Protists: Diversity, Structure, and Evolutionary Significance

Chapter 28: Protists

Eukaryotic Origins and Endosymbiosis

Protists are a diverse group of mostly unicellular eukaryotes. Their evolutionary history is closely tied to the origin of eukaryotic cells and the process of endosymbiosis.

• Origin of Eukaryotes: Eukaryotes are thought to have originated about 2.7 billion years ago, though the oldest microfossils with clear eukaryotic features are about 1.5 billion years old.

• Internal Membranes: The nucleus and endoplasmic reticulum likely evolved from infoldings of the plasma membrane.

• Endosymbiotic Theory: Mitochondria and chloroplasts originated from free-living bacteria that were engulfed by ancestral eukaryotic cells. Mitochondria evolved from aerobic bacteria, while chloroplasts (plastids) evolved from photosynthetic bacteria. Some plastids arose via secondary endosymbiosis (engulfment of a eukaryotic alga).

• Evidence for Endosymbiosis: Mitochondria and chloroplasts contain their own DNA, which closely resembles prokaryotic DNA.

• Mitosis: Evolved in eukaryotes, with mechanisms varying among lineages.

Defining Protists

"Kingdom Protista" is considered paraphyletic, meaning it does not include all descendants of a common ancestor. Protists are grouped together for convenience, but they are highly diverse and not a true evolutionary group.

• Over 200,000 species, mostly single-celled, with diverse evolutionary relationships.

• Modern classification divides eukaryotes into four supergroups (see below).

Protist Cell Structure and Locomotion

• Cell Surfaces:

  • Some have only a plasma membrane (e.g., amoebas).

  • Others have a plasma membrane plus an extracellular matrix (ECM), such as diatoms and radiolarians, which secrete glassy silica shells.

• Locomotion:

  • Flagella or cilia (e.g., euglenoids, ciliates).

  • Pseudopodia (e.g., amoebas, radiolarians, foraminiferans).

  • Other specialized means.

Protist Nutrition and Reproduction

• Nutritional Strategies:

  • Phototrophs (photosynthetic).

  • Heterotrophs:

    • Osmotrophs: absorb soluble food.

    • Phagotrophs: ingest visible food particles into food vacuoles.

  • Mixotrophs: combine photosynthesis and heterotrophy.

• Reproduction:

  • Asexual: mitosis, budding, schizogony.

  • Sexual: meiosis and genetic recombination.

• Colonial Protists: Some form colonies, providing a possible evolutionary bridge to multicellularity.

Supergroup Excavata

Defined by cytoskeletal features and, in some, an "excavated" feeding groove. Includes several groups of flagellated protists, some lacking typical mitochondria.

Diplomonads

• Unicellular, with two nuclei and multiple flagella.

• Possess modified mitochondria called mitosomes.

• Example: Giardia intestinalis (causes diarrhea; transmitted via contaminated water).

Parabasalids

• Move with flagella and have undulating membranes.

• Reduced mitochondria called hydrogenosomes.

• Examples:

  • Trichonympha (lives in termite guts, digests cellulose).

  • Trichomonas vaginalis (causes a sexually transmitted disease in humans).

Euglenozoa

• Euglenoids:

  • Free-living, with a pocket at one end containing one or two flagella.

  • Flexible pellicle allows shape change.

  • About one-third have chloroplasts (autotrophic); others are heterotrophic.

  • Reproduce by mitosis; no known sexual reproduction.

  • Example: Euglena (photosynthetic).

• Kinetoplastids:

  • Single, large mitochondrion (kinetoplast).

  • Evade immune response by frequently changing surface proteins.

  • Examples:

    • Trypanosoma (causes African sleeping sickness, Chagas disease).

    • Leishmania (causes leishmaniasis).

Supergroup SAR (Stramenopila, Alveolata, Rhizaria)

A large, diverse clade, possibly originating via secondary endosymbiosis. Divided into three subgroups: Alveolata, Stramenopila, and Rhizaria.

Alveolata

• Characterized by membrane-bound sacs (alveoli) under the plasma membrane.

• Includes dinoflagellates, apicomplexans, and ciliates.

Dinoflagellates

• Photosynthetic unicells with two flagella in grooves of cellulose plates.

• Marine and freshwater; some are bioluminescent.

• Responsible for "red tides" (harmful algal blooms) that can produce toxins affecting marine life and humans.

• Reproduce mainly asexually.

Apicomplexans

• Spore-forming parasites of animals.

• Have an apical complex of organelles for penetrating host cells.

• Example: Plasmodium (causes malaria; transmitted by Anopheles mosquitoes).

Ciliates

• Unicellular heterotrophs with numerous cilia arranged in rows or spirals.

• Possess a pellicle, food and contractile vacuoles.

• Two types of nuclei: micronucleus (sexual reproduction) and macronucleus (cell function).

• Conjugation involves exchange of micronuclei between cells.

Stramenopila

• Characterized by "hairy" flagella (at least in some life stages).

• Includes brown algae, golden algae, diatoms, and oomycetes.

Brown Algae

• Large, multicellular seaweeds (kelps) with differentiated thallus (blade, stipe, holdfast).

• Produce algin (used as a thickener in foods).

• Exhibit alternation of generations (sporophyte and gametophyte stages).

Golden Algae

• Biflagellated, mostly unicellular (some colonial), photosynthetic or mixotrophic.

• Can form protective cysts under adverse conditions.

Diatoms

• Unicellular, photosynthetic phytoplankton with double shells of opaline silica.

• Shells (frustules) resemble a box with a lid; highly resistant to pressure.

• Deposits form diatomaceous earth (used as a filtering medium).

Oomycetes (Water Molds)

• Parasitic or saprobic; formerly classified as fungi.

• Produce motile zoospores with two unequal flagella.

• Sexual reproduction via male and female gametes.

• Examples:

  • Phytophthora infestans (causes potato late blight; responsible for the Irish potato famine).

  • Saprolegnia (fish and amphibian pathogen).

Rhizaria

• Move and feed using threadlike pseudopodia.

• Includes radiolarians, foraminiferans, and cercozoans.

Radiolarians

• Glassy silica exoskeletons with radial or bilateral symmetry.

• Pseudopodia extend through openings in the shell.

• Contribute to siliceous ooze on the seafloor.

Foraminiferans

• Heterotrophic, mostly marine; some with symbiotic algae.

• Produce tests (shells) of calcium carbonate and other materials.

• Fossilized foraminiferans form limestone deposits (e.g., White Cliffs of Dover).

• Use podia for feeding, test construction, and locomotion.

Cercozoans

• Amoeboid and flagellated protists with threadlike pseudopodia.

• Found in marine, freshwater, and soil environments.

• Most are parasites or predators.

Supergroup Archaeplastida

Includes red algae, green algae, and land plants. This group is united by the presence of primary plastids derived from cyanobacteria.

Red Algae (Rhodophyta)

• No centrioles or flagella.

• Reproduce by alternation of generations.

• Contain pigments (phycoerythrin) that give a red color.

• Example: Porphyra ("nori" used in sushi).

Green Algae

• Divided into chlorophytes and charophytes (the latter are most closely related to land plants).

• Chlorophytes include over 7,000 species, mostly freshwater but also marine and terrestrial.

• Unicellular: Chlamydomonas (simple, resembles gametes or zoospores).

• Colonial: Volvox (forms colonies), filamentous forms (pond scum).

• Multicellular: Ulva (sea lettuce; true multicellularity), Caulerpa (large "supercell" with many nuclei).

• Complex life cycles with both asexual and sexual reproduction; biflagellated gametes with cup-shaped chloroplasts.

• Ulva exhibits alternation of generations.

Supergroup Unikonta

Includes animals, fungi, and some protists. Amoebozoans are a major group within Unikonta.

Amoebozoans

• Move using lobe- or tube-shaped pseudopodia (formed by actin and myosin microfilaments).

• Can move in any direction.

Slime Molds

• Once grouped with fungi; originated independently at least three times.

• Plasmodial Slime Molds: Multinucleate, moving mass of cytoplasm (plasmodium); feeds on bacteria and organic matter; forms sporangia under adverse conditions.

• Cellular Slime Molds: Individual amoeboid cells; aggregate to form a "slug" when food is scarce.

Tubulinids (formerly Gymnamoebas)

• Large, free-living amoebas found in soil, freshwater, and saltwater.

• Heterotrophs feeding on bacteria, other protists, and detritus.

Entamoebas

• Parasitic amoebas of vertebrates and some invertebrates.

• Entamoeba histolytica causes amebic dysentery; can be transmitted via contaminated food, water, or utensils, and may cause brain abscesses.

Opisthokonts

• Includes animals, fungi, and several protist groups.

• Choanoflagellates: Most similar to the common ancestor of sponges; have a single flagellum surrounded by a collar of microvilli; feed by straining bacteria from water.

Summary Table: Major Protist Groups and Key Features

Group	Key Features	Examples	Notable Diseases/Uses
Diplomonads	Two nuclei, mitosomes, multiple flagella	Giardia intestinalis	Giardiasis (diarrhea)
Parabasalids	Hydrogenosomes, undulating membrane	Trichonympha, Trichomonas vaginalis	STD (trichomoniasis)
Euglenozoa	Flagella, pellicle, chloroplasts (some)	Euglena, Trypanosoma	Sleeping sickness, Chagas disease
Dinoflagellates	Cellulose plates, two flagella	Various marine species	Red tides, toxins
Apicomplexans	Apical complex, parasitic	Plasmodium	Malaria
Ciliates	Cilia, two nuclei, pellicle	Paramecium	Model organism
Brown Algae	Multicellular, algin, alternation of generations	Kelp	Food thickener
Diatoms	Silica shells, photosynthetic	Various planktonic species	Diatomaceous earth
Oomycetes	Water molds, motile spores	Phytophthora infestans	Potato blight
Radiolarians	Silica skeleton, pseudopodia	Marine plankton	Siliceous ooze
Foraminiferans	Calcium carbonate tests, podia	Marine plankton	Limestone deposits
Red Algae	No flagella, phycoerythrin pigment	Porphyra	Nori (sushi)
Green Algae	Chlorophyll, diverse forms	Chlamydomonas, Volvox, Ulva	Model for multicellularity
Amoebozoans	Lobe-shaped pseudopodia	Slime molds, Entamoeba	Amebic dysentery
Choanoflagellates	Collar of microvilli, single flagellum	Various aquatic species	Closest protist relatives of animals

Additional info: The classification and evolutionary relationships among protists are still being revised as new molecular and morphological data become available. Many protists play crucial ecological roles as primary producers, decomposers, and pathogens.