Unit 2: Algae

1. General characteristics

Definition: Algae are a large, diverse group of eukaryotic (mostly) or prokaryotic (Cyanobacteria), photosynthetic (autotrophic) organisms. They are thallophytes, meaning their body (thallus) is simple and not differentiated into true roots, stems, or leaves. The study of algae is Phycology.

• Habitat: Primarily aquatic (freshwater and marine), but also found in diverse habitats (soil, snow, tree bark).
• Nutrition: Photosynthetic. They are the primary producers in aquatic ecosystems.
• Pigments: All contain Chlorophyll-a, plus a variety of accessory pigments (Chl-b, c, d, carotenoids, phycobilins) which are used for classification.
• Cell Wall: Usually present, composed of cellulose, algin, carrageenan, or silica.
• Reserve Food: Stored as polysaccharides (e.g., starch, laminarin, floridean starch) or oils.
• Reproductive Organs: Typically unicellular. When multicellular, they lack a protective jacket of sterile cells (unlike in Bryophytes).

2. Classification

Algae are classified into major divisions based on a combination of key features:

1. Photosynthetic Pigments: The type of chlorophylls and accessory pigments.
2. Reserve Food: The chemical nature of the stored food.
3. Flagella: The number, type (whiplash, tinsel), and insertion (apical, lateral) of flagella.
4. Cell Wall Composition: The main chemical components.

3. Ecology and occurrence

Algae are found almost everywhere on Earth with light and moisture.

• Aquatic Algae:
  • Plankton (Phytoplankton): Microscopic algae that float in the upper layers of freshwater (limnoplankton) and marine water (haloplankton). E.g., Diatoms, *Chlamydomonas*.
  • Benthic: Algae attached to the bottom surface (rocks, mud). E.g., Seaweeds (*Laminaria*), *Chara*.
• Terrestrial Algae (Edaphic):
  • Algae growing on or in soil. They help in soil binding and fertility. E.g., *Nostoc*, *Vaucheria*.
• Algae of Unusual Habitats:
  • Cryophytes: Grow on snow and ice. E.g., *Chlamydomonas nivalis* (causes "red snow").
  • Thermophytes: Grow in hot springs. E.g., *Oscillatoria* (a Cyanobacterium).
  • Epiphytes: Grow on other plants. E.g., *Oedogonium*.
  • Epizoophytes: Grow on animals. E.g., Algae on a turtle's shell.
  • Symbionts:
    • Lichens: Alga + Fungus.
    • Inside animals: *Zoochlorella* inside *Hydra*.

4. Economic importance

Positive Importance (Beneficial)

• Primary Producers: Algae are the "grass of the sea." They form the base of all aquatic food chains and produce 50-70% of the Earth's oxygen via photosynthesis.
• Human Food:
  • Seaweeds: Rich in vitamins and minerals. E.g., *Porphyra* (Nori), *Laminaria* (Kombu), *Chondrus*.
  • Single-Cell Protein (SCP): *Chlorella* and *Spirulina* are cultivated as health supplements, rich in protein.
• Industrial Products (Phycocolloids):
  • Agar: From red algae (*Gelidium*, *Gracilaria*). Used as a gelling agent for microbiological media, and in food.
  • Carrageenan: From red algae (*Chondrus*). Used as a thickener in ice cream, toothpaste, and cosmetics.
  • Alginates: From brown algae (*Laminaria*). Used as a thickener, stabilizer, and emulsifier in foods (ice cream) and paints.
• Diatomaceous Earth: Fossilized cell walls (frustules) of diatoms. A gritty powder used in filters (for pools, sugar), as a mild abrasive (toothpaste), and as a natural insecticide.
• Agriculture:
  • Biofertilizers: Nitrogen-fixing blue-green algae (e.g., *Nostoc*, *Anabaena*) are used in rice paddies.
  • Fodder: Seaweeds are used as animal feed.
• Research: *Chlamydomonas* and *Chlorella* are used as model organisms in genetic and photosynthetic research.
• Biofuels: Algae are being researched for producing biodiesel, as many species store large amounts of oil.

Negative Importance (Harmful)

• Algal Blooms: Eutrophication (nutrient pollution from sewage/fertilizer) causes a rapid, dense growth of algae.
• Harmful Effects:
  • When the algae die, their decomposition by bacteria consumes all the dissolved oxygen, creating "dead zones" that kill fish.
  • Some blooms (e.g., dinoflagellates, cyanobacteria) produce toxins that are harmful to fish, wildlife, and humans ("Red Tides").

5. Range of thallus organization

Algal bodies show a great diversity in form, from single cells to complex structures.

1. Unicellular: The entire organism is a single cell.
   • Motile: Have flagella (e.g., *Chlamydomonas*).
   • Non-motile (Coccoid): Spherical, no flagella (e.g., *Chlorella*).
2. Colonial: A collection of individual cells.
   • Coenobium: A colony with a fixed number of cells and a definite shape (e.g., *Volvox*).
   • Aggregate: Irregular clumps of cells in a gelatinous matrix (e.g., *Tetraspora*).
3. Filamentous: Cells are arranged in a chain.
   • Unbranched: A single, simple thread (e.g., *Spirogyra*, *Oedogonium*).
   • Branched: The filament shows branching (e.g., *Cladophora*).
   • Heterotrichous: A complex form with both a prostrate (creeping) and an erect (upright) system (e.g., *Coleochaete*).
4. Siphonaceous (Coenocytic):
   • The thallus is a single, large, multinucleate cell (a coenocyte) without cross-walls. E.g., *Vaucheria*.
5. Parenchymatous:
   • A complex, tissue-like thallus formed by cell division in multiple planes. E.g., *Ulva* (sea lettuce), *Laminaria*.

6. Reproduction in Chlorophyta, Phaeophyta and Rhodophyta

This covers the types of sexual reproduction, which is a key evolutionary feature.

Chlorophyta (Green Algae)

Green algae show the entire range of sexual reproduction, representing an evolutionary progression.

• Isogamy: Fusion of two gametes that are morphologically identical (same size and shape) but physiologically different (+ and - strains).
  • Example: *Spirogyra* (by conjugation), *Chlamydomonas* (simpler species).
• Anisogamy: Fusion of two gametes that are motile but differ in size. The female gamete (macrogamete) is larger than the male gamete (microgamete).
  • Example: Some species of *Chlamydomonas*.
• Oogamy: The most advanced form. Fusion of a large, non-motile female gamete (egg) with a small, motile male gamete (sperm/antherozoid).
  • Example: *Oedogonium*, *Volvox*, *Coleochaete*.

Phaeophyta (Brown Algae)

Brown algae also show a progression, but most are oogamous.

• Isogamy: Seen in simpler forms (e.g., *Ectocarpus*).
• Anisogamy: Seen in some intermediate forms.
• Oogamy: The dominant and most common method. Found in all the advanced, large seaweeds.
  • Example: *Laminaria*, *Fucus*, *Sargassum*.

Rhodophyta (Red Algae)

Red algae have a unique and highly specialized form of oogamy. Flagella are completely absent in all stages.

• Advanced Oogamy:
  • The male gamete is a non-motile spermatium. It is passively carried by water currents.
  • The female organ is a carpogonium, which has a long, sticky, receptive neck called a trichogyne to catch the spermatium.
  • Fertilization leads to a complex post-fertilization development, often resulting in a triphasic life cycle (Gametophyte -> Carposporophyte -> Tetrasporophyte).
  • Example: *Polysiphonia*.

7. Significant contributions of Phycologists

F.E. Fritsch (Felix Eugen Fritsch)

• A British phycologist, often called the "Father of Modern Phycology."
• His most famous work is the two-volume book, "The Structure and Reproduction of the Algae" (1935, 1945).
• This book became the "bible" for phycologists, compiling all known information about algae at the time.
• He proposed a comprehensive classification system for algae (Fritsch classification) based on morphology, pigments, and reproduction, dividing algae into 11 classes. This system was the standard for decades.

G. M. Smith (Gilbert Morgan Smith)

• An American phycologist.
• He published "The Fresh-water Algae of the United States" (1933, 1950), a standard reference for identifying American freshwater algae.
• He also published "Cryptogamic Botany" (Vol. 1 on Algae and Fungi), which was a widely used textbook.
• He is well-known for his research on the morphology and reproduction of the green algae, especially the colonial forms (Volvocales).

M.O.P. Iyenger (Mandayam Osuri Parthasarathy Iyengar)

• An Indian phycologist, revered as the "Father of Indian Phycology" or "Father of Algology in India."
• He established a major center for algological research at the University of Madras.
• He discovered and described many new genera and species of algae from India, particularly from the Chlorophyta.
• He did pioneering work on the morphology, cytology, and life-cycles of many green algae, including *Coleochaete* and *Oedogonium*. He also established the order Conjugales (for *Spirogyra*, etc.).