Unit 1: Algae

1. General characteristics

Definition: Algae (singular: alga) are a large, diverse group of simple, typically autotrophic (photosynthetic) organisms. They are "plant-like" but lack true roots, stems, leaves, and complex reproductive organs like flowers. The study of algae is called Phycology.

• Habitat: Primarily aquatic (both freshwater and marine), but also found in moist terrestrial habitats (soil, tree bark).
• Thallus: Their body is a simple structure called a thallus, which is not differentiated into true roots, stems, or leaves.
• Nutrition: Overwhelmingly photosynthetic (autotrophic), containing chlorophyll-a and other pigments. They are the primary producers in aquatic ecosystems.
• Cell Type: Can be prokaryotic (e.g., Cyanobacteria or blue-green algae) or eukaryotic (all other algal groups).
• Reproductive Organs: Simple, often unicellular. If multicellular (like in brown algae), all cells of the reproductive structure are fertile, and there is no protective jacket of sterile cells.
• Life Cycle: Show a wide variety of life cycles, including haplontic, diplontic, and haplodiplontic (alternation of generations).

2. Ecology and distribution

Algae are ubiquitous and can be found in almost any environment containing light and moisture.

• Aquatic Habitats (Main):
  • Phytoplankton: Free-floating, microscopic algae in freshwater (limnoplankton) and marine (haloplankton) environments. They form the base of the aquatic food web.
  • Benthic: Algae attached to surfaces (rocks, sand, other plants) at the bottom of a body of water (e.g., seaweeds).
• Terrestrial Habitats:
  • Edaphic: Algae growing on or in the soil (e.g., *Nostoc*, *Vaucheria*).
  • Epiphytic: Growing on the surface of other plants (e.g., *Oedogonium* on larger aquatic plants).
  • Epizoic: Growing on the bodies of animals (e.g., algae on a turtle's shell).
  • Lithophytic: Growing on rocks (e.g., *Coleochaete*).
• Unusual Habitats (Extremophiles):
  • Cryophytic: Algae growing in snow and ice, causing "red snow" (e.g., *Chlamydomonas nivalis*).
  • Thermophilic: Algae growing in hot springs (e.g., Cyanobacteria like *Oscillatoria*).
  • Halophilic: Algae growing in high-salt environments like salt lakes (e.g., *Dunaliella salina*).
• Symbiotic Habitats:
  • Lichens: A mutualistic association between an alga (phycobiont) and a fungus (mycobiont).
  • Endosymbionts: Living inside other organisms (e.g., *Chlorella* inside *Hydra* or *Paramecium*).

3. Range of thallus organization

Algal bodies show a vast range of complexity, from single cells to large, differentiated structures.

1. Unicellular: The entire organism is a single cell.
   • Motile: Have flagella for movement (e.g., *Chlamydomonas*).
   • Non-motile (Coccoid): Spherical, lack flagella (e.g., *Chlorella*).
2. Colonial: A collection of individual cells held together, often in a gelatinous matrix.
   • Coenobium (Definite): A colony with a fixed number of cells arranged in a specific way (e.g., *Volvox*, *Pandorina*).
   • Aggregate (Indefinite): Irregular clumps of cells (e.g., *Tetraspora*).
3. Filamentous: Cells are arranged in a chain or thread (filament).
   • Unbranched: A single, simple chain (e.g., *Spirogyra*, *Oedogonium*).
   • Branched: The filament shows branching (e.g., *Cladophora*).
   • Heterotrichous: A complex form with two parts: a prostrate (creeping) system and an erect (upright) branched system (e.g., *Coleochaete*).
4. Siphonaceous (Coenocytic): The thallus is a single, large, multinucleate cell without cross-walls (septa). It can be large and branched (e.g., *Vaucheria*).
5. Parenchymatous: A thallus formed by the division of cells in multiple planes, creating a solid, tissue-like structure (e.g., *Ulva* - the sea lettuce).
6. Pseudoparenchymatous: A thallus that *looks* parenchymatous but is actually formed by the aggregation and interweaving of many filaments (e.g., *Polysiphonia*).

4. Cell structure

Algal cell structure is divided into two main types:

Prokaryotic Cell (Cyanophyta/Blue-Green Algae)

• Nucleus: No true, membrane-bound nucleus. The genetic material (DNA) is in a region called the nucleoid.
• Organelles: No membrane-bound organelles (no chloroplasts, mitochondria, ER).
• Photosynthesis: Occurs on thylakoids that lie free in the cytoplasm.
• Pigments: Chlorophyll-a and Phycobilins (phycocyanin, phycoerythrin).
• Cell Wall: Composed of peptidoglycan (like bacteria). Often surrounded by a gelatinous mucilage sheath.

Eukaryotic Cell (All other algae)

• Nucleus: A true, membrane-bound nucleus containing the chromosomes.
• Organelles: All standard membrane-bound organelles are present.
• Chloroplast: The site of photosynthesis. It is membrane-bound and contains the thylakoids.
  • Shape: Varies greatly and is a key identification feature (e.g., cup-shaped in *Chlamydomonas*, spiral in *Spirogyra*, reticulate (net-like) in *Oedogonium*).
  • Pyrenoid: A dense, protein-rich body found within the chloroplast of many algae (especially green algae). It is the site of starch synthesis.
• Cell Wall: Located outside the plasma membrane. Its composition varies by group but is often made of cellulose, algin, or silica.
• Motility: If motile, they possess flagella (typically 9+2 microtubule arrangement).
• Eyespot (Stigma): A light-sensitive pigmented spot, allowing the alga to sense light and move towards it (phototaxis).

5. Pigments

Photosynthetic pigments are the primary basis for classifying algal divisions. All photosynthetic algae have Chlorophyll-a.

6. Reserve food

The primary product of photosynthesis is sugar, which is stored as a specific polysaccharide. The type of storage product is another key taxonomic feature.

7. Reproduction (vegetative, asexual, sexual)

Algae exhibit all three major types of reproduction.

1. Vegetative Reproduction

Simple multiplication of the thallus without any special spores or gametes.

• Fragmentation: The thallus breaks into two or more pieces, and each piece grows into a new individual (e.g., *Spirogyra*).
• Cell Division (Fission): In unicellular algae, the cell simply divides into two (mitosis) (e.g., *Chlamydomonas*).
• Hormogonia: Short, motile fragments of a filament that break off and form new filaments (e.g., *Nostoc*).
• Tubers/Bulbils: Small, food-filled structures that can survive unfavorable conditions.

2. Asexual Reproduction

Involves the formation of specialized spores that can germinate into a new plant directly.

• Zoospores: Motile (flagellated) spores. Very common in aquatic algae (e.g., *Oedogonium*, *Vaucheria*).
• Aplanospores: Non-motile (non-flagellated) spores with a thin wall.
• Hypnospores: Thick-walled aplanospores formed to survive long periods of drought or extreme temperatures.
• Akinetes: Thick-walled, food-filled vegetative cells (not spores) that rest, common in Cyanophyta (e.g., *Nostoc*).

3. Sexual Reproduction

Involves the fusion of two gametes (syngamy) to form a zygote. This introduces genetic variation.

• Isogamy: Fusion of two gametes that are identical in size, shape, and structure (morphologically similar). They are just designated as + and - strains (e.g., *Spirogyra* - by conjugation).
• Anisogamy: Fusion of two gametes that are motile but differ in size. The female gamete (macrogamete) is larger than the male gamete (microgamete).
• Oogamy: The most advanced form. Fusion of a large, non-motile female gamete (egg or ovum) with a small, motile male gamete (sperm or antherozoid) (e.g., *Oedogonium*, *Vaucheria*, *Polysiphonia*).

8. Salient features and classification of Lee (only upto groups)

Robert Edward Lee (2008, in "Phycology") proposed a classification based on ultrastructure, biochemistry, and molecular data. It classifies algae into several "groups" or "divisions".

Salient Features Used by Lee:

• Cell Structure: Prokaryotic vs. Eukaryotic.
• Photosynthetic Pigments: The specific types of chlorophylls, carotenoids, and phycobilins.
• Reserve Food Product: The chemical nature of the storage polysaccharide.
• Flagellar Apparatus: The number, type (whiplash, tinsel), and insertion point of flagella.
• Chloroplast Ultrastructure: The arrangement of thylakoids and the number of membranes surrounding the chloroplast (e.g., 2 membranes = primary endosymbiosis; 4 membranes = secondary endosymbiosis).

Outline of Lee's Classification (Major Groups/Divisions):

1. Group: Prokaryotic Algae
   • Division: Cyanophyta (Blue-green algae)
   • Division: Prochlorophyta (e.g., *Prochloron*. Have Chl-a & b, but are prokaryotic)
2. Group: Eukaryotic Algae (with chloroplasts from primary endosymbiosis)
   • Division: Glaucophyta (Retain a peptidoglycan wall in their chloroplast, called a "cyanelle")
   • Division: Rhodophyta (Red algae. No flagella, have phycobilins)
   • Division: Chlorophyta (Green algae. Chl-a & b, starch)
3. Group: Eukaryotic Algae (with chloroplasts from secondary endosymbiosis)
   • Division: Euglenophyta (Pellicle, Chl-a & b, paramylon)
   • Division: Dinophyta (Dinoflagellates. Two dissimilar flagella, peridinin)
   • Division: Cryptophyta (Have a "nucleomorph", proteinaceous periplast)
   • Division: Heterokontophyta (Also called Stramenopiles. Have two different flagella - one tinsel, one whiplash. This large group includes:)
     • Class: Phaeophyceae (Brown algae)
     • Class: Xanthophyceae (Yellow-green algae)
     • Class: Bacillariophyceae (Diatoms)
   • Division: Haptophyta (Have a "haptonema", a peg-like appendage)

9. Significant contributions of important Phycologists

1. 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.

2. 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).

3. M.O.P. Iyengar (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.).

10. Role of algae in environment, agriculture, biotechnology and industry

Environment

• Primary Production: Algae (especially phytoplankton) are the base of most aquatic food webs.
• Oxygen Production: They perform 50-70% of all global photosynthesis, producing the majority of the Earth's oxygen.
• Carbon Sequestration: They fix vast amounts of atmospheric CO2, helping to regulate the global climate.
• Symbiosis: They form critical symbiotic relationships (e.g., lichens, corals).
• Harmful Algal Blooms (HABs): Eutrophication can lead to "blooms" of algae (e.g., red tides) that deplete oxygen and release toxins, killing fish.

Agriculture

• Biofertilizers: Nitrogen-fixing Cyanobacteria (e.g., *Nostoc*, *Anabaena*) are used to enrich the soil, especially in rice paddies.
• Soil Conditioners: Seaweed extracts and calcified algae (lithothamnion) are used to improve soil structure and water retention.
• Animal Feed: Seaweeds and microalgae (*Chlorella*, *Spirulina*) are cultivated as a nutrient-rich supplement for livestock and aquaculture.

Biotechnology and Industry

• Phycocolloids (Gelling Agents):
  • Agar: From red algae (*Gelidium*, *Gracilaria*). Used as a gelling agent for microbiological culture media, and in food (jellies).
  • Carrageenan: From red algae (*Chondrus crispus*). Used as a thickener and stabilizer in dairy products (ice cream, chocolate milk), toothpaste.
  • Alginates (Algin): From brown algae (*Macrocystis*, *Laminaria*). Used as a thickener in foods, paints, and in dental impressions.
• Food Source (Human):
  • Seaweeds: *Porphyra* (Nori), *Laminaria* (Kombu), *Undaria* (Wakame) are major food staples, especially in Asia.
  • Microalgae: *Spirulina* and *Chlorella* are grown as "superfoods" or health supplements due to their high protein and vitamin content.
• Other Products:
  • Diatomaceous Earth: Fossilized cell walls of diatoms. Used in filters (swimming pools, sugar refining), as a mild abrasive (toothpaste), and as an insecticide.
  • Biofuels: Algae are being heavily researched for producing biodiesel and bioethanol, as they grow fast and produce high amounts of oil.
  • Pigments: Natural pigments like β-carotene (from *Dunaliella*) and phycocyanin (from *Spirulina*) are used as food colorants.