Unit 2: Divisions of Algae

1. Cyanophyta and Xanthophyta

General characteristics, distribution, thallus, cell, reproduction of Cyanophyta

• General Characteristics: Commonly known as blue-green algae. They are prokaryotic, lacking a true nucleus and membrane-bound organelles.
  • Pigments: Chlorophyll-a and phycobilins (C-phycocyanin - blue; C-phycoerythrin - red), which give them their characteristic color.
  • Cell Structure: Prokaryotic. Photosynthesis occurs on thylakoids free in the cytoplasm. Cell wall contains peptidoglycan.
  • Reserve Food: Cyanophycean starch (like glycogen).
  • Motility: Flagella are completely absent in all life stages. Some show gliding movement.
• Distribution: Ubiquitous. Found in freshwater, marine, and terrestrial environments. Many are extremophiles (hot springs, saline lakes).
• Range of Thallus: Unicellular (e.g., *Chroococcus*), colonial (e.g., *Microcystis*), and filamentous (e.g., *Nostoc*, *Oscillatoria*).
• Reproduction:
  • Vegetative: Cell division (unicells), fragmentation, and hormogonia (short, motile fragments of a filament).
  • Asexual: By akinetes (thick-walled, resting cells that survive harsh conditions).
  • Sexual Reproduction: True sexual reproduction is absent. Genetic recombination occurs via bacterial-like conjugation, transformation, and transduction.

General characteristics, distribution, thallus, cell, reproduction of Xanthophyta

• General Characteristics: Commonly known as yellow-green algae. They are eukaryotic.
  • Pigments: Chlorophyll-a and Chlorophyll-c. They lack fucoxanthin, and their yellow-green color comes from an excess of xanthophylls (like diatoxanthin).
  • Cell Structure: Eukaryotic. Cell wall is often composed of cellulose and pectin.
  • Reserve Food: Chrysolaminarin (Leucosin) and oils. They do not store starch.
  • Motility: Motile cells have two unequal flagella (heterokont): one long tinsel (hairy) flagellum and one short whiplash (smooth) flagellum.
• Distribution: Mostly freshwater, but also found in soil and marine environments.
• Range of Thallus: Unicellular, colonial, filamentous, and siphonaceous (coenocytic) (e.g., *Vaucheria*).
• Reproduction:
  • Vegetative: Fragmentation.
  • Asexual: By zoospores (heterokont) or aplanospores.
  • Sexual: Can be isogamous, anisogamous, or oogamous (e.g., *Vaucheria*).

Morphology and life-cycle of *Nostoc* (Cyanophyta)

Morphology:

• *Nostoc* is a colonial, filamentous blue-green alga.
• The colonies are gelatinous, spherical, or irregular "balls" (often called "star jelly" or "witch's butter").
• Inside the mucilage are numerous, long, unbranched filaments called trichomes.
• The trichome is like a string of beads, composed of two cell types:
  1. Vegetative Cells: Small, spherical, photosynthetic cells.
  2. Heterocysts: Large, thick-walled, pale-yellow, empty-looking cells. Their sole function is anaerobic nitrogen fixation (using the nitrogenase enzyme).

Life-cycle (Reproduction):

*Nostoc* has no sexual reproduction. It reproduces vegetatively and asexually.

1. Colony Fragmentation: The main colony can break apart, with each piece forming a new colony.
2. Hormogonia Formation: The trichome breaks into short, motile fragments (hormogonia) that glide out of the parent colony, settle, and grow into new filaments.
3. Akinete Formation: During unfavorable conditions (drought), vegetative cells enlarge, fill with food (cyanophycean starch), and develop thick walls. These akinetes are highly resistant resting cells. When conditions improve, they germinate to form new filaments.

Morphology and life-cycle of *Vaucheria* (Xanthophyta)

Morphology:

• The *Vaucheria* thallus is a siphonaceous filament: a long, branched, tube-like structure with no cross-walls (septa), except during reproduction.
• It is coenocytic, containing numerous nuclei and chloroplasts in a continuous cytoplasm.
• It grows on mud, damp soil ("water felt"), or in shallow freshwater.

Life-cycle (Reproduction):

*Vaucheria* reproduces asexually and sexually (oogamous).

• Asexual Reproduction:
  • Occurs when conditions are favorable.
  • The tip of a filament swells and is cut off by a septum.
  • The protoplast inside rounds up to form a single, large, multinucleate, multiflagellate zoospore called a synzoospore.
  • The synzoospore is released, swims, settles, and germinates directly into a new siphonaceous thallus.
• Sexual Reproduction (Oogamous):
  • Occurs during unfavorable conditions (e.g., changes in light, temperature).
  • The sex organs, antheridia (male) and oogonia (female), develop on the filament.
  • Oogonium: A large, spherical structure with a single, large, non-motile egg (oosphere). It is cut off by a septum.
  • Antheridium: A curved, tube-like structure that develops near the oogonium. It produces many small, biflagellate antherozoids (sperm). It is also cut off by a septum.
  • Fertilization: The antherozoids are released, and one enters the oogonium through a pore to fuse with the egg, forming a zygote (oospore).
  • The oospore develops a thick wall and becomes a resting stage. After a dormant period, it undergoes meiosis (it is the only diploid stage) and germinates into a new haploid *Vaucheria* thallus.

2. Chlorophyta

General characteristics, Ecology, Distribution, Cell structure, Reproduction

• General Characteristics: Commonly known as green algae. They are eukaryotic and are believed to be the ancestors of higher plants.
  • Pigments: Dominant pigments are Chlorophyll-a and Chlorophyll-b, plus carotenoids (β-carotene, lutein). This is the *same* pigment combination as higher plants.
  • Cell Structure: Eukaryotic. Chloroplasts have thylakoids stacked into grana and often contain pyrenoids (for starch synthesis).
  • Reserve Food: True Starch (stored inside the chloroplast).
  • Cell Wall: Composed primarily of cellulose.
  • Motility: Motile cells typically have two (or four) equal, whiplash flagella inserted at the apex.
• Ecology and Distribution: Extremely diverse. Most are freshwater (e.g., *Spirogyra*, *Volvox*), but many are marine (e.g., *Ulva*), terrestrial (e.g., *Trentepohlia*), or symbiotic (e.g., *Chlorella*).
• Reproduction: All three types are common.
  • Vegetative: Fragmentation, cell division.
  • Asexual: By zoospores, aplanospores.
  • Sexual: Shows the full range: isogamy (*Spirogyra*), anisogamy, and oogamy (*Oedogonium*, *Coleochaete*).

Morphology and life-cycle of *Oedogonium*

Morphology:

• A common, unbranched filamentous green alga found in quiet freshwater.
• The filament is attached to a substrate by a specialized basal cell called a holdfast.
• The vegetative cells are cylindrical, with a distinct reticulate (net-like) chloroplast and multiple pyrenoids.
• A unique feature is the presence of cap cells. These are apical "rings" on certain cells, which indicate that the cell has divided.

Life-cycle (Reproduction):

*Oedogonium* reproduces vegetatively, asexually, and by advanced oogamy.

• Vegetative: By fragmentation.
• Asexual Reproduction:
  • By a large, multinucleate, multiflagellate zoospore (similar to *Vaucheria*'s synzoospore but with a ring of flagella at one end).
  • It is formed within a vegetative cell, escapes, swims, settles, and germinates into a new filament.
• Sexual Reproduction (Oogamous):
  • Oogonium: A single, spherical vegetative cell enlarges and becomes an oogonium, containing one large, non-motile egg.
  • Antheridium: One or more short, box-like cells that divide to produce two small, multiflagellate antherozoids (sperm) each.
  • Species Types:
    • Macrandrous: Antheridia and oogonia are on the same filament (monoecious) or different filaments (dioecious) of normal size.
    • Nannandrous: A specialized "dwarf male" filament (nannandrium) attaches to the female filament and produces antheridia. This dwarf male grows from a special motile spore called an androspore.
  • Fertilization: An antherozoid enters the oogonium and fuses with the egg to form a zygote (oospore).
  • The zygote develops a thick, often ornamented, wall and becomes a resting stage. After a dormant period, it undergoes meiosis to produce four haploid zoospores, each of which grows into a new haploid filament.

Morphology and life-cycle of *Coleochaete*

Morphology:

• A freshwater green alga, usually epiphytic (grows on other aquatic plants).
• The thallus is heterotrichous (showing both a prostrate, creeping system and an erect system).
• In some species (like *C. scutata*), the prostrate system is dominant, forming a flat, disc-shaped, parenchymatous thallus.
• A unique feature is that many cells bear a single, long, unbranched seta (hair) that comes out of a sheath.

Life-cycle (Reproduction):

*Coleochaete* is significant because its advanced oogamy and post-fertilization events hint at the evolution of land plants.

• Asexual Reproduction: By biflagellate zoospores.
• Sexual Reproduction (Oogamous):
  • Oogonium: A flask-shaped structure with a long neck (trichogyne). It contains one egg.
  • Antheridium: Small, colorless, box-like cells that produce a single biflagellate antherozoid.
  • Fertilization: The antherozoid fuses with the egg in the oogonium.
  • Post-Fertilization (Important!):
    1. The zygote is retained on the parent thallus.
    2. Neighboring vegetative filaments grow around the zygote to form a protective jacket (cortex). This entire structure is called a spermocarp. This is NOT seen in *Oedogonium*.
    3. The zygote (now diploid, 2n) undergoes meiosis to form 8-32 haploid cells.
    4. Each haploid cell develops into a zoospore, which is released and grows into a new haploid thallus.

3. Evolutionary significance of *Prochloron*

*Prochloron* is a genus of prokaryotic algae (Division: Prochlorophyta). Its discovery was a major evolutionary puzzle.

• What it is: A prokaryote (like Cyanophyta).
• The Puzzle: Unlike Cyanophyta (which have Chl-a + phycobilins), *Prochloron* has Chlorophyll-a and Chlorophyll-b. It has no phycobilins.
• Significance (Endosymbiotic Theory):
  1. This is the exact same pigment combination as Chlorophyta (green algae) and higher plants.
  2. The Endosymbiotic Theory states that the eukaryotic chloroplast originated from a prokaryote (a cyanobacterium) being engulfed by a host cell.
  3. Before *Prochloron*, the problem was that cyanobacteria have the "wrong" pigments (no Chl-b).
  4. The discovery of *Prochloron* (a prokaryote with Chl-a and Chl-b) provided a "missing link." It suggested that the ancestor of the green algal and plant chloroplast may not have been a typical cyanobacterium, but rather a *Prochloron*-like organism.

In summary: *Prochloron* is evolutionarily significant because it is a prokaryote that has the pigments (Chl-a and Chl-b) of green algae and plants, making it a perfect candidate for the prokaryotic ancestor of the green plant chloroplast.

4. Phaeophyta & Rhodophyta

Characteristic features and classification (Phaeophyta)

• General Characteristics: Commonly known as brown algae. They are eukaryotic and almost exclusively marine (seaweeds).
  • Pigments: Chlorophyll-a, Chlorophyll-c, and a large amount of the xanthophyll Fucoxanthin, which masks the other pigments and gives the characteristic brown color.
  • Reserve Food: Laminarin (a polysaccharide) and Mannitol (a sugar alcohol). No starch.
  • Cell Wall: Contains cellulose and unique phycocolloids called alginates (alginic acid).
  • Motility: Motile cells (zoospores and gametes) are heterokont, with two unequal, laterally-inserted flagella (one tinsel, one whiplash).
  • Thallus: The most complex of all algae. They are all multicellular, ranging from branched filaments (e.g., *Ectocarpus*) to massive, parenchymatous thalli (e.g., *Kelp*, *Sargassum*) with complex internal differentiation (sieve tubes).
• Classification: Based on life cycle and thallus structure. An outline of major classes/orders:
  • Class Phaeophyceae: (All brown algae)
    • Order Ectocarpales: Simple, filamentous thallus (e.g., *Ectocarpus*). Isomorphic alternation of generations.
    • Order Laminariales: Complex, parenchymatous thallus (Kelps). Heteromorphic alternation of generations.
    • Order Fucales: Complex thallus (e.g., *Fucus*, *Sargassum*). Diplontic life cycle (no free-living gametophyte).

Characteristic features and classification (Rhodophyta)

• General Characteristics: Commonly known as red algae. They are eukaryotic and mostly marine.
  • Pigments: Chlorophyll-a, Chlorophyll-d, and phycobilins (R-Phycoerythrin - red; R-Phycocyanin - blue). The phycoerythrin allows them to absorb blue/green light and live at great depths.
  • Reserve Food: Floridean Starch (stored in the cytoplasm, not chloroplast).
  • Cell Wall: Contains cellulose and unique phycocolloids, agar and carrageenan. Many are "coralline" algae, depositing calcium carbonate in their walls.
  • Motility: Flagella are completely absent in all life stages (vegetative cells, spores, and gametes).
  • Thallus: Can be unicellular, filamentous, or pseudoparenchymatous. Filaments are often held together by pit connections.
• Classification: Based on life cycle and pit connections.
  • Subclass Bangiophycidae: Simple thallus, simple pit connections (e.g., *Porphyra*).
  • Subclass Florideophycidae: Complex thallus, complex pit connections, complex triphasic life cycle (e.g., *Polysiphonia*).

5. Morphology and life-cycle of *Polysiphonia*

Morphology:

• A marine red alga, common in temperate regions.
• The thallus is filamentous, branched, and polysiphonous ("many siphons").
• This means the main axes are composed of a central siphon (filament) surrounded by a ring of pericentral siphons (filaments).
• The thallus is pseudoparenchymatous, as these siphons are interwoven.
• It is heterotrichous, with a prostrate system for attachment and an erect, bushy system.

Life-cycle (Triphasic):

*Polysiphonia* has a very complex locycle with three distinct phases (generations). It is isomorphic (the gametophyte and tetrasporophyte look identical).

1. Phase 1: The Gametophytes (n, Haploid)
   • Plants are dioecious (separate male and female plants).
   • Male Gametophyte (n): Develops clusters of small branches called spermatangia. These produce non-motile male gametes called spermatia (n), which are released into the water.
   • Female Gametophyte (n): Develops a special reproductive structure called the carpogonium. This is a flask-shaped cell with a long receptive neck (trichogyne) and a swollen base containing the egg (n).
2. Fertilization
   • A spermatium (n) drifts in the current and sticks to the trichogyne.
   • The male nucleus travels down the trichogyne and fuses with the egg nucleus.
   • This forms the zygote (2n), which is retained on the female gametophyte.
3. Phase 2: The Carposporophyte (2n, Diploid)
   • The zygote (2n) does not undergo meiosis. It divides mitotically to form a *new* generation that is parasitic on the female gametophyte.
   • This new 2n generation is the carposporophyte.
   • It develops into a structure that produces spores called carpospores (2n).
   • The entire structure (carposporophyte + surrounding female tissue) is called the cystocarp (a small, urn-shaped body on the female plant).
   • The diploid (2n) carpospores are released.
4. Phase 3: The Tetrasporophyte (2n, Diploid)
   • The diploid (2n) carpospores germinate and grow into a free-living diploid plant called the tetrasporophyte.
   • This plant looks *identical* to the gametophyte plants (hence, isomorphic).
   • On its branches, the tetrasporophyte develops tetrasporangia.
   • Inside each tetrasporangium, the diploid nucleus undergoes meiosis to produce four haploid (n) tetraspores.
5. Back to Phase 1
   • The haploid (n) tetraspores are released.
   • Two of them will grow into male gametophytes (n), and two will grow into female gametophytes (n).
   • The cycle then repeats.

6. Reproduction in *Petrisiphonia*

(Note: "Petrisiphonia" is likely a misspelling in the syllabus. The most logical assumption, given its proximity to the *Polysiphonia* topic, is that it is a typo for *Polysiphonia*. *Petrosiphonia* is a real, but obscure, red algal genus in the same family, and its reproduction is virtually identical to *Polysiphonia*. Therefore, this section will detail the reproductive structures of *Polysiphonia* as covered in the life cycle above.)

Summary of Reproductive Structures in *Polysiphonia*:

1. Sexual Reproductive Structures (on Gametophytes, n)

• Spermatangium (Male):
  • Develops on the male gametophyte.
  • Forms as a dense, colorless cluster of small branches, often near the tips of the thallus.
  • Each small branch (spermatangial filament) consists of fertile cells (spermatangial mother cells) that each produce one spermatium (n).
  • The spermatium is a small, non-motile, uninucleate male gamete.
• Carpogonium (Female):
  • Develops on the female gametophyte from a pericentral cell.
  • It is a flask-shaped, unicellular structure.
  • The base is swollen and contains the egg nucleus (n).
  • The neck is a long, slender, receptive extension called the trichogyne, which projects out of the thallus to catch spermatia.

2. Asexual/Propagative Structures (on Sporophytes, 2n)

• Cystocarp / Carposporophyte (2n):
  • This is the structure that results from fertilization. It is diploid (2n) and remains attached to the female gametophyte (n).
  • The carposporophyte is the spore-producing part, consisting of filaments that bear carposporangia.
  • The cystocarp is the entire fruiting body: the carposporophyte *plus* the protective, urn-shaped covering (pericarp) that grows from the surrounding female gametophyte tissue.
  • Each carposporangium produces one diploid carpospore (2n) by mitosis.
• Tetrasporangium (on Tetrasporophyte, 2n):
  • Develops on the free-living tetrasporophyte plant (2n).
  • They are spherical structures, usually one per segment, embedded in the thallus.
  • The diploid nucleus of the tetrasporangium undergoes MEIOSIS.
  • This produces four haploid tetraspores (n), which are arranged in a tetrahedral manner.
  • These tetraspores are the beginning of the new haploid gametophyte generation.