7.2 Major Groups of Protists

This section outlines the major groups within the Kingdom Protista, a diverse group of eukaryotic organisms. They are broadly categorized into animal-like (Protozoa), plant-like (Algae), and fungi-like (Myxomycota and Oomycota) protists based on their characteristics.

7.2.1 Protozoa: The Animal-Like Protists

Protozoa are a polyphyletic group of microscopic, single-celled organisms that exhibit animal-like behaviors such as movement and heterotrophic nutrition.

• Habitat: Found in a wide range of aquatic (freshwater, marine) and terrestrial environments (soil, decaying organic matter). Many are parasitic.
  • Freshwater examples: Amoeba, Paramecium
  • Marine examples: Radiolaria
  • Parasitic examples: Plasmodium, Entamoeba histolytica
• Structure:
  • Unicellular, consisting of a single mass of cytoplasm containing all necessary organelles.
  • Vacuoles: Specialized membrane-bound sacs.
    • Food vacuoles: Digest ingested food particles using enzymes.
    • Contractile vacuoles: Regulate water balance (osmoregulation) by expelling excess water, especially in freshwater species.
    • Storage vacuoles: Store nutrients or waste products.
  • Some, like foraminifera, possess a protective outer shell.
• Reproduction: Occurs through both asexual and sexual methods.
• Locomotion: Classified based on their method of movement.
  • Pseudopodia ("false feet"): Cytoplasmic extensions for movement and feeding (e.g., Amoeba).
  • Flagella: Long, whip-like tails (e.g., Trypanosoma).
  • Cilia: Short, hair-like structures that beat in unison (e.g., Paramecium).
  • No specific locomotion: Parasitic forms often lack motility structures (e.g., Plasmodium).
• Survival: Form resistant cysts to survive harsh, unfavorable environmental conditions.

Subgroups of Protozoa

• Amoebas:
  • Free-living or parasitic protozoa that use pseudopodia for movement and feeding.
  • Examples: Amoeba proteus (free-living), Entamoeba histolytica (parasitic, causes amoebic dysentery).

• Zooflagellates:
  • Protozoa that move using one or more flagella.
  • Example: Giardia intestinalis, a parasite causing giardiasis.

• Choanoflagellates:
  • Sessile (stalk-anchored) marine or freshwater flagellates.
  • Characterized by a single flagellum surrounded by a collar of microvilli, resembling the collar cells of sponges.

• Ciliates:
  • Protozoa covered in numerous short, hair-like cilia used for movement and feeding.
  • Examples: Paramecium, Stentor, Vorticella.

• Foraminifera and Actinopods:
  • Marine protozoa that produce protective shells called tests.
    • Foraminifera: Shells made of calcium carbonate.
    • Actinopods: Shells made of silica.

• Apicomplexans:
  • A large group of parasitic protozoa with complex life cycles.
  • Named for the specialized structures at their apex used to invade host cells.
  • Example: Plasmodium, the parasite that causes malaria.

7.2.2 Algae: The Plant-Like Protists

Algae are a diverse group of photosynthetic organisms responsible for 50-60% of global photosynthesis.

• Structure:
  • Can be unicellular, colonial, filamentous, or complex multicellular.
  • Coenocytic: Some filamentous algae have a multinucleated structure lacking cross-walls between cells.
  • Thallus: The body of a multicellular alga, which is undifferentiated and lacks true roots, stems, or leaves.
  • Cell Wall: Often contains cellulose.
• Photosynthetic Pigments: Possess chlorophyll 'a' and various accessory pigments like carotenoids, xanthophyll, and phycoerythrin.
• Reproduction: Exhibit diverse life cycles. Most have a motile, flagellated stage.

Subgroups of Algae

• Euglenoids:
  • Freshwater organisms. About one-third have chloroplasts and are photosynthetic; the others are heterotrophic (ingest/absorb food).
  • Example: Euglena.

• Dinoflagellates:
  • Mostly unicellular, with two flagella.
  • Cellulose plates impregnated with silica form a protective armor.
  • Some are bioluminescent. Some cause "red tides" (harmful algal blooms).

• Diatoms:
  • Numerous unicellular algae in marine and freshwater environments.
  • Characterized by a two-part silica cell wall (frustule) that fits together like a box.

• Brown Algae (Phaeophyta):
  • Multicellular, found in cooler marine waters.
  • Range from small filaments to giant kelps up to 75 meters long.
  • Example: Laminaria (kelp).

• Red Algae (Rhodophyta):
  • Multicellular, found primarily in warm seawater.
  • Anchored to surfaces by a holdfast.
  • Lack flagellated cells in their life cycle.

• Green Algae (Chlorophyta):
  • Believed to be ancestors of land plants due to shared characteristics (cellulose cell wall, chlorophylls a and b).
  • Exhibit great diversity in form (unicellular, colonial, filamentous, multicellular).
  • Examples: Chlamydomonas, Spirogyra, Volvox, Ulva.

7.2.4 Fungi-Like Protists

These protists are heterotrophs that absorb nutrients from dead or decaying organic matter (saprotrophs) or from living hosts (parasites).

Myxomycota: Slime Molds

• The vegetative stage is a plasmodium: a diploid, multinucleated mass of cytoplasm enveloped in a slime sheath.
• Life Cycle:
  1. Under unfavorable conditions (e.g., drought), the plasmodium differentiates to form stalked reproductive structures called sporangia.
  2. Within the sporangia, meiosis occurs, producing haploid spores.
  3. Spores are released and germinate into haploid flagellated or amoeboid cells.
  4. Two of these cells fuse to form a diploid zygote.
  5. The zygote grows and its nucleus divides repeatedly without cytokinesis to form a new multinucleated plasmodium.

Oomycota: Water Molds

• Includes water molds, white rusts, and downy mildews.
• Live as parasites or saprotrophs in aquatic or moist terrestrial environments.
• Key difference from Fungi: Cell walls are made of cellulose, not chitin.
• Structure: Composed of filamentous structures called hyphae. The hyphae are coenocytic (multinucleated and lack septa/cross-walls).
• Reproduction:
  • Asexual: Produces motile, flagellated zoospores inside a sporangium.
  • Sexual: Involves two types of gametangia: the female oogonium (contains eggs) and the male antheridium (contains male nuclei). Fertilization leads to the formation of a thick-walled, resistant zygote called an oospore.
• Example: Phytophthora infestans, the causative agent of late blight in potatoes and the Irish Potato Famine.

Possible Questions/Answers

• Q: What are the three main types of vacuoles in protozoa and their functions? A:

  1. Food Vacuole: Digests food particles.
  2. Contractile Vacuole: Regulates water balance by expelling excess water.
  3. Storage Vacuole: Stores nutrients or waste products.

• Q: How do algae differ from true plants? A: Algae typically have unicellular sex organs, do not protect their zygotes within the parent organism, and do not form embryos, unlike true plants.

• Q: What is a key difference between the cell walls of Oomycota (water molds) and true fungi? A: The cell walls of Oomycota are made primarily of cellulose, whereas the cell walls of true fungi are made of chitin.

• Q: What is a plasmodium in the context of slime molds? A: It is the main vegetative stage, consisting of a diploid, multinucleated mass of cytoplasm that moves and feeds like an amoeba.

Biological Significance: Protists represent a crucial evolutionary link between prokaryotes and multicellular eukaryotes. They are fundamental to global ecosystems as primary producers and decomposers, and they have profound direct impacts on human health, agriculture, and industry.