Origin and Diversification of Eukaryotes

CAMPBELL BIOLOGY IN FOCUS
© 2014 Pearson Education, Inc.
Urry • Cain • Wasserman • Minorsky • Jackson • Reece
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
25
The Origin and
Diversification
of Eukaryotes

Overview: Shape Changers
 Protist is the informal name of the diverse group
of mostly unicellular eukaryotes
 Some protists, like the ciliate Didinium, are able
to perform dramatic shape changes due to the
structural complexity of their cells

Figure 25.1

Concept 25.1: Eukaryotes arose by endosymbiosis
more than 1.8 billion years ago
 Early eukaryotes were unicellular
 Eukaryotic cells have organelles and are structurally
more complex than prokaryotic cells
 A well-developed cytoskeleton enables eukaryotic
cells to have asymmetrical forms and to change
shape

The Fossil Record of Early Eukaryotes
 Chemical evidence for the presence of eukaryotes
dates back to 2.7 billion years ago
 The earliest fossils of eukaryotic cells are 1.8
billion years old

 Initial diversification of eukaryotes occurred 1.8 to 1.3
billion years ago
 Novel features of eukaryotes, including complex
multicellularity, sexual life cycles, and photosynthesis,
arose 1.3 billion to 635 million years ago
 The first large, multicellular eukaryotes represented
by the Ediacaran biota evolved 635 to 535 million
years ago

Figure 25.2
4.5 bya 3.5 1.52.5 500 mya
1.8 bya 1.5 bya 1.3 bya 1.2 bya
20 µm
20µm
25 µm 25 µm
(a) A 1.8-billion-
year-old fossil
eukaryote
(b) Tappania, a
1.5-billion-year-old
fossil that may
represent an early
alga or fungus
(c) Bangiomorpha,
an ancient red
alga
20µm
(d) Proterocladus,
classified as a
green alga
(e) Bonneia, a
vase-shaped
eukaryote
(f) An early
member of
the Ediacaran
biota
(g) Spriggina
floundersi,
an early
animal with
many body
segments
750 mya 635 mya 600 mya
550
mya
535
mya
1 cm
0.5 cm

Figure 25.2a
4.5 bya 3.5 1.52.5 500 mya
1.8 bya 1.5 bya
20 µm
20µm
(a) A 1.8-billion-
year-old fossil
eukaryote
(b) Tappania, a
1.5-billion-year-old
fossil that may
represent an early
alga or fungus
Initial diversification

Figure 25.2aa
20 µm
(a) A 1.8-billion-year-old
fossil eukaryote

Figure 25.2ab
(b) Tappania, a 1.5-billion-year-old
fossil that may represent an
early alga or fungus
20µm

Figure 25.2b
4.5 bya 3.5 1.52.5 500 mya
Appearance of novel features
1.3 bya 1.2 bya 750 mya
20µm
25 µm 25 µm
(c) Bangiomorpha,
an ancient red
alga
(d) Proterocladus,
classified as a
green alga
(e) Bonneia, a
vase-shaped
eukaryote

Figure 25.2ba
25 µm
(c) Bangiomorpha, an
ancient red alga

Figure 25.2bb
25 µm
(d) Proterocladus,
classified as a
green alga

Figure 25.2bc
20µm
(e) Bonneia, a vase-shaped
eukaryote

Figure 25.2c
4.5 bya 3.5 1.52.5 500 mya
Rise of large eukaryotes
635 mya 600 mya
550
mya
535
mya
1 cm
0.5 cm
(f) An early
member of
the Ediacaran
biota
(g) Spriggina
floundersi,
an early
animal with
many body
segments

Figure 25.2ca
1 cm
(f) An early member of
the Ediacaran biota

Figure 25.2cb
0.5 cm
(g) Spriggina floundersi,
an early animal with
many body segments

 Eukaryotic cells exhibited greater structural
complexity than prokaryotic cells as early as 1.5
billion years ago
 The earliest multicellular eukaryotic fossils are of red
algae, which date back to 1.2 billion years ago, but
large, multicellular eukaryotes did not arise until 635
million years ago
 Severe ice ages from 750 to 580 million years ago
may have hindered the rise of large eukaryotes

Endosymbiosis in Eukaryotic Evolution
 DNA sequence data indicate that eukaryotes are
“combination” organisms
 Eukaryotic genes and characteristics show evidence
of both archaeal and bacterial origins
 Evidence of mixed origins may be a consequence
of endosymbiosis, a symbiotic relationship in which
one organism lives inside the body or cell of another
organism

Table 25.1

Origin of Mitochondria and Plastids
 Endosymbiont theory proposes that mitochondria
and plastids were formerly small prokaryotes that
began living within larger cells
 An endosymbiont is a cell that lives within a host cell
 Prokaryote ancestors to mitochondria and plastids
probably entered the host cell as undigested prey or
internal parasites

 The relationship between endosymbiont and host
cells was mutually beneficial
 Anaerobic host cells benefited from endosymbionts’
ability to take advantage of an increasingly aerobic
world
 Heterotrophic host cells benefited from the nutrients
produced by photosynthetic endosymbionts
 In the process of becoming more interdependent, the
host and endosymbionts would have become a
single organism

 Serial endosymbiosis supposes that mitochondria
evolved before plastids through a sequence of
endosymbiotic events

Figure 25.3
Cytoplasm
DNA
Nucleus
Engulfing
of aerobic
bacterium
Engulfing
of photo-
synthetic
bacterium
Mitochondrion
Mito-
chondrion
Plastid
Plasma
membrane
Endoplasmic
reticulum
Nuclear
envelope
Ancestral
prokaryote
Ancestral
heterotrophic
eukaryote
Ancestral
photosynthetic
eukaryote

 Key evidence supporting an endosymbiotic origin of
mitochondria and plastids:
 Inner membranes are similar to plasma membranes of
prokaryotes
 Division is similar in these organelles and some
prokaryotes
 DNA structure is similar to that of prokaryotes
 These organelles transcribe and translate their own
DNA
 Their ribosomes are more similar to prokaryotic than
eukaryotic ribosomes

 DNA sequence analysis indicates that mitochondria
arose from an alpha proteobacterium
 Eukaryotic mitochondria descended from a single
common ancestor
 Plastids arose from an engulfed cyanobacterium
 Some photosynthetic protists may have been
engulfed to become endosymbionts themselves

 The ancestral host cell may have been an archaean
or a “protoeukaryote,” from a lineage related to, but
diverged from, archaeal ancestors

Plastid Evolution: A Closer Look
 There is now considerable evidence that much
protist diversity has its origins in endosymbiosis
 Mitochondria arose first through descent from a
bacterium that was engulfed by a cell from an
archaeal lineage
 The plastid lineage evolved later from a
photosynthetic cyanobacterium that was engulfed
by a heterotrophic eukaryote

Figure 25.4
Cyano-
bacterium
Membranes
are represented
as dark lines in
the cell.
Red alga
Primary
endo-
symbiosis
1 2
3
Nucleus
Heterotrophic
eukaryote
One of these
membranes
was lost in
red and
green algal
descendants.
Green
alga
Secondary
endo-
symbiosis
Secondary
endo-
symbiosis
Plastid
Euglenids
Chlorarachniophytes
Stramenopiles
Plastid
Secondary
endo-
symbiosis
Dinoflagellates

Figure 25.4a
Cyano-
bacterium
Membranes
are represented
as dark lines in
the cell. Red alga
Primary
endosymbiosis
1 2 3
Nucleus
Heterotrophic
eukaryote
One of these
membranes
was lost in
red and
green algal
descendants.
Green alga

Figure 25.4b
Secondary endosymbiosis
Red alga
Plastid
Dinoflagellates
Stramenopiles

Figure 25.4c
Secondary endosymbiosis
Plastid
Euglenids
Green
alga
Chlorarachniophytes

 The plastid-bearing lineage of protists evolved into
red and green algae
 Like cyanobacteria, plastids of red algae and green
algae have two membranes
 Transport proteins in the membranes of red and
green algae are homologous to those found in
cyanobacteria
 On several occasions during eukaryotic evolution,
red and green algae underwent secondary
endosymbiosis, in which they were ingested by a
heterotrophic eukaryote

Concept 25.2: Multicellularity has originated
several times in eukaryotes
 The evolution of eukaryotic cells allowed for a
greater range of unicellular forms
 A second wave of diversification occurred when
multicellularity evolved and gave rise to algae,
plants, fungi, and animals

Multicellular Colonies
 The first multicellular forms were colonies, collections
of cells that are connected to one another but show
little or no cellular differentiation
 Multicellular colonies consist of simple filaments,
balls, or cell sheets
 Colonial cells may be attached by shared cell walls
or, in cells that lack rigid walls, held together by
proteins that physically connect adjacent cells

Figure 25.5
Pediastrum

Independent Origins of Complex Multicellularity
 Multicellular organisms with differentiated cells
originated multiple times over the course of
eukaryotic evolution
 Genetic and morphological evidence indicates that
lineages of red, green, and brown algae, plants,
fungi, and animals arose independently from different
single-celled ancestors

 Volvox is a multicellular green algae that forms a
monophyletic group with a single-celled alga
(Chlamydomonas) and several colonial species
 Volvox and all colonial members of this group have
proteins homologous to those found in the cell wall
of Chlamydomonas
 Multicellularity in Volvox may have originated
through evolution of increasingly complex colonial
forms descended from a single-celled common
ancestor
Video: Chlamydomonas

Figure 25.6
Flagellum
Cytoplasm
Outer cell wall
Inner cell wall
Outer cell wall
Cytoplasm
Extracellular matrix (ECM)
Chlamydomonas
Gonium
Pandorina
Volvox

 Only a few novel genes account for morphological
differences between Volvox and Chlamydomonas
 The transition to multicellularity may result from
changes in how existing genes are used rather
than the origin of large numbers of novel genes

Steps in the Origin of Multicellular Animals
 Choanoflagellates are the closest living relatives of
animals
 The common ancestor of choanoflagellates and
living animals may have resembled present-day
choanoflagellates

Figure 25.7
Individual
choanoflagellate
Choano-
flagellates
Other
animals
Collar cell
(choanocyte)
Sponges
Animals
OTHER
EUKARY-
OTES

 The origin of multicellularity in animals required
mechanisms for cells to adhere to each other and to
communicate with each other (cell signaling)
 Molecular similarities in domains of proteins
functioning in cell adherence (cadherins) and cell
signaling have been found between modern
choanoflagellates and representative animals

Figure 25.8
Hydra
Mouse
“CCD” domain
Choano-
flagellate
Fruit
fly

 The transition to multicellularity in animals involved
new ways of using proteins encoded by genes
found in choanoflagellates rather than the evolution
of many novel genes

Concept 25.3: Four “supergroups” of eukaryotes
have been proposed based on morphological and
molecular data
 Eukaryote diversity is influenced by their hybrid
origins and the independent origins of complex
multicellularity
 One hypothesis divides all eukaryotes into four
supergroups

Four Supergroups of Eukaryotes
 It is no longer thought that amitochondriates (lacking
mitochondria) are the oldest lineage of eukaryotes
 Many have been shown to have mitochondria and
have been reclassified
 Our understanding of the relationships among protist
groups continues to change rapidly
 One hypothesis divides all eukaryotes (including
protists) into four supergroups

Video: Amoeba Pseudopodia
Video: Volvox Colony
Video: Amoeba
Video: Volvox Female Spheroid
Video: Volvox Flagella
Video: Volvox Daughter
Video: Volvox Inversion 1
Video: Volvox Sperm
Video: Volvox Inversion 2

Figure 25.9
Stramen-
opiles Diplomonads
Parabasalids
Euglenozoans
Excavata
Alveo-
lates
Rhiza-
rians
Diatoms
Brown algae
Dinoflagellates
Apicomplexans
Ciliates
Forams
Cercozoans
“SAR”clade
Green
algae
Amoebo-
zoans
Red algae
Chlorophytes
Charophytes
Land plants
Archaeplastida
Opisthokonts
Unikonta
Gymnamoebas
Slime molds
Nucleariids
Fungi
Choanoflagellates
Animals
100 µm
50 µm
“SAR” Clade
Excavata 5 µm Archaeplastida
20 µm
50 µm
Unikonta
100 µm

Figure 25.9a
Stramen-
opiles
Diplomonads
Parabasalids
Euglenozoans
Excavata
Alveo-
lates
Rhiza-
rians
Diatoms
Brown algae
Dinoflagellates
Apicomplexans
Ciliates
Forams
Cercozoans
“SAR”clade
Green
algae
Amoebo-
zoans
Red algae
Chlorophytes
Charophytes
Land plants
Archaeplastida
Opisthokonts
Unikonta
Gymnamoebas
Slime molds
Nucleariids
Fungi
Choanoflagellates
Animals

Figure 25.9aa
Stramen-
opiles
Diplomonads
Parabasalids
Euglenozoans
Excavata
Alveo-
lates
Rhiza-
rians
Diatoms
Brown algae
Dinoflagellates
Apicomplexans
Ciliates
Forams
Cercozoans
“SAR”clade

Figure 25.9ab
Green
algae
Amoebo-
zoans
Red algae
Chlorophytes
Charophytes
Land plants
Archaeplastida
Opisthokonts
Unikonta
Gymnamoebas
Slime molds
Nucleariids
Fungi
Choanoflagellates
Animals

Figure 25.9b
Excavata “SAR” Clade Archaeplastida
50 µm
50 µm
20 µm
5 µm
Unikonta
100 µm
100 µm

Figure 25.9ba
Giardia intestinalis, a
diplomonad parasite
5 µm

Figure 25.9bb
Diatom diversity
50 µm

Figure 25.9bc
Globigerina, a rhizarian in the SAR supergroup
100 µm

Figure 25.9bca
Globigerina, a rhizarian in the SAR supergroup
100 µm

Figure 25.9bcb

Figure 25.9bd
Volvox, a multicellular freshwater green
alga
20 µm
50 µm

Figure 25.9bda
Volvox, a multicellular freshwater green
alga
50 µm

Figure 25.9bdb
20 µm

Figure 25.9be
100 µm
A unikont amoeba

Excavates
 The supergroup Excavata is characterized by its
cytoskeleton
 Some members have a feeding groove
 This monophyletic group includes the diplomonads,
parabasalids, and euglenozoans

Figure 25.UN02
Diplomonads
Parabasalids
Euglenozoans
Excavata
SAR clade
Archaeplastida
Unikonta

Diplomonads and Parabasalids
 These two groups lack plastids and have modified
mitochondria, and most live in anaerobic
environments
 Diplomonads
 Have reduced mitochondria called mitosomes
 Derive energy from anaerobic biochemical pathways
 Are often parasites, for example, Giardia intestinalis
 Move using multiple flagella

 Parabasalids
 Have reduced mitochondria called hydrogenosomes
that generate some energy anaerobically
 Include Trichomonas vaginalis, a sexually transmitted
parasite

Figure 25.10
The parabasalid parasite, Trichomonas vaginalis
(colorized SEM)
Undulating
membrane
Flagella
5µm

Euglenozoans
 Euglenozoa is a diverse clade that includes
predatory heterotrophs, photosynthetic autotrophs,
and parasites
 The main feature distinguishing them as a clade is a
spiral or crystalline rod inside their flagella
 This clade includes the euglenids and kinetoplastids
Video: Euglena Motion
Video: Euglena

Figure 25.11
Euglenozoan flagellum
Ring of microtubules
(cross section)
Crystalline rod
(cross section)
8 µm
0.2 µm
Flagella

Figure 25.11a
Ring of microtubules
(cross section)
Crystalline rod
(cross section)
0.2 µm

 Euglenids have one or two flagella that emerge from
a pocket at one end of the cell
 Some species can be both autotrophic and
heterotrophic

 Kinetoplastids have a single mitochondrion with an
organized mass of DNA called a kinetoplast
 They include free-living consumers of prokaryotes
in aquatic ecosystems and parasites of animals,
plants, and other protists
 This group includes Trypanosoma, which causes
sleeping sickness in humans

Figure 25.12
Trypanosoma, the kinetoplastid that causes sleeping
sickness
9 µm

The “SAR” Clade
 The “SAR” clade is a diverse monophyletic
supergroup named for the first letters of its three
major clades, stramenopiles, alveolates, and
rhizarians

Figure 25.UN03
Archaeplastida
Unikonta
SARclade
Rhizarians
Alveolates
Stramenopiles
Excavata
Diatoms
Brown algae
Dinoflagellates
Apicomplexans
Ciliates
Forams
Cercozoans

Stramenopiles
 The subgroup stramenopiles includes the most
important photosynthetic organisms on Earth
 Diatoms and brown algae are members of the
stramenopiles clade
 Diatoms are highly diverse, unicellular algae with a
unique two-part, glass-like wall of silicon dioxide
Video: Various Diatoms
Video: Diatoms Moving

Figure 25.13
The diatom Triceratium morlandii
(colorized SEM)
40µm

 Brown algae are the largest and most complex
algae
 All are multicellular, and most are marine
 Brown algae include many species commonly called
“seaweeds”

 Brown algal seaweeds have plantlike structures: the
rootlike holdfast, which anchors the alga, and a
stemlike stipe, which supports the leaflike blades
 Similarities between algae and plants are examples
of analogous structures

Figure 25.14
Blade
Stipe
Holdfast

Alveolates
 The alveolates are a subgroup of the SAR clade
that have membrane-bounded sacs (alveoli) just
under the plasma membrane
 Dinoflagellates and ciliates are members of the
alveolata clade

Figure 25.15
Flagellum Alveoli
Alveolate
0.2µm

Figure 25.15a
Flagellum Alveoli
0.2µm

 Dinoflagellates have two flagella, and each cell is
reinforced by cellulose plates
 They are abundant components of both marine and
freshwater phytoplankton
 They are a diverse group of aquatic phototrophs,
mixotrophs, and heterotrophs
 Toxic “red tides” are caused by dinoflagellate
blooms
Video: Dinoflagellate

Figure 25.16
Flagella
Pfiesteria shumwayae, a dinoflagellate
3µm

 Ciliates, a large varied group of protists, are named
for their use of cilia to move and feed
 Most ciliates are predators of bacteria or small
protists
Video: Paramecium Cilia
Video: Paramecium Vacuole
Video: Vorticella Cilia
Video: Vorticella Habitat
Video: Vorticella Detail

Figure 25.17
Contractile
vacuole
Cilia
Oral groove
Cell mouth
Micronucleus
Food
vacuolesMacronucleus
50 µm

Figure 25.17a
50 µm

 Many species of rhizarians are amoebas
 Amoebas move and feed by pseudopodia; some
but not all belong to the clade Rhizaria
 Forams and cercozoans are members of the
rhizarian clade
Rhizarians

 Foraminiferans, or forams, are named for porous
shells, called tests
 Pseudopodia extend through the pores in the test
 Many forams have endosymbiotic algae
 Forams include both marine and freshwater species

 Cercozoans include amoeboid and flagellated
protists with threadlike pseudopodia
 They are common in marine, freshwater, and soil
ecosystems
 Most are heterotrophs, including parasites and
predators

 Paulinella chromatophora is an autotroph with a
unique photosynthetic structure called a
chromatophore
 This structure evolved from a different
cyanobacterium than the plastids of other
photosynthetic eukaryotes

Figure 25.18
5 µm
Chromatophore

Archaeplastids
 Plastids arose when a heterotrophic protist acquired
a cyanobacterial endosymbiont
 The photosynthetic descendants of this ancient
protist evolved into red algae and green algae
 Land plants are descended from the green algae
 Archaeplastida is the supergroup that includes red
algae, green algae, and land plants

Figure 25.UN04
Archaeplastida
Unikonta
SAR clade
Excavata
Chlorophytes
Charophytes
Red algae
Green algae
Land plants

Red Algae
 Red algae are reddish in color due to an accessory
pigment called phycoerythrin, which masks the
green of chlorophyll
 The color varies from greenish-red in shallow water
to dark red or almost black in deep water
 Red algae are usually multicellular; the largest are
seaweeds
 Red algae are the most abundant large algae in
coastal waters of the tropics
 Red algae reproduce sexually

Figure 25.19
Bonnemaisonia
hamifera
8 mm
Red algae
Nori

Figure 25.19a
Bonnemaisonia
hamifera 8 mm

Figure 25.19b
Nori

Figure 25.19c

Green Algae
 Green algae are named for their grass-green
chloroplasts
 Plants are descended from the green algae
 Green algae are a paraphyletic group
 The two main groups are charophytes and
chlorophytes
 Charophytes are most closely related to land plants

 Most chlorophytes live in fresh water, although many
are marine and some are terrestrial
 Nearly all species of chlorophytes reproduce
sexually
 Some unicellular chlorophytes are free-living while
others live symbiotically within other eukaryotes
 Larger size and greater complexity are found in
multicellular species including Volvox and Ulva

Figure 25.20
(a) Ulva, or sea
lettuce
(b) Caulerpa, an
intertidal
chlorophyte
Multicellular chlorophytes
2 cm

Figure 25.20a
(a) Ulva, or sea lettuce
2 cm

Figure 25.20b
(b) Caulerpa, an intertidal chlorophyte

Unikonts
 The supergroup Unikonta includes animals, fungi,
and some protists
 This group includes two clades: the amoebozoans
and the opisthokonts (animals, fungi, and related
protists)
 The root of the eukaryotic tree remains controversial
 It is unclear whether unikonts separated from other
eukaryotes relatively early or late

Figure 25.UN05
Unikonta
Archaeplastida
SAR clade
Excavata
Gymnamoebas
Slime molds
Nucleariids
Fungi
Animals
Choanoflagellates

Figure 25.21
Results
Common
ancestor
of all
eukaryotes
Choanoflagellates
Animals
Fungi
Amoebozoans
Unikonta
Excavata
Diplomonads
SAR clade
Euglenozoans
Stramenopiles
Alveolates
Rhizarians
Plants
Green algae
Red algae
Archaeplastida
DHFR-TS
gene
fusion

Amoebozoans
 Amoebozoans are amoebas that have lobe- or
tube-shaped, rather than threadlike, pseudopodia
 Most amoebozoans are free-living, but those that
belong to the genus Entamoeba are parasites

 Slime molds are free-living amoebozoans that
were once thought to be fungi
 DNA sequence analyses indicate that slime molds
belong in the clade Amoebozoa

 Dictyostelium is an example of a cellular slime mold
 Cellular slime molds consist of solitary cells that feed
individually but can aggregate to form a fruiting body

Figure 25.22
Spores
(n)
Fruiting
bodies (n)
Emerging
amoeba
(n)
Solitary amoebas
(feeding stage) (n)
ASEXUAL
REPRODUCTION
SEXUAL
REPRODUCTION
MEIOSIS
FERTILIZATION
Zygote
(2n)
Amoebas
(n)
Aggregated
amoebas
600 µm
Migrating
aggregate
200 µm
Key
Haploid (n)
Diploid (2n)

Figure 25.22a-1
Solitary amoebas
(feeding stage) (n)
SEXUAL
REPRODUCTION
FERTILIZATION
Zygote
(2n)
Key
Haploid (n)
Diploid (2n)

Figure 25.22a-2
Solitary amoebas
(feeding stage) (n)
SEXUAL
REPRODUCTION
MEIOSIS
FERTILIZATION
Zygote
(2n)
Amoebas
(n)
Key
Haploid (n)
Diploid (2n)

Figure 25.22a-3
Solitary amoebas
(feeding stage) (n)
ASEXUAL
REPRODUCTION
SEXUAL
REPRODUCTION
MEIOSIS
FERTILIZATION
Zygote
(2n)
Amoebas
(n)
Aggregated
amoebas
Key
Haploid (n)
Diploid (2n)

Figure 25.22a-4
Fruiting
bodies (n)
Solitary amoebas
(feeding stage) (n)
ASEXUAL
REPRODUCTION
SEXUAL
REPRODUCTION
MEIOSIS
FERTILIZATION
Zygote
(2n)
Amoebas
(n)
Aggregated
amoebas
Migrating
aggregate
Key
Haploid (n)
Diploid (2n)

Figure 25.22a-5
Spores
(n)
Fruiting
bodies (n)
Emerging
amoeba
(n)
Solitary amoebas
(feeding stage) (n)
ASEXUAL
REPRODUCTION
SEXUAL
REPRODUCTION
MEIOSIS
FERTILIZATION
Zygote
(2n)
Amoebas
(n)
Aggregated
amoebas
Migrating
aggregate
Key
Haploid (n)
Diploid (2n)

Figure 25.22b
200 µm

Figure 25.22c
600 µm

Opisthokonts
 Opisthokonts include animals, fungi, and several
groups of protists

Concept 25.4: Single-celled eukaryotes play key
roles in ecological communities and affect
human health
 The majority of the eukaryotic lineages are
composed of protists
 Protists exhibit a wide range of structural and
functional diversity

Structural and Functional Diversity in Protists
 Single-celled protists can be very complex, as all
biological functions are carried out by organelles in
each individual cell

 Protists are found in diverse aquatic environments
 Some protists reproduce asexually, while others
reproduce sexually, or by the sexual processes of
meiosis and fertilization

 Protists show a wide range of nutritional diversity,
including
 Photoautotrophs, which contain chloroplasts
 Heterotrophs, which absorb organic molecules or
ingest larger food particles
 Mixotrophs, which combine photosynthesis and
heterotrophic nutrition

Photosynthetic Protists
 Many protists are important producers that obtain
energy from the sun
 In aquatic environments, photosynthetic protists and
prokaryotes are the main producers
 In aquatic environments, photosynthetic protists are
limited by nutrients
 These populations can explode when limiting
nutrients are added

Figure 25.23
Other
consumers
Herbivorous
plankton
Carnivorous
plankton
Protistan
producers
Prokaryotic
producers

 Diatoms are a major component of the phytoplankton
 After a diatom population has bloomed, many dead
individuals fall to the ocean floor undecomposed
 This removes carbon dioxide from the atmosphere
and “pumps” it to the ocean floor

 The biomass and growth of photosynthetic protists
and prokaryotes have declined as sea surface
temperature has increased
 Warmer surface temperatures may prevent nutrient
upwelling, a process critical to the growth of marine
producers

 If sea surface temperature continues to warm due
to global warming, this could have large effects on
 Marine ecosystems
 Fishery yields
 The global carbon cycle

Figure 25.24
NI
SI
S SP
SA
EA
NA
AEPNP
Sea-surface temperature (SST) change (°C)
−2.50 −1.25 0.00 1.25 2.50
(a) (b)
Southern (S)
South Pacific (SP)
Equatorial Pacific (EP)
North Pacific (NP)
South Indian (SI)
North Indian (NI)
South Atlantic (SA)
Equatorial Atlantic (EA)
North Atlantic (NA)
Arctic (A)
Chlorophyll change (mg/m2
• yr)
−0.02 −0.01 0.00 0.01 0.02

Figure 25.24a
Sea-surface temperature (SST) change (°C)
−2.50 −1.25 0.00 1.25 2.50
SA
SP
EA
NA
AEPNP
NI
SI
S
(a)

Figure 25.24b
−0.02 −0.01 0.00 0.01 0.02
Southern (S)
South Pacific (SP)
Equatorial Pacific (EP)
North Pacific (NP)
South Indian (SI)
North Indian (NI)
South Atlantic (SA)
Equatorial Atlantic (EA)
North Atlantic (NA)
Arctic (A)
Chlorophyll change (mg/m2
• yr)
(b)

Symbiotic Protists
 Some protist symbionts benefit their hosts
 Dinoflagellates nourish coral polyps that build reefs
 Wood-digesting protists digest cellulose in the gut of
termites

10µm
Figure 25.25

 Some protists are parasitic
 Plasmodium causes malaria
 Pfiesteria shumwayae is a dinoflagellate that
causes fish kills
 Phytophthora ramorum causes sudden oak death
 Phytophthora infestans causes potato late blight

Effects on Human Health
 Protists that cause infectious disease can pose
major challenges to human immune systems and
public health
 Trypanosoma is an excavate that causes sleeping
sickness in humans

 Trypanosomes evade immune responses by
switching surface proteins
 A cell produces millions of copies of a single protein
 The new generation produces millions of copies of a
different protein
 These frequent changes prevent the host from
developing immunity

 Entamoebas are amoebozoan parasites of vertebrates
and some invertebrates
 Entamoeba histolytica causes amebic dysentery, the
third-leading cause of human death due to eukaryotic
parasites

 Apicomplexans are alveolate parasites of
animals, and some cause serious human diseases
 Most have sexual and asexual stages that require
two or more different host species for completion

 The apicomplexan Plasmodium is the parasite that
causes malaria
 Plasmodium requires both mosquitoes and humans
to complete its life cycle
 Approximately 900,000 people die each year from
malaria
 Efforts are ongoing to develop vaccines that target
this pathogen

Figure 25.26
0.5 µm
Apex
Red blood
cell
Merozoite
Liver
Liver
cell
Red
blood
cells
Merozoite
(n)
Zygote
(2n)
Oocyst
MEIOSIS
Sporozoites
(n)
Inside mosquito Inside human
FERTILIZATION
Gametes
Gametocytes
(n)
Key
Haploid (n)
Diploid (2n)

Figure 25.26a
0.5 µm
Apex
Red blood
cell
Merozoite

Figure 25.UN01

Figure 25.UN06

Origin and Diversification of Eukaryotes

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