Green Algae Chlorophyceae: Structure, Reproduction, Uses

Green algae, the class Chlorophyceae, are a group of mostly freshwater algae coloured green by the same pigments found in land plants: chlorophyll a and chlorophyll b. They store food as starch and have cellulose walls, making them the closest algal relatives of land plants.

Green algae matter because they are major primary producers in ponds, lakes and rivers, forming the base of freshwater food chains. Species like Chlamydomonas and Spirogyra are classic laboratory organisms for studying photosynthesis and reproduction.

Key Concepts

Green algae show a remarkable range of body forms. Chlamydomonas is unicellular and swims with two flagella. Volvox forms hollow spherical colonies of thousands of cells. Spirogyra and Ulothrix are unbranched filaments, while Ulva (sea lettuce) forms flat sheets. Spirogyra is famous for its spiral, ribbon-shaped chloroplasts, one or more per cell.

• Zoospore: a motile asexual spore with two or four flagella, produced in favourable conditions.
• Isogamy: sexual fusion of two similar-sized gametes, as in Chlamydomonas.
• Anisogamy: fusion of two different-sized motile gametes.
• Oogamy: fusion of a large non-motile egg with a small motile sperm, as in Volvox.
• Conjugation: Spirogyra's sexual process, where contents of one cell flow through a tube into a neighbouring cell and fuse.

How It Works

Green algae reproduce in three ways. In vegetative reproduction, filaments simply break into fragments, each growing into a new filament. In asexual reproduction, cells produce zoospores that swim away, settle and grow into new individuals. In sexual reproduction, gametes fuse to form a thick-walled zygospore, which survives harsh conditions and later divides by meiosis to release new haploid cells. Like plants, green algae photosynthesise: 6CO2 + 6H2O + light energy gives C6H12O6 + 6O2, storing energy as starch.

Worked Example

Using the photosynthesis equation, how much glucose can green algae produce from 264 grams of carbon dioxide?

1. Molar mass of CO2 = 12 + (2 x 16) = 44 g/mol. Moles of CO2 = 264 / 44 = 6 mol.
2. The equation shows 6 mol of CO2 produce 1 mol of glucose (C6H12O6).
3. Molar mass of glucose = (6 x 12) + (12 x 1) + (6 x 16) = 72 + 12 + 96 = 180 g/mol.

Answer: 1 mol of glucose = 180 grams. This is the same chemistry land plants use, which is why green algae are called the ancestors of plants.

Common Misconceptions

• Misconception: All green algae are single-celled. Correction: They range from single cells (Chlamydomonas) to colonies (Volvox), filaments (Spirogyra) and leafy sheets (Ulva).
• Misconception: Green algae are true plants. Correction: They share pigments and storage products with plants but lack true roots, stems and leaves, and most live in water.
• Misconception: Algae only grow in dirty water. Correction: Many green algae thrive in clean freshwater and are signs of a healthy pond ecosystem.

Real-World Applications

• Chlorella is grown as a protein-rich health supplement and studied as a future food source.
• Green algae are researched for biofuels, since they grow fast and store energy-rich oils and starch.
• Ulva (sea lettuce) is eaten as food in several coastal regions.
• They are used in sewage treatment ponds, where they absorb nutrients and release oxygen.

Green algae (Chlorophyceae) are freshwater organisms with chlorophyll a and b, starch storage and cellulose walls, appearing as single cells, colonies, filaments or sheets. They reproduce vegetatively, asexually by zoospores, and sexually from isogamy to oogamy. Their plant-like chemistry makes them vital food producers and promising sources of food, fuel and clean water.