28 protists

Chapter 28

Protists

PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece

Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Overview: A World in a Drop of Water
• Even a low-power microscope
– Can reveal an astonishing menagerie of
organisms in a drop of pond water

Figure 28.1

50 µm

• These amazing organisms
– Belong to the diverse kingdoms of mostly
single-celled eukaryotes informally known as
protists

• Advances in eukaryotic systematics
– Have caused the classification of protists to
change significantly


• Concept 28.1: Protists are an extremely
diverse assortment of eukaryotes
• Protists are more diverse than all other
eukaryotes
– And are no longer classified in a single
kingdom

• Most protists are unicellular
– And some are colonial or multicellular


• Protists, the most nutritionally diverse of all
eukaryotes, include
– Photoautotrophs, which contain chloroplasts
– Heterotrophs, which absorb organic molecules
or ingest larger food particles
– Mixotrophs, which combine photosynthesis
and heterotrophic nutrition


• Protist habitats are also diverse in habitat
• And including freshwater and marine
species
(a) The freshwater ciliate Stentor,
a unicellular protozoan (LM)

100 µm
100 µm

4 cm

(b) Ceratium tripos, a unicellular marine dinoflagellate (LM)

(c) Delesseria sanguinea, a multicellular marine red alga
500 µm

Figure 28.2a–d

(d) Spirogyra, a filamentous freshwater green alga (inset LM)


• Reproduction and life cycles
– Are also highly varied among protists, with
both sexual and asexual species


• A sample of protist diversity

Table 28.1

Endosymbiosis in Eukaryotic Evolution
• There is now considerable evidence
– That much of protist diversity has its origins in
endosymbiosis


• The plastid-bearing lineage of protists
– Evolved into red algae and green algae

• On several occasions during eukaryotic
evolution
– Red algae and green algae underwent
secondary endosymbiosis, in which they
themselves were ingested


• Diversity of plastids produced by secondary
endosymbiosis
Plastid

Alveolates

Dinoflagellates

Apicomplexans
Secondary
endosymbiosis

Cyanobacterium

Ciliates

Red algae

Primary
endosymbiosis

Stramenopiles
Heterotrophic
eukaryote

Plastid

Euglenids
Secondary
endosymbiosis

Green algae

Figure 28.3

Chlorarachniophytes

Diplomonadida

Figure 28.4

Cercozoa

Ancestral eukaryote

Plants

Charophyceans

(Opisthokonta)
Chlorophytes

Red algae

Metazoans

Choanoflagellates

Amoebozoa
Fungi

Cellular slime molds

Plasmodial slime molds

Entamoebas

Gymnamoebas

Plantae

Chlorophyta

Rhodophyta

Animalia

Fungi

Radiolarians Radiolaria

Foraminiferans

Stramenopila

Chlorarachniophytes

Brown algae

Golden algae

Diatoms

Oomycetes

Ciliates

Apicomplexans

Euglenozoa

Alveolata

Dinoflagellates

Euglenids

Kinetoplastids

Parabasalids Parabasala

Diplomonads

• Concept 28.2: Diplomonads and parabasalids
have modified mitochondria

• A tentative phylogeny of eukaryotes
– Divides eukaryotes into many clades
(Viridiplantae)

• Diplomonads and parabasalids
– Are adapted to anaerobic environments
– Lack plastids
– Have mitochondria that lack DNA, an electron
transport chain, or citric-acid cycle enzymes


Diplomonads
• Diplomonads
– Have two nuclei and multiple flagella

Figure 28.5a

(a) Giardia intestinalis, a diplomonad (colorized SEM)


5 µm

Parabasalids
• Parabasalids include trichomonads
– Which move by means of flagella and an
undulating part of the plasma membrane
Flagella

Undulating membrane
5 µm
Figure 28.5b (b) Trichomonas vaginalis, a parabasalid (colorized SEM)

• Concept 28.3: Euglenozoans have flagella with
a unique internal structure
• Euglenozoa is a diverse clade that includes
– Predatory heterotrophs, photosynthetic
autotrophs, and pathogenic parasites


• The main feature that distinguishes protists in
this clade
– Is the presence of a spiral or crystalline rod of
unknown function inside their flagella
Flagella

0.2 µm

Crystalline rod

Figure 28.6

Ring of microtubules

Kinetoplastids
• Kinetoplastids
– Have a single, large mitochondrion that
contains an organized mass of DNA called a
kinetoplast
– Include free-living consumers of bacteria in
freshwater, marine, and moist terrestrial
ecosystems


• The parasitic kinetoplastid Trypanosoma
– Causes sleeping sickness in humans

Figure 28.7

9 µm

Euglenids
• Euglenids
– Have one or two flagella that emerge from a
pocket at one end of the cell
– Store the glucose polymer paramylon
Long flagellum

Eyespot: pigmented
organelle that functions
as a light shield, allowing
light from only a certain
direction to strike the
light detector

Light detector: swelling near the
base of the long flagellum; detects
light that is not blocked by the
eyespot; as a result, Euglena moves
toward light of appropriate
intensity, an important adaptation
that enhances photosynthesis

Short flagellum
Euglena (LM)

Nucleus

Contractile vacuole

5 µm
Plasma membrane

Figure 28.8

Pellicle: protein bands beneath
the plasma membrane that
provide strength and flexibility
(Euglena lacks a cell wall)


Chloroplast
Paramylon granule

• Concept 28.4: Alveolates have sacs beneath
the plasma membrane
• Members of the clade Alveolata
– Have membrane-bounded sacs (alveoli) just
under the plasma membrane
0.2 µm

Figure 28.9

Flagellum

Alveoli

Dinoflagellates
• Dinoflagellates
– Are a diverse group of aquatic photoautotrophs
and heterotrophs
– Are abundant components of both marine and
freshwater phytoplankton


• Each has a characteristic shape
– That in many species is reinforced by internal
plates of cellulose

• Two flagella
– Make them spin as they move through the
water

Figure 28.10

3 µm

Flagella

• Rapid growth of some dinoflagellates
– Is responsible for causing “red tides,” which
can be toxic to humans


Apicomplexans
• Apicomplexans
– Are parasites of animals and some cause
serious human diseases
– Are so named because one end, the apex,
contains a complex of organelles specialized
for penetrating host cells and tissues
– Have a nonphotosynthetic plastid, the
apicoplast


• Most apicomplexans have intricate life cycles
– With both sexual and asexual stages that often require
two or more different host species for completion
2 The sporozoites enter the person’s
liver cells. After several days, the sporozoites
undergo multiple divisions and become
merozoites, which use their apical complex
to penetrate red blood cells (see TEM below).

1 An infected Anopheles
mosquito bites a person,
injecting Plasmodium
sporozoites in its saliva.

Inside mosquito

Inside human

Sporozoites
(n)
7 An oocyst develops
from the zygote in the wall
of the mosquito’s gut. The
oocyst releases thousands
of sporozoites, which
migrate to the mosquito’s
salivary gland.

Merozoite

Liver

Liver cell

Apex

Oocyst
MEIOSIS
Zygote
(2n)

Red blood
cell

Merozoite
(n)
Red blood
cells

FERTILIZATION
Gametes
Key

3 The merozoites divide asexually inside the
red blood cells. At intervals of 48 or 72 hours
(depending on the species), large numbers of
merozoites break out of the blood cells, causing
periodic chills and fever. Some of the merozoites
infect new red blood cells.

Gametocytes
(n)

Haploid (n)
Diploid (2n)

Figure 28.11

0.5 µm

4 Some merozoites
form gametocytes.
6 Gametes form from gametocytes.
Fertilization occurs in the mosquito’s
digestive tract, and a zygote forms.
The zygote is the only diploid stage
in the life cycle.


5 Another Anopheles mosquito
bites the infected person and picks
up Plasmodium gametocytes along
with blood.

Ciliates
• Ciliates, a large varied group of protists
– Are named for their use of cilia to move and
feed
– Have large macronuclei and small micronuclei


• The micronuclei
– Function during conjugation, a sexual process
that produces genetic variation

• Conjugation is separate from reproduction
– Which generally occurs by binary fission


• Exploring structure and function in a ciliate
FEEDING, WASTE REMOVAL, AND WATER BALANCE
Paramecium, like other freshwater
protists, constantly takes in water
by osmosis from the hypotonic environment.
Bladderlike contractile vacuoles accumulate
excess water from radial canals and periodically
expel it through the plasma membrane.

Contractile Vacuole

Paramecium feeds mainly on bacteria.
Rows of cilia along a funnel-shaped oral
groove move food into the cell mouth,
where the food is engulfed into food
vacuoles by phagocytosis.
Oral groove
Cell mouth

50 µm

Thousands of cilia cover
the surface of Paramecium.
Micronucleus

Food vacuoles combine with
lysosomes. As the food is digested,
the vacuoles follow a looping path
through the cell.

Macronucleus

Figure 28.12


The undigested contents of food
vacuoles are released when the
vacuoles fuse with a specialized
region of the plasma membrane
that functions as an anal pore.

CONJUGATION AND REPRODUCTION
1 Two cells of compatible
mating strains align side
by side and partially fuse.

2 Meiosis of micronuclei
produces four haploid
micronuclei in each cell.

3 Three micronuclei in each cell
disintegrate. The remaining micronucleus in each cell divides by mitosis.

MEIOSIS
4 The cells swap
one micronucleus.

Macronucleus

Compatible
mates

Haploid
micronucleus

Diploid
micronucleus
Diploid
micronucleus

MICRONUCLEAR
FUSION

5

9 Two rounds of cytokinesis
partition one macronucleus
and one micronucleus
into each of four daughter cells.

8 The original macronucleus disintegrates.
Four micronuclei
become macronuclei,
while the other four
remain micronuclei.

7 Three rounds of
mitosis without
cytokinesis
produce eight
micronuclei.


6 Micronuclei fuse,
forming a diploid
micronucleus.

The cells
separate.

Key
Conjugation
Reproduction

• Concept 28.5: Stramenopiles have “hairy” and
smooth flagella
• The clade Stramenopila
– Includes several groups of heterotrophs as
well as certain groups of algae


• Most stramenopiles
– Have a “hairy” flagellum paired with a “smooth”
flagellum

Hairy
flagellum
Smooth
flagellum

Figure 28.13

5 µm


Oomycetes (Water Molds and Their Relatives)
• Oomycetes
– Include water molds, white rusts, and downy
mildews
– Were once considered fungi based on
morphological studies


• Most oomycetes
– Are decomposers or parasites
– Have filaments (hyphae) that facilitate nutrient
uptake


• The life cycle of a water mold
1 Encysted zoospores
land on a substrate and
germinate, growing into
a tufted body of hyphae.

2 Several days later,
the hyphae begin to
form sexual structures.

3 Meiosis produces
eggs within oogonia
(singular, oogonium).

Germ tube
Cyst
9 Each zoosporangium produces
about 30
biflagellated
zoospores
asexually.

MEIOSIS
ASEXUAL
REPRODUCTION
Zoospore
(2n)

8 The ends
of hyphae
form tubular
zoosporangia.

Zoosporangium
(2n)

4 On separate branches of the
same or different individuals, meiosis
produces several haploid sperm nuclei
contained within antheridial hyphae.
Oogonium
Egg nucleus
(n) Antheridial
hypha with
sperm nuclei
(n)

SEXUAL
FERTILIZATION
Zygote
REPRODUCTION
germination
Zygotes
(oospores)
(2n)

Key
Haploid (n)
Diploid (2n)

7 The zygotes germinate
and form hyphae, and the
cycle is completed.

Figure 28.14

6 A dormant period
follows, during which the
oogonium wall usually
disintegrates.

5 Antheridial hyphae grow like
hooks around the oogonium and
deposit their nuclei through
fertilization tubes that lead to the
eggs. Following fertilization, the
zygotes (oospores) may develop
resistant walls but are also
protected within the wall of the
oogonium.

• The ecological impact of oomycetes can be
significant
– Phytophthora infestans causes late blight of
potatoes


Diatoms
• Diatoms are unicellular algae

Figure 28.15

3 µm

– With a unique two-part, glass-like wall of
hydrated silica

• Diatoms are a major component of
phytoplankton
– And are highly diverse

Figure 28.16

50 µm

• Accumulations of fossilized diatom walls
– Compose much of the sediments known as
diatomaceous earth


Golden Algae
• Golden algae, or chrysophytes
– Are named for their color, which results from
their yellow and brown carotenoids

• The cells of golden algae
– Are typically biflagellated, with both flagella
attached near one end of the cell


• Most golden algae are unicellular
– But some are colonial
25 µm

Figure 28.17

Brown Algae
• Brown algae, or phaeophytes
– Are the largest and most complex algae
– Are all multicellular, and most are marine


• Brown algae
– Include many of the species commonly called
seaweeds

• Seaweeds
– Have the most complex multicellular anatomy
of all algae
Blade

Stipe

Figure 28.18

Holdfast

• Kelps, or giant seaweeds
– Live in deep parts of the ocean

Figure 28.19

Human Uses of Seaweeds
• Many seaweeds
– Are important commodities for humans
– Are harvested for food
(a) The seaweed is
grown on nets in
shallow coastal
waters.

(b) A worker spreads
the harvested seaweed on bamboo
screens to dry.

Figure 28.20a–c

(c) Paper-thin, glossy sheets
of nori make a mineral-rich wrap
for rice, seafood, and vegetables
in sushi.

Alternation of Generations
• A variety of life cycles
– Have evolved among the multicellular algae

• The most complex life cycles include an
alternation of generations
– The alternation of multicellular haploid and
diploid forms


• The life cycle of the brown alga Laminaria
1 The sporophytes of this seaweed
are usually found in water just below
the line of the lowest tides, attached
to rocks by branching holdfasts.
2 In early spring, at the end of
the main growing season, cells on
the surface of the blade develop
into sporangia.

Sporangia

3 Sporangia produce
zoospores by meiosis.
Sporophyte
(2n)
7 The zygotes
grow into new
sporophytes,
starting life
attached to
the remains of
the female
gametophyte.

Developing
sporophyte

MEIOSIS
Zoospores

Female
Gametophytes
(n)

Zygote
(2n)
Egg
Mature female
gametophyte
(n)
Key

Figure 28.21

Male

FERTILIZATION

Haploid (n)
Diploid (2n)


Sperm
6 Sperm fertilize
the eggs.

4 The zoospores are all
structurally alike, but
about half of them develop
into male gametophytes
and half into female
gametophytes. The
gametophytes look
nothing like the sporophytes, being short,
branched filaments that
grow on the surface of
subtidal rocks.
5 Male gametophytes release
sperm, and female gametophytes
produce eggs, which remain
attached to the female gametophyte. Eggs secrete a chemical
signal that attracts sperm of the
same species, thereby increasing
the probability of fertilization in
the ocean.

• Concept 28.6: Cercozoans and radiolarians
have threadlike pseudopodia
• A newly recognized clade, Cercozoa
– Contains a diversity of species that are among
the organisms referred to as amoebas

• Amoebas were formerly defined as protists
– That move and feed by means of pseudopodia

• Cercozoans are distinguished from most other
amoebas
– By their threadlike pseudopodia

Foraminiferans (Forams)
• Foraminiferans, or forams
– Are named for their porous, generally
multichambered shells, called tests
20 µm

Figure 28.22

• Pseudopodia extend through the pores in the
test
• Foram tests in marine sediments
– Form an extensive fossil record


Radiolarians
• Radiolarians are marine protists
– Whose tests are fused into one delicate piece,
which is generally made of silica
– That phagocytose microorganisms with their
pseudopodia


• The pseudopodia of radiolarians, known as
axopodia
– Radiate from the central body

Axopodia
Figure 28.23

200 µm

• Concept 28.7: Amoebozoans have lobeshaped pseudopodia
• Amoebozoans
– Are amoeba that have lobe-shaped, rather
than threadlike, pseudopodia
– Include gymnamoebas, entamoebas, and
slime molds


Gymnamoebas
• Gymnamoebas
– Are common unicellular amoebozoans in soil
as well as freshwater and marine
environments


• Most gymnamoebas are heterotrophic
– And actively seek and consume bacteria and
other protists
Pseudopodia
40 µm

Figure 28.24

Entamoebas
• Entamoebas
– Are parasites of vertebrates and some
invertebrates

• Entamoeba histolytica
– Causes amebic dysentery in humans


Slime Molds
• Slime molds, or mycetozoans
– Were once thought to be fungi

• Molecular systematics
– Places slime molds in the clade Amoebozoa


Plasmodial Slime Molds
• Many species of plasmodial slime molds
– Are brightly pigmented, usually yellow or
orange
4 cm

Figure 28.25

• At one point in the life cycle
– They form a mass called a plasmodium
1 The feeding stage
is a multinucleate
plasmodium that lives
on organic refuse.

2 The plasmodium
takes a weblike form.

Feeding
plasmodium
Zygote
(2n)

3 The plasmodium erects
stalked fruiting bodies (sporangia)
when conditions become harsh.

Mature
plasmodium
(preparing to fruit)
Young
sporangium

SYNGAMY

1 mm
Mature
sporangium

Amoeboid cells
(n)

Flagellated cells
(n)

Figure 28.26

7 The cells unite
in pairs (flagellated
with flagellated
and amoeboid with
amoeboid), forming
diploid zygotes.

6 These cells are
either amoeboid or
flagellated; the two
forms readily convert
from one to the other.


Germinating
spore

Spores
(n)

Key

MEIOSIS

Haploid (n)
Diploid (2n)
Stalk

5 The resistant spores disperse
through the air to new locations
and germinate, becoming active
haploid cells when conditions
are favorable.

4

Within the bulbous
tips of the sporangia,
meiosis produces haploid
spores.

• The plasmodium
– Is undivided by membranes and contains
many diploid nuclei
– Extends pseudopodia through decomposing
material, engulfing food by phagocytosis


Cellular Slime Molds
• Cellular slime molds form multicellular
aggregates
– In which the cells remain separated by their
membranes


• The life cycle of Dictyostelium, a cellular slime
mold
1 In the feeding
9 In a favorable
environment, amoebas
stage of the life
emerge from the spore
cycle, solitary haploid
coats and begin feeding. amoebas engulf bacteria.

8 Spores
are released.

2 During sexual reproduction, two haploid
amoebas fuse and
form a zygote.

SYNGAMY

7 Other
cells crawl
up the stalk
and develop
into spores.

Emerging
Spores amoeba
(n)
Solitary amoebas
(feeding stage)

600 µm

Zygote
(2n)

SEXUAL
REPRODUCTION
MEIOSIS
Amoebas

Fruiting
bodies

ASEXUAL
REPRODUCTION

4 The resistant
wall ruptures,
releasing new
haploid amoebas.

Aggregated
amoebas

5 When food is depleted,
hundreds of amoebas
congregate in response to a
chemical attractant and form
a sluglike aggregate (photo
below left). Aggregate
formation is the beginning
of asexual reproduction.

Migrating
aggregate

6 The aggregate migrates for a
while and then stops. Some of the
cells dry up after forming a stalk that
supports an asexual fruiting body.

Figure 28.27

3 The zygote
becomes a giant
cell (not shown)
by consuming
haploid amoebas.
After developing a
resistant wall, the
giant cell undergoes
meiosis followed by
several mitotic
divisions.

Key
200 µm

Haploid (n)
Diploid (2n)

• Dictyostelium discoideum
– Has become an experimental model for
studying the evolution of multicellularity


• Concept 28.8: Red algae and green algae are
the closest relatives of land plants
• Over a billion years ago, a heterotrophic protist
acquired a cyanobacterial endosymbiont
– And the photosynthetic descendants of this
ancient protist evolved into red algae and
green algae


Red Algae
• Red algae are reddish in color
– Due to an accessory pigment call
phycoerythrin, which masks the green of
chlorophyll


• Red algae
– Are usually multicellular; the largest are
seaweeds
– Are the most abundant large algae in coastal
waters of the tropics
(b) Dulse (Palmaria palmata). This edible
species has a “leafy” form.

(c) A coralline alga. The cell walls of
coralline algae are hardened by calcium
carbonate. Some coralline algae are
members of the biological communities
around coral reefs.

(a) Bonnemaisonia hamifera. This red alga

Figure 28.28a–c has a filamentous form.


Green Algae
• Green algae
– Are named for their grass-green chloroplasts
– Are divided into two main groups: chlorophytes
and charophyceans
– Are closely related to land plants


• Most chlorophytes
– Live in fresh water, although many are marine

• Other chlorophytes
– Live in damp soil, as symbionts in lichens, or in
snow

Figure 28.29

• Chlorophytes include
– Unicellular, colonial, and multicellular forms
20 µm

50 µm

(a) Volvox, a colonial freshwater chlorophyte. The colony is a hollow
ball whose wall is composed of hundreds or thousands of
biflagellated cells (see inset LM) embedded in a gelatinous
matrix. The cells are usually connected by strands of cytoplasm;
if isolated, these cells cannot reproduce. The large colonies seen
here will eventually release the small “daughter” colonies within
them (LM).

(b) Caulerpa, an intertidal chlorophyte.
The branched filaments lack cross-walls
and thus are multinucleate. In effect,
the thallus is one
huge “supercell.”

Figure 28.30a–c

(c) Ulva, or sea lettuce. This edible seaweed has a multicellular
thallus differentiated into leaflike blades and a rootlike holdfast
that anchors the alga against turbulent waves and tides.


• Most chlorophytes have complex life cycles
– With both sexual and asexual reproductive
stages
7 These daughter cells develop flagella
and cell walls and then emerge as
swimming zoospores from the wall of
the parent cell that had enclosed them.
The zoospores grow into mature haploid
cells, completing the asexual life cycle.

Flagella

1 In Chlamydomonas,
mature cells are haploid and
contain a single cup-shaped
chloroplast (see TEM at left).

2 In response to a
shortage of nutrients, drying
of the pond, or some other
stress, cells develop into gametes.

3 Gametes of opposite
mating types (designated
+ and –) pair off and
cling together. Fusion of
the gametes (syngamy)
forms a diploid zygote.

−

1 µm

Cell wall

+

Nucleus

+
Zoospores
ASEXUAL
REPRODUCTION

Regions
of single
chloroplast

Mature cell
(n)
SEXUAL
REPRODUCTION

+
Key
Haploid (n)
Diploid (2n)

+

6 When a mature cell reproduces asexually, it resorbs its
flagella and then undergoes two
rounds of mitosis, forming four
cells (more in some species).

Figure 28.31

−
SYNGAMY

Zygote
(2n)

−
MEIOSIS

4 The zygote secretes
a durable coat that
protects the cell against
harsh conditions.

5 After a dormant period, meiosis
produces four haploid individuals (two
of each mating type) that emerge from
the coat and develop into mature cells.

28 protists

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