotrophs. Balantidium is
a parasite of humans, while Paramecium is a well-known pond-water ciliate.
Apicomplexans are all pathogens of animals. The name of this group refers to
the complex of special intracellular
organelles, located at the apices of the infective stages of these microbes,
that enables them to penetrate a host cell.
Plasmodium, the cause of malaria, is an example.
Dinoflagellates are unicellular microbes that have photosynthetic pigments
such as carotene. They make up a
large proportion of freshwater and marine plankton, and some are responsible
for red tides. Toxins produced by
some dinoflagellates can cause serious disease in humans who ingest or even
merely come in contact with them.
Rhizaria
Unicellular eukaryotes called amoebae are protozoa that move and feed by
means of pseudopodia and reproduce via
binary fission. Beyond these common features, amoebae exhibit little
uniformity. Some taxonomists classify amoebae in two kingdoms: Rhizaria, and
Amoebozoa.
Amoebae in the Rhizaria have threadlike pseudopodia. The marine amoebae,
foraminifera, have a porous shell
composed of calcium carbonate. Over 90% of known foraminifera are fossil
species.
Amoebae in the group radiolaria have shells composed of silica. Their
threadlike pseudopodia radiate like spokes
due to a stiff internal structure of microtubules. Radiolarians live in
marine water as part of the floating plankton.
Amoebozoa
Amoebozoa have lobe-shaped pseudopodia and no shells. The normally freeliving amoebae Naegleria and Acanthamoeba can each cause disease of the
brains in humans and animals that swim in water containing them. Entamoeba
causes potentially fatal dysentery.
Another group of amoebozoa, the slime molds, was once considered fungi, but
their
nucleotide sequences show that they are amoebozoa. Slime molds lack cell
walls and are phagocytic in their nutrition. Slime molds can be either
plasmodial slime molds or cellular slime molds. Plasmodial slime molds are
composed of multinucleate filaments of cytoplasm. Cellular slime molds are
composed of myxamoebae that
phagocytize bacteria and yeasts.
Fungi (pp. 357–366)
Fungi are chemoheterotrophic eukaryotes with cell walls that are usually
composed of a strong, flexible, nitrogenous
polysaccharide called chitin. The study of fungi is mycology.
The Significance of Fungi
Fungi decompose dead organisms and recycle their nutrients. Mycorrhizae at
the roots of about 90% of all vascular
plants assist the plants to absorb water and dissolved minerals. Humans
consume edible fungi and use others in the
manufacture of foods, beverages, and pharmaceuticals. Fungi are important
research tools in the study of metabolism, growth, and development. Fungi
produce useful medications. About 30% of fungi produce mycoses, fungal
diseases of plants, animals, and humans.
Morphology of Fungi
The vegetative (nonreproductive) body of a fungus is called its thallus. The
thalli of molds are large and composed
of long, branched, tubular filaments called hyphae. Septate hyphae are
divided into cells by cross walls called septa,
whereas aseptate hyphae are undivided and coenocytic. The thalli of yeasts
are small, globular, and composed of a
single cell. Dimorphic fungi produce both yeastlike and moldlike thalli.The
hyphae of a mold intertwine to form a
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tangled mass called a mycelium, which is typically subterranean. Mushrooms
and other fruiting bodies of molds are
only small visible extensions of a vast underground mycelium.
Nutrition of Fungi
Most fungi are saprobes; that is, they acquire nutrients by absorption from
dead organisms. Other fungi obtain nutrients from living organisms by using
modified hyphae called haustoria that penetrate the host tissue to withdraw
nutrients. Most fungi are aerobic, though many yeasts are facultative
anaerobes that obtain energy from fermentation.
Some fungi may use ionizing radiation as an energy source.
Reproduction of Fungi
Fungi reproduce asexually both by budding and via asexual spores, which are
categorized
according to their mode of development:
 Sporangiospores form inside a sac called a sporangium.
 Chlamydospores form with a thickened cell wall inside hyphae.
 Conidiospores are produced at the tips or sides of hyphae, but not within a
sac.
Most fungi also reproduce sexually via spores. In the process, haploid cells
from parental thalli fuse to form a dikaryon. After several hours to many
years, pairs of nuclei within a
dikaryon fuse; meiosis restores haploid nuclei, and the nuclei are
partitioned into new spores. Fungi vary in ways
their dikaryons form and the site of meiosis.
Classification of Fungi
Taxonomists classify fungi into four major subgroups: division Zygomycota,
division
Ascomycota, division Basidiomycota, and division Deuteromycetes.
Division Zygomycota
Fungi in the division Zygomycota are coenocytic molds called zygomycetes.
Most are
saprobes. They reproduce asexually via sporangiospores. The distinctive
feature of most zygomycetes is the formation of rough-walled sexual
structures called zygosporangia that develop from the fusion of sexually
compatible
hyphae. Following meiosis, one of the four meiotic nuclei survives to produce
a haploid sporangium filled with haploid spores.
Microsporidia are zygomycetes that are obligate intracellular parasites, some
of which are known to cause disease in immunocompromised persons. For example
Nosema are obligatory intracellular parasites of insects.
Division Ascomycota
The division Ascomycota contains about 32,000 known species of molds and
yeasts. Sexual reproduction results in
the formation of haploid ascospores within sacs called asci. Asci occur in
fruiting bodies called ascocarps. They reproduce asexually by conidiospores.
Sexual reproduction produces a dikaryon, in which nuclei fuse to produce
diploids, followed by meiosis to produce haploids within a sac called an
ascus. The haploids undergo mitosis to produce
ascospores. Ascomycetes include most of the fungi that spoil food, as well as
plant pathogens such as the agents of
Dutch elm disease. Others are beneficial in production of pharmaceuticals,
for baking and brewing, and research.
Division Basidiomycota
In the division Basidiomycota are almost 22,000 species of fungi. Mushrooms
and other fruiting bodies of basidiomycetes, called basidiocarps, consist of
tightly woven hyphae that extend into multiple projections called basidia,
the ends of which produce sexual basidiospores. Many basidiomycetes are
toxic, while others are edible. Most basidiomycetes decompose cellulose and
lignin in dead plants and return nutrients to the soil. Many produce
hallucinatory chemicals or toxins. Some are important plant pathogens.
Copyright © 2014 Pearson Education, Inc.
CHAPTER 12
Characterizing and Classifying Eukaryotes
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Deuteromycetes
The deuteromycetes, formerly classified in the division Deuteromycota, are an
informal grouping of fungi having
no known sexual stage. Recently, the analysis of rRNA sequences has revealed
that most deuteromycetes belong in
the division Ascomycota, and thus modern taxonomists have abandoned
Deuteromycota as a formal taxon. The term
continues to have clinical significance.
Lichens
Lichens are economically and environmentally important organisms composed of
fungi
living in partnership with photosynthetic microbes, either cyanobacteria or
green algae. The fungus of a lichen reproduces by spores, which must
germinate and develop into hyphae that capture an appropriate alga or
cyanobacterium. Alternatively, wind, rain, and small animals disperse bits of
lichen called soredia to new locations where they
can establish a new lichen if there is a suitable substrate. Lichens occur in
three basic shapes: foliose are leaflike,
with margins that grow free from the substrate; crustose grow appressed to
their substrates; and fruticose are either
erect or hanging cylinders. Lichens create soil, provide nitrogen, and many
animals eat them. Humans use lichens in
the production of food, dyes, clothing, perfumes, and medicines, as well as
to monitor air quality.
Algae (pp. 367–371)
Algae are simple, eukaryotic, phototrophic organisms that, like plants, carry
out oxygenic photosynthesis using chlorophyll a. They have sexual
reproductive structures in which every cell becomes a gamete. The study of
algae is
called phycology.
Distribution of Algae
Most algae are aquatic, living in the photic zone of fresh, brackish, and
salt bodies of water. Algae have accessory
photosynthetic pigments to trap energy from various wavelengths of light,
resulting in photosynthetic organisms that
are not green.
Morphology of Algae
Algae are either unicellular, colonial, or have simple multicellular bodies
called thalli, which are commonly composed of branched filaments or sheets.
The thalli of large algae (seaweeds) are complex, with branched holdfasts to
anchor them to rocks, stemlike stipes, and leaflike blades. They may be
buoyed in the water by gas-filled bulbs
called pneumocysts. They do not have a vascular system.
Reproduction of Algae
Unicellular algae reproduce asexually or sexually. In unicellular algae, two
cells fuse to form a zygote, which then
undergoes meiosis. Multicellular algae typically reproduce either asexually
by fragmentation or by spore formation.
Many multicellular algae reproduce sexually by an alternation of generations
in which a haploid thallus alternates
with a diploid thallus.
Classification of Algae
The classification of algae is not settled. Historically, taxonomists have
used differences in photosynthetic pigments,
storage products, and cell wall composition to classify algae into several
groups named for the colors of their photosynthetic pigments: Chlorophyta,
Rhodophyta, Chrysophyta, and Phaeophyta.
Division Chlorophyta (Green Algae)
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Chlorophyta are green algae that share numerous characteristics with plants:
they have chlorophylls a and b, they
store sugar and starch as food reserves, and many have cell walls composed of
cellulose whereas others have walls
of protein or lack walls entirely. Their 18S rRNA sequences are comparable.
Most are unicellular or filamentous and
live in freshwater. Trebouxia is the most common algae in lichens.
Kingdom Rhodophyta (Red Algae)
Rhodophyta contain the pigment phycoerythrin, the storage molecule glycogen,
and cell walls of agar or carrageenan, substances used as thickening agents
for the production of microbiological media as well as numerous foods
such as ice cream, syrups, and salad
dressings.
Phaeophyta (Brown Algae)
Phaeophyta produce motile gametes with two types of flagella. They contain
brown pigments called xanthopylls as
well as carotene and chlorophyll a and c. They use the polysaccharide
laminarin and oils as food reserves and have
cell walls composed of cellulose and alginic acid, which is used in numerous
foods as a thickener.
Chrysophyta (Golden Algae, Yellow-Green Algae, and Diatoms)
Chrysophyta are diverse in terms of cell wall composition and pigments, but
all use the polysaccharide chrysolaminarin as a storage product. All
chrysophytes contain more orange-colored carotene than chlorophyll, which
accounts for their more golden coloring.
Diatoms have cell walls made of silica, arranged in nesting halves called
frustules. They are a major component
of phytoplankton and are the major source of the world’s oxygen.
Water Molds (p. 371)
Scientists once classified the microbes commonly known as water molds as
fungi because they resemble filamentous
fungi in having finely branched filaments; however, they are not true molds.
Water molds have tubular cristae in their mitochondria, cell walls of
cellulose, spores with two different kinds of
flagella, and they have diploid bodies. They are placed in the kingdom
Stramenopila along with diatoms, other
chrysophytes, and brown algae. Water molds decompose dead animals and return
nutrients to the environment.
Some are crop pathogens, such as Phytophthora infestans, which devastated the
potato crop in Ireland in the mid19th century, causing a famine that killed
over 1 million people.
Other Eukaryotes of Microbiological Interest: Parasitic
Helminths and Vectors (pp. 372–373)
Microbiologists are interested in two groups of eukaryotes that are not in
fact microorganisms. The first are parasitic
helminths, which are significant because their infective stages are usually
microscopic. The second is arthropod
vectors, animals that carry pathogens and have segmented bodies, hard
external skeletons, and jointed legs. Disease
vectors belong to two classes of arthropods. The first class, Arachnida,
includes ticks and mites. Adult arachnids
have four pairs of legs. Ticks are the most important arachnid vectors. The
second class, Insecta, includes fleas, lice,
flies, mosquitoes, and kissing bugs. Adult insects have three pairs of legs,
and many can fly. Mosquitoes are the
most important arthropod vectors of disease; they carry the pathogens for
diseases such as malaria, yellow fever, and
viral encephalitis.
Copyright © 2014 Pearson Education, Inc.
CHAPTER 12
Characterizing and Classifying Eukaryotes
77
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