Algae Notes

Algae Overview

Algae are photosynthetic organisms, both prokaryotic and eukaryotic, crucial in plant evolution. They lack specialized structures like roots, stems, and leaves, and do not form embryos, reproducing via gamete fusion in open water.

Habitat

Algae are mostly aquatic, found in marine and freshwater environments, either floating (planktonic) or attached (benthic). Some terrestrial species exist in wet soils with mechanisms to survive drying.

Forms

Algae occur as unicells (with or without flagella), filamentous chains, colonial structures, coenobium (fixed number of cells), siphonaceous (tubular, multinucleate), and parenchymatous (large, multicellular).

Reproduction

Algae reproduce sexually and asexually. Asexual reproduction occurs through fragmentation and spore formation. Sexual reproduction involves:

  • Isogamy: fusion of equal-sized motile gametes.
  • Anisogamy: fusion of unequal gametes (small male, large female, both motile).
  • Oogamy: fusion of small, motile male gamete with a large, stationary female gamete.

Major Groups of Algae

  1. Blue-green algae (Cyanophyta/Cyanobacteria)
  2. Green algae (Chlorophyta)
  3. Diatoms
  4. Euglenoids
  5. Brown algae
  6. Red algae

Blue-Green Algae (Cyanobacteria)

  • Oldest group of oxygen-producing photosynthetic organisms.
  • Prokaryotic; lack organelles and chloroplasts.
  • Contain chlorophyll ‘a’ and phycocyanin.
  • Cell wall similar to bacteria (murein).
  • Asexual reproduction via mitosis or fragmentation.
  • Body form: unicells, colonies, or filaments with differentiated cells (vegetative, akinetes, heterocysts).
  • Important in nitrogen fixation.

Green Algae (Chlorophyta)

  • Largest, most common, and diverse group, especially in freshwaters.
  • Contain chlorophyll a & b and carotenoids.
  • Cell wall mainly cellulose; store starch.
  • Exhibit both asexual and sexual reproduction with alternation of generations.

Other Algae Types

  • Diatoms: Have silica cell walls.
  • Euglenoids: Unicellular with flagella and a soft cell covering.
  • Brown and Red Algae: Mostly marine macroalgae (seaweeds) with chlorophyll a & c (brown) or a & d (red).

Ecological and Economic Importance

  1. Primary producers: contribute 50-60% of primary production in aquatic environments.
  2. Nitrogen fixers (blue-greens).
  3. Some are toxic, forming harmful algal blooms.
  4. Used as food or fertilizers.
  5. Source of useful compounds (auxins, gibberellins, vitamins, proteins, fibre).
  6. Source of Agar & carrageenans (thickening agents).
  7. Diatomaceous earth: used in water filters, beer filtration, reflective road signs, and abrasives.

Green Algae as Ancestors of Land Plants

  • Charales (Green Algae) are considered ancestors of land plants.
  • Evidence includes:
    1. Biochemical: starch, cellulose, chlorophyll a & b, oxygen production.
    2. Molecular: similar rRNA & DNA sequences, peroxisome contents, mitosis and cytokinesis.
    3. Structural: plasmodesmata, egg retention, chloroplast structure (grana), branching from the tip.