Chapter 28: Protists
1. General Features of Protists
2. Survey of the Protista
A. The Excavata
B. The SAR Clade
C. The Archaeplastida
D. The Unikonta
1. General Features of Protists
All Protists are Eukaryotes
Eukaryotic organisms consist of 1 or more eukaryotic cells:
Prokaryotic cell
• eukaryotic cells contain a “true
nucleus” and other membranebound organelles
Most eukaryotes are
single-celled organisms,
most of which are protists
(i.e., not fungi, plants or
animals).
Nucleus
Eukaryotic cell
Organelles
Protists have Diverse Characteristics
Protists are much more diverse that plants, animals & fungi:
• most are unicellular, though some are multicellular
• some are photoautotrophs (commonly called “algae”)
• some are chemoheterotrophs (commonly called “protozoa”)
• some have characteristics of both (mixotrophs)
• protists can reproduce sexually, asexually, or both
Endosymbiosis in Eukaryotic Evolution
Plasma membrane
The endosymbiont theory proposes
that mitochondria & chloroplasts are
derived from internalized prokaryotes.
Cytoplasm
DNA
Ancestral
prokaryote
Nucleus
Endoplasmic
reticulum
Photosynthetic
prokaryote
Mitochondrion
Nuclear envelope
Aerobic heterotrophic
prokaryote
Mitochondrion
Plastid
Ancestral
heterotrophic eukaryote
Ancestral photosynthetic
eukaryote
A Closer Look at Plastid Evolution
Dinoflagellates
Membranes
are represented
as dark lines
in the cell.
Secondary
endosymbiosis
Red alga
Cyanobacterium
1 2
Plastid
3
Primary
endosymbiosis
Stramenopiles
Nucleus
Heterotrophic
eukaryote
Secondary
endosymbiosis
One of these
membranes
was lost in
red and
green algal
descendants.
Plastid
Euglenids
Secondary
endosymbiosis
Green alga
Chlorarachniophytes
Protists No Longer Constitute a Single Kingdom
The traditional grouping of eukaryotes involved 4 kingdoms:
PROTISTA
FUNGI
PLANTAE
ANIMALIA
However this is not consistent with phylogenies based on recent
molecular analysis which supports the following 4 supergroups…
Figure 28.2
■ Excavata
Parabasalids
Euglenozoans
5 μm
4 Supergroups
Excavata
Diplomonads
Stramenopiles
■ “SAR” Clade
Diatoms
Golden algae
Giardia intestinalis,
Brown algae
Apicomplexans
Ciliates
“SAR” clade
Alveolates
Dinoflagellates
Forams
Rhizarians
Cercozoans
■ Archaeplastida
Radiolarians
Green
algae
Chlorophytes
Charophytes
Land plants
50 μm
3. Archaeplastida
• includes all land plants
Volvox, a colonial freshwater green alga
Tubulinids
■ Unikonta
Opisthokonts
Fungi
4. Unikonta
• includes fungi & animals
Entamoebas
Nucleariids
1. Excavata
2. SAR Clade
Unikonta
Amoebozoans
Slime molds
The evolutionary
history of all
eukaryotes
currently consists
of 4 supergroups:
Diatom diversity
Archaeplastida
Red algae
50 μm
Choanoflagellates
Animals
100 μm
A unikont amoeba
All eukaryotes that are not
plants, animals or fungi are
considered to be Protists.
2A. Survey of the Protista
The Excavata
Parabasalids
Euglenozoans
Excavata
Diplomonads
SAR clade
Archaeplastida
Unikonta
General Characteristics of Excavates
Below are some of the characteristics seen in Excavates:
• some members have an “excavated” feeding grooves
that are associated with unique cytoskeletal structures
• many have modified “unclassical” mitochondria
• most have 2 or more flagella
• includes the following major groups:
DIPOMONADS
PARABASALIDS
EUGLENOZOANS
Diplomonads
Diplomonads have
reduced mitochondria
called mitosomes and
derive energy by
anaerobic processes.
They also have 2 nuclei
and multiple flagella.
Many are parasites such
as Giardia intestinalis
shown here:
Parabasalids
Parabasalids have reduced mitochondria called hydrogenosomes and
thrive in anaerobic environments.
• Trichomonas vaginalis, a sexually transmitted human pathogen
shown below is one example
Flagella
Undulating
membrane
5 μm
Euglenozoans
The Euglenozoa is a diverse clade that includes predatory heterotrophs,
photosynthetic autotrophs, mixotrophs, and parasites.
• a characteristic feature of this
clade is the crystalline rod
structure within their flagella
Flagella
0.2 μm
• members of this clade include:
8 μm
Crystalline rod
(cross section)
Ring of microtubules
(cross section)
KINETOPLASTIDS
EUGLENIDS
Kinetoplastids
Kinetoplastids have a single mitochondrion containing an unusual
organized mass of DNA called a kinetoplast.
• includes the genus Trypanosoma
which contains parasites that
cause “sleeping sickness” and
Chaga’s disease
RBC
9 μm
Euglenids
Euglenids have 1 or 2 flagella, and some members of this clade, such
as the genus Euglena, are both autotrophic and heterotrophic
(i.e., mixotrophs).
Long flagellum
Eyespot
Short flagellum
Light
detector
Contractile vacuole
Nucleus
Chloroplast
Euglena (LM)
5 μm
Plasma
membrane
Pellicle
2B. Survey of the Protista
The SAR Clade
Excavata
Stramenopiles
Dinoflagellates
Apicomplexans
Alveolates
Ciliates
Forams
Cercozoans
Radiolarians
Rhizarians
SAR clade
Diatoms
Golden algae
Brown algae
Archaeplastida
Unikonta
General Characteristics of the SAR Clade
The SAR clade is a highly diverse monophyletic supergroup
based on DNA similarities that includes the following:
STRAMENOPILES
ALVEOLATES
RHIZARIANS
Stramenopiles
Most members of the Stramenopile clade have both “hairy”
and smooth flagella.
Hairy
flagellum
Smooth
flagellum
• this clade includes the
following ecologically
important groups of
photosynthetic
organisms:
DIATOMS
GOLDEN ALGAE
BROWN ALGAE
5 μm
Diatoms
Diatoms have unique glass-like walls made of silicon dioxide
and are a significant portion of phytoplankton in the oceans.
40 μm
• their remains are the main components of
white sediments referred to as
“diatomaceous earth” (e.g., the “white cliffs
of Dover”)
Golden Algae
Flagellum
Outer container
• contain yellow and
brown carotenoids
which give them
their characteristic
color
Living cell
• typically have two
flagella
25 μm
• most are singlecelled though some
are colonial
Brown Algae
Brown algae are all multicellular
and are the largest algae.
• includes many “seaweeds”
such as kelp
• although brown algae are
not plants, many have
plant-like structures shown
in this image
Blade
Stipe
Holdfast
Haploid (n)
Diploid (2n)
Sporangia
MEIOSIS
10 cm
Sporophyte
(2n)
Developing
sporophyte
Zygote
Mature female
(2n)
gametophyte
(n)
FERTILIZATION
Zoospore
Female
Gametophytes
(n)
Male
Egg
Sperm
Alternation of
Generations
• many multicellular
algae have a life cycle
similar to that of
plants called the
“Alternation of
Generations” – the
alternation between
multicellular haploid
and diploid forms
Alveolates
Members of the Alveolate clade have membrane enclosed
sacs called alveoli must beneath the plasma membrane.
Alveoli
• this clade includes the
following:
DINOFLAGELLATES
Alveolate
0.2 μm
Flagellum
APICOMPLEXANS
CILIATES
Dinoflagellates
Flagella
Dinoflagellates are also an
important component of
phytoplankton and have 2
flagella, one of which wraps
around the cell and causes it to
rotate.
• are responsible for the algal
blooms known as “red tides”
(a) Dinoflagellate
flagella
3 μm
(b) Red tide in the Gulf
of Carpentaria in
northern Australia
Apicomplexans
The Apicomplexans are parasites of animals that have
complex life cycles with multiple hosts.
• the most significant genus in this
clade is Plasmodium, the cause of
malaria
• nearly 1 million people each year
die from malaria
Inside human
Inside mosquito
Sporozoites
(n)
Plasmodium
Life Cycle
Merozoite
Liver
Liver
cell
Apex
Oocyst
MEIOSIS
Zygote
(2n)
Merozoite
(n)
Red blood
cell
Red
blood
cells
• humans are the
intermediate host (in
which reproduction
is asexual)
FERTILIZATION
Gametes
Gametocytes
(n)
0.5 μm
• mosquitoes are
the definitive host
(in which sexual
reproduction
occurs)
Haploid (n)
Diploid (2n)
Ciliates
The Ciliates clade is a large group characterized by cilia used for
locomotion and/or feeding and 2 nuclei, one macronucleus and
one micronucleus.
• the genus Paramecium is a good
example
Contractile
vacuole
50 μm
Cilia
Micronucleus
Macronucleus
(a) Feeding, waste removal, and water balance.
Oral groove
Cell mouth
• Ciliates also engage
in a form of sexual
reproduction called
conjugation…
Food
vacuoles
Conjugation in Ciliates
Compatible
mates
Conjugation
Asexual
reproduction
MEIOSIS
Diploid
micronucleus
The original macronucleus disintegrates.
Haploid
micronucleus
Diploid
micronucleus
MICRONUCLEAR
FUSION
(b) Conjugation and reproduction.
Rhizarians
The Rhizarians are mostly amoebas containing pseudopodia that
are threadlike (unlike the Amoebozoa in the Unikonta clade.
• this clade includes the
following:
RADIOLARIANS
FORAMS
CERCOZOANS
Radiolarians
Radiolarians have
delicate, symmetrical
endoskeletons made
typically of silica.
• the pseudopodia
extend from the cell
to catch prey
Pseudopodia
200 μm
Forams
Foraminiferans (“hole bearers”), or forams for short, are singlecelled heterotrophs that characteristically have multi-chambered
shells called tests.
• foram tests are found throughout
the fossil record and their tests
can be analyzed to estimate
ocean temperatures in ancient
times
Cercozoans
Chromatophore
• most are heterotrophs,
though a few such as
Paulinella
chromatophora are
autotrophs that contain
unique photosynthetic
structures called
chromatophores
5 μm
2C. Survey of the Protista
The Archaeplastida
Excavata
SAR clade
Chlorophytes
Green algae
Charophytes
Land plants
Archaeplastida
Red algae
Unikonta
The Archaeplastida
The supergroup Archaeplastida contains the following:
RED ALGAE
GREEN ALGAE
LAND PLANTS
• molecular and other data indicate the land plants are
descendants of the green algae, though only the red and
green algae are considered to be protists
Red Algae
► Bonnemaisonia
hamifera
20 cm
The red algae contain, in addition to
chlorophyll, a red accessory pigment
called phycoerythrin.
8 mm
◀ Dulse (Palmaria palmata)
▼ Nori
• phycoerythrin absorbs non-red light
• many red algae live in deep water
where red light does not penetrate
This group includes many seaweeds
some of which are edible.
Green Algae
Green algae contain
green chloroplasts (due
to chlorophyll) and
includes 2 main groups:
2 cm
CHLOROPHYTES
• most live in fresh water
CHAROPHYTES
• closest relatives to plants
(b) Caulerpa, an
intertidal
chlorophyte
(a) Ulva, or sea lettuce
Chlorophyte Life Cycle
Most chlorophytes have complex life cycles with both sexual and asexual
reproductive stages:
Flagella
−
1 μm
Cell wall
+
Nucleus
ASEXUAL
REPRODUCTION
Zygote
(2n)
(TEM)
−
+
+
Haploid (n)
Diploid (2n)
−
Mature cell
FERTILIZATION
(n)
SEXUAL
REPRODUCTION
Zoospore
Cross
section
of cupshaped
chloroplast
Gamete
+
(n)
−
MEIOSIS
2D. Survey of the Protista
The Unikonta
Excavata
SAR clade
Opisthokonts
Slime molds
Tubulinids
Entamoebas
Nucleariids
Fungi
Choanoflagellates
Animals
Unikonta
Amoebozoa
Archaeplastida
The Unikonta
This group contains 2 major clades:
AMOEBOZOA
• all of which are protists:
SLIME MOLDS
TUBULINIDS
ENTAMOEBAS
OPISTHOKONTS
• includes animals and fungi in addition to some protists:
NUCLEARIIDS
CHOANOFLAGELLATES
Amoebozoans
Amoebozoans have tube or lobe-shaped pseudopodia (unlike
the Rhizaria).
Amoeba proteus, a tubulinid
Entamoeba histolytica
Slime Molds
Plasmodial slime molds form multicellular aggregates in which
there is no separation of nuclei by membranes (i.e., a
plasmodium), thus they are not considered to be multicellular.
Cellular slime molds form multicellular aggregates in which
cells are separated by their membranes.
• cells feed individually and can aggregate to migrate and
form a fruiting body
• species such as Dyctiostelium discoideum are studied to better
understand the origins of multicellularity
Cellular Slime
Mold Life Cycle
Spores
(n)
FERTILIZATION
Emerging
amoeba (n)
Solitary amoebas
(feeding stage)
(n)
600 μm
SEXUAL Zygote
REPRO- (2n)
DUCTION
MEIOSIS
ASEXUAL
REPRODUCTION
Amoebas
(n)
Aggregated
amoebas
Fruiting
bodies
(n)
Migrating
aggregate
200 μm
Haploid (n)
Diploid (2n)
Zygote
(2n)
Feeding
plasmodium
FERTILIZATION
Mature
plasmodium
(preparing to fruit)
Flagellated
cells
(n)
Young
sporangium
Amoeboid
cells (n)
Mature
sporangium
Germinating
spore
Spores (n)
MEIOSIS
Stalk
Haploid (n)
Diploid (2n)
4 cm
Plasmodial
Slime Mold
Life Cycle