The Protists

Chapter 20

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• Acrasiomycota

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

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

HeterokontsGroup

Taxonomic name

Number of

species Key characteristics

Brown algae Phaeophyta 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

Diatoms Bacillariophyta 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

Xanthophyta Xanthophyta 400Two flagella (various); cellulose cell wall; chlorophyll a (+ c in some), oil stored; mainly unicellular; mainly in freshwater

Chrysophyta Chrysophyta 300Two 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 group

Heterokonts Plants

Common name

Kelp Seaweed

Habitat Marine; cool, shallow water

Marine; warm water; greater depths

PigmentsChlorophylls a and c, fucoxanthin

Chlorophyll a, phycobilins

Food storage

Mannitol or laminaran Floridean starch

Cell wall Cellulose and alginatesCellulose; 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