CHAPTER
12
Characterizing and
Classifying Eukaryotes
Chapter Outline
General Characteristics of Eukaryotic Organisms (pp. 351–355)
Reproduction of Eukaryotes
Classification of Eukaryotic Organisms
Protozoa (pp. 355–361)
Distribution of Protozoa
Morphology of Protozoa
Fungi (pp. 361–370)
The Significance of Fungi
Morphology of Fungi
Nutrition of Fungi
Reproduction of Fungi
Classification of Fungi
Lichens
Algae (pp. 370–373)
Distribution of Algae
Morphology of Algae
Water Molds (pp. 373–374)
Other Eukaryotes of Microbiological Interest: Parasitic Helminths
and Vectors (pp. 374–376)
Chapter 12 Characterizing and Classifying Eukaryotes
Chapter Summary
General Characteristics of Eukaryotic Organisms (pp. 351-355)
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, 6 are eukaryotic microbes. Our discussion of the general characteristics of eukaryotic
organisms 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
Nuclear Division
Typically, a eukaryotic nucleus has either one or two complete copies of the chromosomal por-
tion 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. There are two
types of nuclear division—mitosis and meiosis.
1. In prophase, the cell’s DNA condenses into visible chromatids, with the two identical DNA
3. In anaphase, sister chromatids separate and move to opposite poles of the spindle as chromo-
4. In telophase, the chromosomes become less compact and the nuclear envelopes form around
the daughter nuclei. Division of the cell contents, cytokinesis, may occur.
2. Late prophase I. Homologous chromosomes exchange DNA in a process called crossing over.
4. Anaphase I. Sister chromatids remain attached to each other and move toward the poles.
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
5. Telophase I. The spindle disintegrates, and cell division usually follows. Note, at this point,
2. Metaphase II. Chromosomes assemble in the middle of the cell and spindle fibers attach.
4. Telophase II. Daughter nuclei form, cell division usually follows to yield four haploid cells.
Thus, four haploid nuclei are produced from a single diploid cell. Overall, meiosis can be
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
Schizogony
Some protozoa, such as Plasmodium, reproduce asexually within red blood cells and liver cells
Classification of Eukaryotic Organisms
The classification of eukaryotic microbes has been difficult and has changed frequently. Histori-
cal schemes based on similarity in morphology and chemistry have been replaced with schemes
Protozoa (pp. 355–361)
Protozoa are a diverse group that are all eukaryotic, unicellular organisms that lack a cell wall.
Protozoology is the scientific study of protozoa.
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. Others live in moist soils. Few are
pathogens.
Chapter 12 Characterizing and Classifying Eukaryotes
Morphology of Protozoa
Protozoa are characterized by great morphologic diversity. Some have cilia, whereas others have
flagella or pseudopodia. Some ciliates have two functionally different nuclei, a macronucleus
Nutrition of Protozoa
Most protozoa are chemoheterotrophic, obtaining nutrients by phagocytizing bacteria, decaying
Reproduction of Protozoa
Most protozoa reproduce asexually only, by binary fission or schizogony; a few protozoa also
Classification of Protozoa
Historically protozoa were classified on the basis of locomotion: Sarcodina (pseudopods), Mas-
zoa, 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,
Diplomonadida
Diplomonadida lack mitochondria, Golgi bodies, and peroxisomes. Diplomonads have two
equal-sized nuclei, multiple flagella, and mitochondria-like mitosomes. An example is Giardia,
a diarrhea-causing pathogen that produces cysts.
Euglenozoa
The group of euglenozoa consists of photoautotrophic, unicellular microbes with chloroplasts
and mitochondria with disk-shaped cristae. 18S rRNA sequences and various structural features
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
means of flagella and euglenoid movement. They have a semi-rigid, proteinaceous, helical pelli-
cle that underlies the cytoplasmic membrane and helps maintain shape. Most euglenids also
Alveolates
Alveolates have cavities called alveoli beneath their cell surfaces. They have tubular mitochon-
dria. They include ciliates, apicomplexans, and dinoflagellates.
Ciliates have cilia for movement and two nuclei. All are chemoheterotrophs. Vorticella uses
its cilia to create a current. Balantidium is a parasite of humans, while Paramecium is a well-
known pond-water ciliate often preyed upon by Didinium.
Rhizaria
Unicellular eukaryotes called amoebae are protozoa that move and feed by means of pseudopo-
dia and reproduce via binary fission. Beyond these common features, amoebae exhibit little uni-
formity. Some taxonomists classify amoebae in two kingdoms: Rhizaria and Amoebozoa.
Amoebozoa
Amoebozoa have lobe-shaped pseudopodia and no shells. The normally free-living amoebae
Naegleria and Acanthamoeba can each cause disease of the brains in humans and animals that
Chapter 12 Characterizing and Classifying Eukaryotes
Fungi (pp. 361–370)
Fungi are chemoheterotrophic eukaryotes with cell walls that are usually composed of a strong,
The Significance of Fungi
Fungi decompose dead organisms and recycle their nutrients. Mycorrhizae associate with the
roots of about 90% of all vascular plants and assist the plants to absorb water and dissolved
minerals. Humans consume edible fungi and use others in the manufacture of foods, beverages,
Morphology of Fungi
The bodies 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 yeasts are small, globular, and composed of a single cell.
Nutrition of Fungi
Fungi obtain nutrients by extracellular digestion and absorption. Most fungi are saprobes; that
is, they acquire nutrients by absorption from dead organisms. Other fungi obtain nutrients from
Reproduction of Fungi
All fungi reproduce asexually by mitosis, and most also reproduce sexually.
Budding and Asexual Spore Formation
Yeasts produce buds after mitosis and form a cell wall to separate parent and daughter nuclei.
Sometimes the daughter cells remain attached and form filaments called pseudohyphae. Can-
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
Sexual Spore Formation
Most fungi also reproduce sexually via spores. Scientists designate fungal mating types as plus
(+) and minus (-) rather than male and female because the bodies are indistinguishable. The
basic steps are:
1. Haploid cells from two parental fungi fuse to form a dikaryon. Dikaryon cells are designated
(n+n). Dikaryotic cells can also reproduce to form (n+n) daughter cells.
3. Meiosis restores haploid nuclei.
Classification of Fungi
Taxonomists classify fungi into four major subgroups: Three of these divisions (divisions are
equivalent to phyla), Zygomycota, Ascomycota, and Basidiomycota are based on the type of sex-
ual spore they produce. A fourth taxon, Deuteromycetes served as a repository for fungi for
which no sexual stage was known.
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
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 repro-
Division Basidiomycota
In the division Basidiomycota are almost 22,000 species of fungi. The mycelium of a
basidiomycete is a dikaryon. Mushrooms and other fruiting bodies of basidiomycetes, called
Chapter 12 Characterizing and Classifying Eukaryotes
basidiocarps, consist of tightly woven hyphae that extend into multiple projections called ba-
sidia, the ends of which undergo meiosis to produce sexual basidiospores. Many basidiomycetes
Deuteromycetes
The deuteromycetes, formerly classified in the division Deuteromycota, are an informal group-
ing of fungi having no known sexual stage. Recently, the analysis of rRNA sequences has
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
Algae (pp. 370–373)
Algae are simple, eukaryotic, phototrophic organisms that, like plants, carry out oxygenic pho-
tosynthesis using chlorophyll a. They have sexual reproductive structures in which every cell
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 unicellular, colonial, or have simple multicellular bodies, which are commonly
composed of branched filaments or sheets. The body of large algae (seaweeds) is complex, with
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
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
Classification of Algae
The classification of algae is not settled. Historically, taxonomists have used differences in pho-
tosynthetic pigments, storage products, and cell wall composition to classify algae into several
groups named for the colors of their photosynthetic pigments: green algae, red algae, brown
algae, golden algae, and yellow-green algae.
Division Chlorophyta (Green Algae)
Chlorophyta are green algae that share numerous characteristics with plants: they have chloro-
phylls a and b, they store sugar and starch as food reserves, and many have cell walls composed
Kingdom Rhodophyta (Red Algae)
Rhodophyta contain the pigment phycoerythrin, the storage molecule glycogen, and cell walls
of agar or carrageenan. They produce nonmotile male gametes called spermatia. Most red
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. The brown algae are primarily
Chrysophyta (Golden Algae, Yellow-Green Algae, and Diatoms)
Chrysophyta are diverse in terms of cell wall composition and pigments, but all use the poly-
saccharide chrysolaminarin as a storage product. All chrysophytes contain more orange-colored
Chapter 12 Characterizing and Classifying Eukaryotes
Water Molds (pp. 373-374)
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
Other Eukaryotes of Microbiological Interest: Parasitic
Helminths and Vectors (pp. 374–376)
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
Arachnids
Adult arachnids have four pairs of legs. Ticks and mites have larval stages with six legs. They
have disk-shaped bodies and are very small. Ticks are the most important arachnid vectors,
spreading bacterial, viral, and protozoan pathogens as they feed on blood. Mites are parasites of
animals and spread some intracellular parasites.
Insects
Adult insects have three pairs of legs and three body parts. Many can fly. Fleas and lice are