The Protists
Chapter 21
Protists
• General characteristics
– Unicellular, colonial, simple multicellular
organisms
– Eukaryotic
– Some exhibit both plant and animal
characteristics
• Euglenoids
• Slime molds
Protists
– Many are photosynthetic
• All have chlorophyll a
• Often called algae
Protists
• Evolutionary relationships of eukaryotes
– Difficult problem
• Splits between many lineages of eukaryotes are
ancient
• Have been events in which organisms (or parts of
organisms) that are not closely related have joined
to form new organisms
• Not many characters are shared by all members of
group
• Shared characters often yield different results
when analyzed cladistically
Protists
– At base of eukaryotic tree
• Group of protists
– Some lack mitochondria and live as parasites on other
organisms
– Rest of eukaryotes divided into two major
clades
• One clade
– Animals, fungi, slime molds, and small group of
amoeboid organisms
– None are photosynthetic
– Most are motile (except fungi)
Protists
• Other clade
– Variety of protist groups
– Includes both photosynthetic and nonphotosynthetic
protists
Photosynthetic Protists
• Commonly called algae
• Variety of life histories, body forms,
ecological roles
• Often named for distinctive colors
• Unicellular, colonial, filamentous, sheetlike
Amoebozoa
• Clade of protists that includes amoebas
and slime molds
• Traits combine aspects of fungi and
animals
– Animal characteristics
• Lack cell walls, engulf food, have motile cells at
some phase of life cycle
– Fungi and plant characteristics
• Form sporangia and nonmotile cells with cell walls
Amoebozoa
– Two groups of slime
molds
Myxomycota
• Myxomycota
• Acrasiomycota
Plasmodial slime
molds
Cellular slime
molds
Contain
thousands of
nuclei with no
membranes
separating them
Smaller, have
fewer nuclei, and
do have
membranes
between the
nuclei
Acrasiomycota
Alveolates
• Common character
– System of alveoli
• Tiny membrane-enclosed sacs found beneath plasma
membrane
• Clade composed of four ecologically and
economically significant lineages
– Ciliates
•
•
•
•
Found in freshwater
Have cilia and engulf food
Often called protozoa
Example: Paramecium
Alveolates
– Foraminifera
• Characterized by hard shell
• Feeding strategies include active predation,
scavenging, using sticky webs to trap food
• Shells
– Common fossils
– Important in dating geological strata and in oil exploration
Alveolates
– Apicomplexa
• Parasitic or pathogenic protists
• Found to contain vestigial plastids
• Examples
– Plasmodium (causes malaria)
– Toxoplasma (causes toxoplasmosis)
Alveolates
– Photosynthetic dinoflagellates
• Most are unicellular, motile, and marine
• Usually have two flagella (both emerge from same
pore)
– One is flat and ribbonlike, encircles cell in groove around
middle, provides rotational movement
– Other flagellum trails behind and provides forward
movement
• Contain pigments
– Chlorophylls a and c
– Brown pigment (fucoxanthin)
Alveolates
• Chloroplasts surrounded by three or four
membranes
– May also contain a remnant nucleus
• Some are bioluminescent
• Overgrowth (bloom) can cause red tide
Euglenoids
• Typically single-celled
• Usually in freshwater but could be in salt
or brackish water or soil
• A few are parasitic
• Lack cell wall
– Flexible strips of proteins and microtubules
under cell membrane
• Have two flagella
Euglenoids
• About one third of the 1,000 species have
chloroplasts
– Contain chlorophylls a and b, carotenoids
– Three membranes around chloroplasts
• May have eyespot
– Red or orange light-sensitive organelle
– Pigment (astaxanthin) is a carotenoid has
antioxidant properties
• Extracted and sold as health supplement
Euglenoids
• Never been observed to reproduce
sexually
• Important in food chains of freshwater
ecosystems
• Ecological indicators of water that is rich in
organic matter
Heterokonts
• Clade also sometimes called
Stramenopiles or Chromista
• All have two unequally sized flagella
• Heterokonts and alveolates share
common ancestor
Heterokonts
• Includes
– Oomycota (water molds and downy mildew)
– Xanthophyta (golden algae)
– Chrysophyta (golden algae)
– Diatoms
– Brown algae
Heterokonts
Group
Brown algae
Diatoms
Xanthophyta
Chrysophyta
Taxonomic
name
Phaeophyta
Bacillariophyta
Xanthophyta
Chrysophyta
Number
of
species
Key characteristics
1,500
Two unequal lateral flagella; cell wall of algin +
cellulose; chlorophylls a and c; filamentous to
complex large kelps; mainly in shallow, cool,
marine water
8,000
Usually no flagella; cell wall of silica + pectin;
chlorophylls a and c; carbohydrates stored as
oil; mainly unicellular and free-floating;
prominent as freshwater or saltwater
phytoplankton
400
Two flagella (various); cellulose cell wall;
chlorophyll a (+ c in some), oil stored; mainly
unicellular; mainly in freshwater
300
Two unequal anterior flagella; cellulose cell
wall (+ silica in some), chlorophylls a and c;
mainly unicellular and in freshwater
Heterokonts
• Oomycota
– Includes egg fungi, downy mildews, and water
molds
– Fungal characteristics
• Hyphae, produce spores, lack chlorophyll
– Algal characteristics
• Cellulose cell walls, swimming spores, some
cellular details, some metabolic pathways
Heterokonts
• Oomycota
– Most are decomposers
– Some are pathogens of important crops
• Downy mildew of grapes
– Plasmopora viticola
– Nearly destroyed French vineyards in nineteenth century
• Potato blight
– Phytophthora infestans
– Changed history of Ireland in 1840s
Heterokonts
• Diatoms
– Important members of phytoplankton
– Cell wall made out of silica
• Two parts (valves) to cell wall
• Fit together like halves of Petri dish
– Shape of cell varies
– Pigments
• Chlorophylls a and c
• Fucoxanthin
Heterokonts
• Diatoms
– Store food reserves as oil
– Some exhibit gliding motion
– Stalked diatoms grow as epiphytes on seaweeds and
kelps
– Can also become attached to nonliving surfaces
– Create algal turfs
• Coat shallow rocks in quiet freshwater or marine habitats
• Have as high a daily productivity per square meter as tropical
rain forest
Heterokonts
• Diatoms
– Dominate surfaces of salt mudflats
– Extensive fossil record
– Indicators for petroleum exploration
– Silica shells form extensive deposits
Heterokonts
• Brown algae
– Almost exclusively marine
– More abundant in cool, shallow waters
– More complex brown algae  kelps
•
•
•
•
Chlorophylls a and c, fucoxanthin
No grana in chloroplasts
Store carbohydrates as mannitol or laminaran
Cell walls composed of cellulose and alginates
Heterokonts
• Brown algae
– Kelps
• Example
–
–
–
–
Macrocystis
Largest known kelp
Fast growth rate
Consists of
» Holdfast – anchors
» Stipes – stem-like structure
» Blades – leaf-like
» Gas-filled air bladder - buoyancy
Heterokonts
• Brown algae
– Kelps
• Outer layer is protective and consists of cells that are also
meristematic and contain chloroplasts  called meristoderm
• Region of cortex beneath meristoderm
– Composed of parenchyma-like cells
– Mucilage-secreting cells line canals through medulla
• Medulla
– Innermost part of stipe
– Loosely packed filaments of cells
Heterokonts
• Brown algae
– Kelps
• Cells that function as sieve elements
– Found in transition zone between cortex and medulla
– Have sieve plates, form callose, adjoin one another to
make continuous tubes
– Mannitol moves through tubes
• No tissue that resembles xylem
The Plants
• Molecular data supports idea that red
algae, green algae, and land plants belong
in same clade
• Green algae
– Not a natural monophyletic group
– Gave rise to land plants
The Plants
• Red algae
– Almost exclusively marine
– Most abundant in warm water
– Can grow to considerable depth
– More complex forms called seaweeds
•
•
•
•
Parenchyma-like tissue
Holdfast for anchoring
Stipes never very long
Blades never have gas bladders
The Plants
• Red algae
• Pigments
– Chlorophyll a and phycobilins
• Cell wall
– Cellulose and sometimes agar or carrageenan
• Food storage molecule
– Floridean starch
Comparison of Red and Brown
Algae
Brown algae
Red algae
Evolutionary
Heterokonts
group
Plants
Common
name
Kelp
Seaweed
Habitat
Marine; cool, shallow
water
Marine; warm water; greater depths
Pigments
Chlorophylls a and c,
fucoxanthin
Chlorophyll a, phycobilins
Food
storage
Mannitol or laminaran
Floridean starch
Cell wall
Cellulose and alginates
Cellulose; sometimes also contains
agar or carrageenan
The Plants
• Green algae
– Mainly in freshwater habitats but could be in
saltwater, on snow, in hot springs, on soil, on leaves
and branches of terrestrial plants
– Shared characteristics
•
•
•
•
•
Chlorophylls a and b, carotenoid accessory pigments
Food stored as starch
Cellulose cell walls
Formation of phragmoplast during mitosis
Asymmetrically attached flagella
The Plants
• Green algae
– Groups
• Chlorophyceae
• Ulvophyceae
• Charophytes
The Plants
• Green algae
– Chlorophyceae
•
•
•
•
Two flagella at anterior end
Single, large, cup-shaped chloroplast
Most have red-colored carotene eyespot
Examples
– Gonium
– Volvox
The Plants
• Green algae
– Ulvophyceae
•
•
•
•
Sea lettuces
Typically small, green seaweeds
Consumed as food in many places
Example
– Caulerpa
» Accidentally spread to areas with no natural limits to
growth
» Multiplied explosively
» Produces toxins that are lethal to urchins and some fish
» Has been discovered off coast of California
The Plants
• Green algae
– Charophytes
• Group of ancient green algae
• Examples
– Coleochaete
» Once thought to be closest living relative to land
plants
– Chara
» According to molecular characters → more closely
related to land plants than Coleochaete
Ecological and Economic
Importance of Algae
• Phytoplankton
– Base of aquatic food chains
– “grasses of the sea”
– Unicellular
– Produce about four times the amount of
photosynthate that is produced by the Earth’s
croplands each year
– Bloom
• Algal overgrowth
Ecological and Economic
Importance of Algae
• Help build tropical reefs
– Coralline algae
• Certain red and green algae
• Create carbonate exoskeleton that becomes part
of reef when alga die
Ecological and Economic
Importance of Algae
– Coralline algae
• Some algae grow symbiotically with coralline
animals
– Mutualistic relationship
» Algae produces sugar and oxygen for animal
» Cells of coral contribute CO2, nitrogen, and minerals
for alga
– Usually dinoflagellate Symbiodinium microadriaticum
– Has photosynthetic rate 10 times greater than
phytoplankton
» Protected and nourished by animal cytoplasm
around it
Ecological and Economic
Importance of Algae
• Medicine, food, and fertilizer
– Laminaria
• Harvested off coast of China as source of iodine
– Porphyra (nori)
• Cultivated
• Supplement to Japanese diet
– limu
• used as food source by Polynesians in Hawaii
Ecological and Economic
Importance of Algae
• Medicine, food, and fertilizer
– Palmaria palmate
• red seaweed, dulce
• Used as food in British isles
– Chondrus crispus
• Irish moss, red algae
• Used to make jelly desert called blancmange
Ecological and Economic
Importance of Algae
• Medicine, food, and fertilizer
– Good fertilizer or cattle feed supplements
•
•
•
•
Compares favorably with manure as a fertilizer
Enhances germination
Increases uptake of nutrients
Seems to give degree of resistance to frost,
pathogens, and insects
Ecological and Economic
Importance of Algae
• Uses of algal cell walls
– Diatomite (diatomaceous earth)
• Rich deposit near Lompoc, California
• Uses of diatomite
– Superior filter or clarifying material
– Added to many materials to provide bulk, improve flow,
increase stability
» Dental impressions, grouting, paint, asphalt, pesticides
– Abrasive
Ecological and Economic
Importance of Algae
• Uses of algal cell walls
– Agar
• Primarily obtained from red algae, Gelidium and
Gracilaria
• Used as a culture medium
• Substance purified from agar (agarose) used for
gel electrophoresis
• Used in baking industry
– Added to icing to retard drying in open air or melting in
cellophane packages
• Used as a bulk laxative
Ecological and Economic
Importance of Algae
• Uses of algal cell walls
– Carrageenan
• Reacts with proteins in milk to make stable,
creamy, thick solution or gel
– Used commercially in ice cream, whipped cream, fruit
syrups, chocolate milk, custard, evaporated milk, bread,
macaroni
• Added to dietetic, low-calorie foods
• Used in toothpaste, pharmaceutical jellies, and
lotions
• Mainly comes from Irish moss, red algae
Ecological and Economic
Importance of Algae
• Uses of algal cell walls
– Algin
• Compound of brown algae
• Strongly absorbs water
• Used as an additive to beer, water-based paints,
textile sizing, ceramic glaze, syrup, toothpaste,
hand lotion
Ecological and Economic
Importance of Algae
– Algin
• Commercially harvested species
– Macrocystis pyrifera – along California coast
– Ascophyllum, Fucus, and Laminaria – off Maritime
Canada, northeastern United States, England, China
coast
– Durvillea – from Australian waters
Algal Reproduction
• Asexual reproduction occurs more often
than sexual reproduction
• Asexual methods of reproduction
– Cell division (single-celled algae)
– Fragmentation (filamentous algae)
– Formation, liberation, germination of motile or
nonmotile spores produced in sporangia
Algal Reproduction
• Three basic life cycles
– Zygotic
• Only diploid phase of life cycle is single-celled
zygote
– Gametic
• Only haploid phase of life cycle is single-celled
gamete
– Sporic
• Multicellular gametophytes and sporophytes
Algal Reproduction
• Zygotic life cycle
– Example: Ulothrix
– Sexual reproduction
• Some of nuclei divide by mitotic divisions to
produce many motile gametes inside wall of parent
cell
• Parent cell is gametangium
• Ulothrix gamete approaches another suitable
gamete
• Cells fuse forming diploid zygote cell with four
flagella
Algal Reproduction
• Zygotic life cycle
• Zygote become spherical, loses flagella, enters
resting stage
• When conditions are right, zygote becomes
metabolically active
• Zygote divides by meiosis and produces + and –
meiospores
• Meiospores are dispersed
• Each meiospore can germinate, divide by mitosis,
produce a + or – haploid plant
Algal Reproduction
• Zygotic life cycle
– Asexual reproduction
• Vegetative cell becomes sporangium
• 16 to 64 pear-shaped mitospores are released
• After period of activity, motile mitospores settle to
bottom of pond, lose flagella, produce new plant by
mitosis
Algal Reproduction
• Gametic life cycle
– Example: diatoms
– Sexual reproduction
• Diploid nucleus undergoes meiosis
• Produces four haploid nuclei (only 1 or 2 survive to
become gametes)
• Gametangia near each other open, gametes
emerge, fuse, form diploid zygote
• Zygote increases in size
• Secretes silica wall around itself, becomes
vegetative diploid diatom
Algal Reproduction
• Gametic life cycle
– Asexual reproduction
• Reproduce by cell division
• New cell wall forms within old one
• Progeny cell that inherits small wall segment of cell
wall makes wall that is even smaller
• Pattern continues until critically small size is
reached
• Cell division stops
• Cell then must reproduce sexually
Algal Reproduction
• Sporic life cycle with isomorphic
generations
– Identical looking gametophytes and
sporophytes
– Example: Ectocarpus
• Haploid phase has gametangia on side branches
• Mitosis within gametangium produces gametes
• Released isogamous gametes (look alike)
represent + and – mating types
Algal Reproduction
• Sporic life cycle with isomorphic
generations
• Plus and minus gametes fuse in open water →
yields diploid zygote
• Zygote settles to bottom, germinates, divides by
mitosis, produces diploid organism
• Diploid sporophyte looks identical to haploid
gametophyte
Algal Reproduction
• Sporic life cycle with isomorphic
generations
• Sporophyte produces two kinds of reproductive
cells
– Asexual sporangia develop on side branches
» Release motile cells (diploid mitospore) capable of
producing new individual (sporophyte) by itself
– Spherical sporangium
» Produces meiospores
» Meiospores germinate, producing gametophytes
Algal Reproduction
• Sporic life cycle with heteromorphic
generations
– Gametophyte and sporophyte are not
identical
– Most highly developed in brown algae
– Example: Laminaria
• Sporophyte generation with well-developed
holdfast and long unbranched stipe with narrow
blades
Algal Reproduction
• Sporic life cycle with heteromorphic
generations
• Sporangia form in groups just below meristoderm
on blade
• Single-celled sporangium undergoes meiosis
• Produces 8 to 64 meiospores
• Meiospores are released, swim, settle to bottom,
produce gametophytes
• Some gametophytes produce female gametes,
some produce male gametes
Algal Reproduction
• Sporic life cycle with heteromorphic
generations
• Cells at tips of some male gametophyte filaments
enlarge and function as antheridia
– Nuclei undergo mitosis producing motile sperm
• Cells at tips of some female gametophyte filaments
enlarge and function as oogonia
– Nuclei undergo mitosis producing one to several large
eggs
– Eggs are extruded but remain attached
Algal Reproduction
• Sporic life cycle with heteromorphic
generations
• Sperm cell fuses with egg
• Produces zygote which forms sporophyte
• Sporophyte separates from gametophyte, carried
by currents to bottom, begins to develop into
mature Laminaria sporophyte