Algae: Features, Types and Life Cycle

Algae are chlorophyll-bearing, simple, thalloid, autotrophic, and largely aquatic (both fresh water and marine) organisms. In the biological hierarchy, they belong to the Kingdom Plantae, though they lack true roots, stems, or leaves.

General Features

• Habitat: They occur in a variety of habitats: moist stones, soils, and wood. Some also occur in association with fungi (Lichen) and animals (e.g., on sloth bear).
• Structure: The size ranges from microscopic unicellular forms like Chlamydomonas to colonial forms like Volvox and filamentous forms like Ulothrix and Spirogyra. Marine forms such as Kelps form massive plant bodies.
• Nutrition: Strictly photoautotrophic; they contain chlorophyll a and other accessory pigments for photosynthesis.
• Cell Wall: Composed of cellulose, galactans, mannans, and minerals like calcium carbonate.

Classification of Algae

Algae are divided into three main classes based on their primary pigment and stored food materials.

1. Chlorophyceae (Green Algae)

• The plant body may be unicellular, colonial, or filamentous.
• Chloroplasts may be discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon-shaped in different species.
• Most members have one or more storage bodies called pyrenoids located in the chloroplasts. Pyrenoids contain protein besides starch.
• Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, and Chara.

2. Phaeophyceae (Brown Algae)

• Found primarily in marine habitats. They show great variation in size and form.
• They possess chlorophyll a, c, carotenoids, and xanthophylls. They vary in color from olive green to shades of brown depending upon the amount of the xanthophyll pigment, fucoxanthin.
• The plant body is usually attached to the substratum by a holdfast, and has a stalk, the stipe, and leaf-like photosynthetic organ, the frond.
• Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, and Fucus.

3. Rhodophyceae (Red Algae)

• Predominantly marine with greater concentrations found in the warmer areas. They occur in both well-lighted regions close to the surface and at great depths in oceans where relatively little light penetrates.
• The red color is due to the predominance of the red pigment, r-phycoerythrin.
• The food is stored as floridean starch which is very similar to amylopectin and glycogen in structure.
• Examples: Polysiphonia, Porphyra, Gracilaria, and Gelidium.

Life Cycles in Algae

Algae exhibit three distinct types of life cycles, involving the alternation of generations between a haploid gametophyte and a diploid sporophyte.

1. Haplontic Life Cycle

• The dominant, free-living phase is the haploid gametophyte.
• The diploid stage is represented only by the single-celled zygote. There are no free-living sporophytes.
• Meiosis in the zygote results in the formation of haploid spores.
• Examples: Volvox, Spirogyra, and Chlamydomonas.

2. Diplontic Life Cycle

• The diploid sporophyte is the dominant, photosynthetic, independent phase of the plant.
• The gametophytic phase is represented by the single to few-celled haploid gametes.
• Example: Fucus (a brown alga).

3. Haplo-diplontic Life Cycle

• Both phases are multicellular and often free-living. They differ in their dominant phases.
• Examples: Ectocarpus, Polysiphonia, and Kelps.

Economic Importance of Algae

Algae play a critical role in the environment and human industry, a common theme in UPSC Prelims.

Ecological Significance

• Carbon Fixation: At least half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis.
• Primary Producers: They are the primary producers of energy-rich compounds which form the basis of the food cycles of all aquatic animals.
• Oxygen Production: They increase the level of dissolved oxygen in their immediate environment.

Commercial Products

• Hydrocolloids: Certain marine brown and red algae produce large amounts of water-holding substances. Algin (brown algae) and Carrageen (red algae) are used commercially.
• Agar: Obtained from Gelidium and Gracilaria, it is used to grow microbes and in preparations of ice-creams and jellies.
• Food Supplements: Chlorella and Spirulina are unicellular algae rich in proteins and are used as food supplements even by space travelers.
• Edible Species: Many species of Porphyra, Laminaria, and Sargassum are among the 70 species of marine algae used as food.

Quick Facts for UPSC Prelims

• Iodine Source: Laminaria and Fucus are significant sources of iodine.
• Space Algae: Chlorella is often called “Space Algae” due to its use in oxygen regeneration and food supply in space missions.
• Water Blooms: Excessive growth of algae (especially blue-green algae, though classified as Cyanobacteria) in nutrient-rich water bodies leads to Eutrophication and oxygen depletion.
• Non-Motile Gametes: Red algae are unique as they do not produce motile (flagellated) stages in their life cycle.