Ch: 27- The Diversification of Life - Protists

What are protists in the classification system?

The “junk drawer”

protists are a

paraphyletic group

paraphyletic group

includes some but not all descendants of a common ancestor

monophyletic group

ancestor + all its descendants

Are protists monophyletic?

No because the group excludes

Is there any single synapomorphy that unites all protists?

No

Are eukaryotes monophyletic?

Yes

synapomorphies of eukaryotes

• nuclear envelope

• mitosis and meiosis

• mitochondrial respiration

• cytoskeleton

ecological impact of protists

Phytoplankton (protists) are major producers in aquatic ecosystems

human health impact of protists 

many protists are parasitic

evolutionary significance of protists

earliest eukaryotes; innovations such as

• nucleus

• mitochondria

• meiosis

• multicellularity

molecular phylogenies 

help classify organisms with few visible traits 

eukaryotic lineages

1. Unikonta

2. Bikonta

Unikonta

cells with one or no flagella

Bikonta

cells with two flagella

Unikonta protists

Amoebozoa, Opisthokonta

Bikonta protists

Alveolata, Stramenopila, Rhizaria, Plantae, Excavata

Major Protists Groups

1. Excavata

2. Rhizaria 

3. Alveolata

4. Stramenopila

5. Amoebozoa 

Excavata

Protist group with

• ventral feeding groove

• two nuclei

• lost or modified mitochondria (disc-like cristae).

Diplomonads

Excavates with

• two nuclei

• modified mitochondria (mitoses)

• many are parasitic 

Euglenids

• Photosynthesis

• free living protists

• invest food particles through cytosome

• disc-like cristae

Rhizaria

Protist Group characterized by

• long, slender pseudopodia

• shells (tests) made of calcium carbonate

• Group defined by sac-like alveoli under the plasma membrane; may have cilia or flagella.

specialized reticulopdia

branching pseudopods that trap food

Foraminifera

Rhizarian protists with

• calcium carbonate shells

• specialized reticulopdia: branching pseudopods that form food traps

Alveolata

Protists group defined by

• sac-like alveoli under the plasma membrane

• may have cilia or flagella

Ciliates

Protists that use cilia for locomotion and feeding; common in aquatic environments

Ciliates example

Paramecium

Dinoglagellates

Protists with

• two flagella

• test made of cellulose

• many are photosynthetic with red/brown pigments

• Can cause red tides and bioluminescence

• produces saxitoxin

Saxitoxin

A toxin produced by some dinoflagellates; blocks sodium channels and can be fatal

Apicomplexa

Parasitic alveolates with an apical complex that allows penetration of host cells

Apicomplexa example

Plasmodium (causes Malaria)

Stramenopila

Group with

• hair-like projections on flagella

• includes photosynthetic and fungus-like organisms.

Oomycetes (Water Molds)

Fungus-like stramenopiles that absorb nutrients; not true fungi (convergent evolution)

Diatoms

Photosynthetic stramenopiles with silicon dioxide shells; contribute to diatomaceous earth used in filters and abrasives.

Diatomaceous Earth

Deposits of fossilized diatoms; used as filters, pesticides, abrasives, and absorbents.

Brown Algae

Multicellular, photosynthetic stramenopiles containing fucoxanthin pigment; includes kelp.

• used as commercial thickening product

Kelp Structure

Holdfast (root), stipe (stem), blades (leaves); not true plant organs.

Amoebozoa

Protists that feed by engulfing prey (phagocytosis) and move with lobe-like pseudopodia (lobopods).

Lobose Amoebae

Free-living amoebas using pseudopodia for movement and feeding.

Cellular slime molds

Amoebozoans that are unicellular until stressed; form multicellular reproductive structures; display altruistic behavior (stalk dies to help reproduction).

Plasmodial slime molds

Amoebozoans capable of problem-solving and spatial memory; can find the shortest route to food sources.

Ingestive feeding

Protists that consume particles or cells directly; includes ciliated and pseudopodia-based feeders

Phagocytosis

Engulfing prey using pseudopodia; common in amoebas.

Absorptive feeding

Protists that absorb nutrients directly across membranes

Saprolegnia

An oomycete that absorbs nutrients from dead organic matter (cotton mold)

Photosynthetic Feeding

Protists that produce energy using chloroplasts and light; may have pigments like chlorophylls or xanthophylls

Primary Endosymbiosis

A eukaryotic cell engulfed a photosynthetic bacterium (cyanobacterium) that became a chloroplast; occurs in ancestral Plantae.
Chloroplasts have two membranes

Evidence for Primary Endosymbiosis

• Chloroplast DNA resembles cyanobacteria

• Endosymbiotic cyanobacteria live in some protists/animals today

• Glaucophyte algae chloroplasts have peptidoglycan layer

Secondary Endosymbiosis

A eukaryotic cell engulfed a photosynthetic eukaryote (alga); the engulfed cell became a chloroplast with four membranes.
Occurred multiple times independently (convergent evolution).

Protists with 2 chloroplast membranes

Plantae 

• red algae

• green algae

Protists with 4 chloroplast membranes

• Rhizaria - Chlorachniophytes

• Excavate - Euglenids

• Alveolata - Dinoflagellates

• Stramenopila - Diatoms, Brown Algae

Amoeboid Movement

Movement by cytoplasmic streaming through pseudopodia.

flagellar movement 

Few, long flagella; slower undulating motion

ciliary movement 

Many short cilia; fast, rotational movement

Asexual reproduction

Occurs by binary fission; produces genetically identical offspring.

Sexual Reproduction

Involves meiosis; protists were the first organisms to evolve sexual reproduction

nuclear envelope origin

Formed from plasma membrane infoldings; allowed separation of transcription and translation.

Mitochondria origin

Formed through endosymbiosis between an anaerobic eukaryote and aerobic bacterium

Endosymbiotic Theory (Mitochondria)

An ancestral eukaryote engulfed an aerobic bacterium, which became the mitochondrion

evidence for endosymbiotic theory

• Mitochondria have their own genome

• DNA resembles proteobacteria

• Mitochondria generate ATP for host cell

Support and Protection Structures

• Diatoms → silicon dioxide shells

• Dinoflagellates → cellulose plates

• Foraminifera → calcium carbonate shells

Multicellularity

Evolved independently in some protists (e.g., brown algae, slime molds).