Protozoa and Protists '

Protozoa

General Characteristics

• Protozoa are microscopic, unicellular, heterotrophic protists.

• Classification confusion arises because some groups contain both photosynthetic and non-photosynthetic members.

  • For example, the phyla Euglenophyta (Euglinids) and Dinophyta (Dinoflagellates) have members that are photoautotrophs, leading phycologists to classify them as algae.

  • However, both groups also contain many heterotrophic members, resulting in zoologists classifying them as protozoa.

Groups of Protozoa

Major Groups of Protists (Table 20-1)

• ### Alveolates

  • Subgroups:

  • Dinoflagellates

    • Locomotion: Swim with two flagella.

    • Nutrition: Autotrophic; photosynthetic.

    • Representative Features: Many are bioluminescent; often have cellulose.

    • Representative Genus: Gonyaulax (known for causing red tide).

  • Apicomplexans

    • Locomotion: Nonmotile.

    • Nutrition: Heterotrophic.

    • Representative Features: All are parasitic and form infectious spores.

    • Representative Genus: Plasmodium (causes malaria).

  • Ciliates

    • Locomotion: Swim with cilia.

    • Nutrition: Heterotrophic.

    • Representative Features: Most complex single cells.

    • Representative Genus: Paramecium (a fast-moving pond dweller).

• ### Cercozoans

  • Subgroups:

  • Foraminifera

    • Locomotion: Extend thin pseudopods.

    • Nutrition: Heterotrophic.

    • Representative Features: Have calcium carbonate shells.

  • Radiolarians

    • Locomotion: Extend thin pseudopods.

    • Nutrition: Heterotrophic.

    • Representative Features: Have silica shells.

• ### Amoebozoans

  • Subgroups:

  • Lobose Amoebas

    • Locomotion: Extend thick pseudopods.

    • Nutrition: Heterotrophic.

    • Representative Features: Have no shells.

  • Acellular Slime Molds

    • Locomotion: Slug-like mass oozes over surfaces.

    • Nutrition: Heterotrophic.

  • Cellular Slime Molds

    • Locomotion: Amoeboid cells extend pseudopods; form a slug-like mass.

    • Nutrition: Heterotrophic.

• ### Excavates

  • Subgroups:

  • Diplomonads

    • Locomotion: Crawl over surfaces; swim with flagella.

    • Nutrition: Heterotrophic.

  • Parabasalids

    • Locomotion: Swim with flagella.

    • Nutrition: Heterotrophic.

  • Euglenozoans

    • Euglenids

    • Locomotion: Swim with one flagellum.

    • Nutrition: Autotrophic; photosynthetic.

    • Kinetoplastids

    • Locomotion: Swim with flagella.

    • Nutrition: Heterotrophic; form multinucleate plasmodium.

    • Characteristics: Lack mitochondria; inhabit soil or water or are parasitic.

Reproduction and Gene Exchange in Protozoa

• The ciliate Paramecium reproduces asexually through cell division.

• The ciliate Euplotes exchanges genetic material across a cytoplasmic bridge, illustrating a form of sexual reproduction.

Supergroup Excavata

• Name Origin: Named for a suspension-feeding groove of an excavated type.

• Feeding Mechanism: Food particles are taken into cells by phagotrophy from the feeding groove region.

• Structure: The feeding groove is composed of rods of microtubules and non-microtubule fibers.

Giardia: The Curse of Campers

• Organism: A diplomonad (genus Giardia) that can infect drinking water, leading to gastrointestinal disorders.

  • Found in the human small intestine.

• Characteristics:

  • Lacks mitochondria.

  • Moves via multiple flagella.

Euglenozoans

• Euglenids lack a rigid covering and swim using flagella.

• Characteristics:

  • Unicellular.

  • Many are photosynthetic (mixotrophy).

  • When in darkness, the green chloroplasts may disappear.

Life Strategies of Alveolates

• The Alveolates can exhibit various life strategies such as parasites, predators, and phytoplankton.

• Characteristic feature includes the presence of alveoli (sacs) located just below the cell membrane that support it.

Alveolates: Apicomplexans (Sporozoans)

• Life History: All are parasites transmitted between hosts via spores, as illustrated by the life cycle of the malaria pathogen Plasmodium.

Alveolates: Ciliates

• Ciliates, such as Paramecium, represent the most complex of the Alveolates.

• Locomotion: Via cilia.

• Key Organelles:

  • Oral Groove: Acts as a mouth.

  • Food Vacuoles:

  • Function as miniature digestive systems and form at the apex.

  • Waste is expelled via exocytosis through an anal pore.

  • Contractile Vacuoles: Regulate water balance by expelling excess water.

Foraminiferans (Forams)

• Features:

  • Chalky shells with numerous interior chambers.

  • Pseudopods extend through openings in the shells.

• Geological Impact: The shells of these organisms can accumulate to form large deposits of limestone, such as the White Cliffs of Dover.

Radiolarians

• Habitat: Beach sand, especially pink sands such as those found in Bermuda, is primarily composed of radiolarians.

• Structure: Characterized by delicate, glassy shells composed mainly of silica, and pseudopods extend through the openings for sensing and capturing food.

Amoebas

• Common Names: Often referred to as lobose amoebas.

• Habitat: Common in freshwater lakes and ponds; some are free-living, while others are parasites.

  • One notable parasitic species causes dysentery.

• Locomotion & Feeding: Move and capture prey using cytoplasmic projections known as pseudopods.

Amoebozoans

• Composition: This group includes amoebas, acellular slime molds, and cellular slime molds (Myxomycota).

• Cellular Slime Molds: Live as independent cells but aggregate into a pseudoplasmodium when food is scarce.