Lecture Notes on Chlamydomonas

Introduction to Green Algae

• Green algae are interesting due to their relation to green plants.
• Focus on Chlamydomonas: simplest unicellular motile green algae.
• Green ponds and lakes appear green due to the presence of Chlamydomonas.

Habitat of Chlamydomonas

• Predominantly found in freshwater (ponds, lakes).
• Can thrive in unusual habitats (e.g., snow and ice).
  • Example: Chlamydomonas nivellus gives red color to snow in the Alps due to released spores.
• Prefers waters rich in ammonium compounds.

Structure of Chlamydomonas Cell

• Shape: Pear-shaped or droplet-shaped structure.
• Cell Wall: Made of glycoproteins (no cellulose).
• Flagella:
  • Two flagella of equal length (whiplash type: broad at base, tapering at the end).
  • Arise from basal granules (also called blepharoplast).
• Apical Papilla: Protoplasmic papilla at the anterior end.
• Organelles:
  • Centrally placed nucleus.
  • Large cup-shaped chloroplast (site of photosynthesis).
  • Two contractile vacuoles for osmoregulation (balance of water and minerals).
  • Presence of usual organelles like mitochondria, Golgi apparatus, ER.

Role of Organelles

• Contractile Vacuoles:
  • Help maintain water balance (osmoregulation).
  • Excretes excess water.
• Chloroplast:
  • Contains pyrenoid (site for starch synthesis and storage).
  • Eye spot (stigma): Light-sensitive organ enabling movement towards light.

Reproduction in Chlamydomonas

Asexual Reproduction

• Can occur in four ways:
  1. Flagellated Zoospores: Formed under favorable conditions (motile).
  2. Aplanospores: Thin-walled, non-motile.
  3. Hypnospores: Thick-walled, non-motile, resting spores (red due to hematochrome).
  4. Parmala Stage Formation:
     • Occurs under unfavorable conditions (withdrawal of flagella, cell division, forms non-motile daughter cells in gelatinous matrix).
     • When conditions improve, daughter cells develop flagella and emerge.

Sexual Reproduction

• Can occur by:
  • Isogamy: Gametes are similar in shape and size.
  • Anisogamy: Gametes are unequal.
  • Oogamy: Larger non-motile female and smaller motile male gametes.
  • Hologamy: Vegetative cells act as gametes.
• Fertilization: Fuses to form diploid zygote, which undergoes meiosis to produce haploid zoospores.
• Chlamydomonas is predominantly haploid; diploid stage is only the zygote.

Summary of Key Points

• Chlamydomonas: simplest unicellular motile green alga found in freshwater.
• Cell Structure: Pear-shaped with two flagella, contractile vacuoles, and cup-shaped chloroplast with pyrenoid.
• Reproduction:
  • Asexual methods include zoospores, aplanospores, hypnospores, and parmala stage.
  • Sexual reproduction includes isogamy, anisogamy, oogamy, and hologamy resulting in haploid spores from a diploid zygote.

Conclusion

• Chlamydomonas showcases fascinating survival strategies and contributes significantly to the world of green algae.