bio2106-2(a)

Lecture 2: Chlorophyta Introduction

  • What are Chlorophyta?
    • Chlorophyta are microorganisms that can manufacture energy from sunlight.
    • Commonly referred to as green algae.

General Characteristics

  • Types of Green Algae:
    • Unicellular: Single cell.
    • Multicellular: Multiple cells.
    • Colonial: Loose aggregation of cells.
    • Coenocytic: One large cell without cross-walls, can be uninucleate or multinucleate.
  • Cellular Features:
    • Possess membrane-bound chloroplasts and nuclei.
  • Habitats:
    • Most are aquatic, predominantly found in freshwater (charophytes) and marine environments (chlorophytes).
    • Some are terrestrial, found in soil, on trees, or rocks (mostly in the Trebouxiophyceae group).
    • Some engage in symbiosis, e.g., with fungi to form lichens, and animals like the freshwater Hydra with symbiotic Chlorella species.
  • Phylum Summary:
    • Charophyta: Freshwater and terrestrial algae closely related to higher plants.
    • Chlorophyta: Marine green algae.
    • Trebouxiophyceae: Terrestrial green algae with unique characteristics.

Reproduction

  • Asexual Reproduction:
    • Methods include:
    • Fission (splitting).
    • Budding.
    • Fragmentation.
    • Zoospores (motile spores).
  • Sexual Reproduction:
    • Common and can be classified as:
    • Isogamous: Gametes are both motile and of the same size.
    • Anisogamous: Gametes are motile but differ in size (female larger).
    • Oogamous: Female gametes are non-motile and egg-like, male gametes are motile.
    • Many green algae exhibit alternation of haploid and diploid phases:
    • Haploid phases form gametangia (sexual reproductive organs).
    • Diploid phases produce zoospores via reduction division (meiosis).
    • Some organisms don’t show alternation of generations; meiosis occurs in the zygote.

Classification of Chlorophyta

  • Taxonomic Classification for Course:
    • Phylum: Chlorophyta
    • Classes:
      • Chlorophyceae
      • Prasinophyceae
      • Ulvophyceae
      • Charophyceae
  • Class Chlorophyceae:
    • Commonly referred to as green algae.
    • Approximately 429 genera and 6500 species.
    • Primarily freshwater forms (90% freshwater, 10% marine).

Features of Chlorophyceae

  1. Habitat:
    • Mostly freshwater; others are brackish or terrestrial.
  2. Thallus Structure Variations:
    • Unicellular: Motile (Chlamydomonas) and non-motile (Chlorella).
    • Colonial: (Volvox).
    • Filamentous: Branched (Cladophora) and unbranched (Spirogyra).
    • Siphonaceous (Vaucheria) and parenchymatous (Ulva).
  3. Flagella:
    • Equal in size and inserted either apically or sub-apically, exhibiting a 9+2 arrangement under electron microscopy.
  4. Cellular Composition:
    • Eukaryotic nature.
    • Typically unicellular, but coenocytic forms (Siphonales, Cladophorales) can contain many nuclei.
  5. Cell Wall Composition:
    • Primarily comprised of cellulose, including hydroxyproline glycosides, xylans, and mannans.
  6. Cellular Structure:
    • Semipermeable cell membrane surrounds the protoplast, containing many small vacuoles to push the nucleus peripherally (primordial utricle).
  7. Eye-spot or Stigma:
    • Present in flagellate cells near the chloroplast.
  8. Chloroplast Structure:
    • Contains pigments and may contain pyrenoid(s).

Order: Volvocales

  1. General Features:
    • Approximately 60 genera and 500 species, mainly freshwater forms.
    • Thallus can be unicellular or a motile colony (e.g., Volvox).
    • Characterized by flagellated motile vegetative cells with 2 or 4 flagella.
    • Cells possess large cup-shaped chloroplasts with a single pyrenoid covered by a starch plate.
  2. Reproductive Methods:
    • Asexual: Bi-flagellated zoospores, aplanospores, or palmella stage.
    • Sexual: Can be isogamous, anisogamous, or oogamous.

Species of Interest: Volvox

  • Structure:
    • Spherical colonies ranging from 500 to 40,000 cells.
    • Comprises numerous somatic cells and fewer reproductive cells.
    • Zygotes are orange.

Reproductive Processes in Volvox

Asexual Reproduction

  1. Gonidia Development:
    • Posterior cells of colony enlarge considerably (up to 10x), lose flagella, and are termed gonidia.
    • Increased number of pyrenoids in gonidia.
  2. Cell Division:
    • Initially longitudinal division to form 2 cells; subsequent divisions arrangement forming a curved plate (plakea stage).
    • Further longitudinal divisions lead to 8 cells forming a hollow sphere.
    • “Inversion of the colony” occurs as the cells shift towards the colony's exterior.
  3. Daughter Colony Formation:
    • After inversion, daughter colonies develop, initially attach to the parent colony's gelatinous wall and later become free.

Sexual Reproduction

  • Reproductive structures form mainly in the posterior part of the colony.
  • Gender Types:
    • Monoecious species: Both antheridia (male) and oogonia (female) on the same colony.
    • Dioecious species: Antheridia and oogonia develop on different colonies.
  • Male Gametangia Development:
    • Antheridial initials enlarge and differentiate, ultimately forming the antherozoid (spermatozoid).
  • Female Gametangia Development:
    • Oogonial initials differentiate into the mature ovum.
  • Fertilization: One antherozoid penetrates an oogonium, leading to zygote formation, which has a thick three-layered wall (exospore, mesospore, endospore).
  • Germination of Zygote:
    • Under favorable conditions, the diploid nucleus undergoes meiotic division forming haploid cells, and new colonies arise.

Order: Chlamydomonadales

Chlamydomonas Structure

  • Flagella:
    • Two anterior flagella that can emerge through one wall aperture or separate canals.
  • Contractile Vacuoles:
    • Two present at the base of flagella for excretory functions.
  • Eye Spot:
    • An orange-red stigma near flagella's origin, sensitive to light, directs organism's movement.
  • Chloroplast:
    • Cup-shaped with varying structures, allows for autotrophic growth through photosynthesis.
  • Pyrenoid:
    • Embedded in chloroplast, contains protein body surrounded by starch grains.
  • Nucleus:
    • Uninucleate, positioned in chloroplast's cup.

Class: Ulvophyceae

Order: Ulotrichales

  • Filament Structure:
    • Filaments that are unbranched, composed of cylindrical cells with varying wall thickness.
    • Cells contain plate-like or band-like chromatophores, with or without pyrenoids.

Ulothrix Structure and Reproduction

  • Cell Division and Reproduction:
    • Cells can divide; during sexual reproduction, biflagellate isogametes are produced and fused.
    • In asexual reproduction, quadriflagellate zoospores attach and grow.

Charophyceae

  • Significance:
    • Considered the closest extant group related to all terrestrial plants, providing a link to Chlorophyceae and non-vascular bryophytes.

Genus: Chara and Nitella

  • Chara:
    • Aquatic, grows erect; resembles a miniature horse tail, having a branched structure.
  • Nitella:
    • Similar to Chara but found in deeper waters, more branched.

Life Cycle and Reproduction in Chara

  • Types of Reproduction:
    • Vegetative (via amylum stars and bulbils) and sexual (oogamous).

Conclusion

  • Green algae demonstrate diverse forms of reproduction and development, with important ecological roles in freshwater and marine environments.
  • Classifications highlight their evolutionary relationship with terrestrial plants.
  • The study of Chlorophyta continues to provide insights into plant evolution and adaptation strategies in various habitats.