Unit 2: Algae
Introduction to Algal Mounts
This unit involves identifying various algae by observing their vegetative (thallus) and reproductive structures. This is done by making a temporary wet mount.
Procedure:
- Place a drop of clean water (or glycerine for semi-permanent) on a glass slide.
- Use a needle or forceps to place a small amount of the algal specimen in the drop.
- If the specimen is filamentous, use two needles to gently tease the filaments apart.
- Gently lower a coverslip over the drop, avoiding air bubbles.
- Observe under the microscope.
Cyanobacteria: Nostoc / Anabaena
Class: Cyanophyceae (Blue-Green Algae)
Identifying Features:
[Image of Nostoc filament with heterocyst and akinete]
- Thallus: Colonial. Filaments (called trichomes) are unbranched and look like a string of beads.
- Colony: In Nostoc, many filaments are embedded in a common, firm mucilaginous ball. Anabaena is often free-floating.
- Cell Types:
- Vegetative Cells: Standard, spherical or barrel-shaped, blue-green, photosynthetic cells.
- Heterocysts: Large, thick-walled, pale-yellow, and empty-looking. They are the site of nitrogen fixation.
- Akinetes: (May not be present) Large, thick-walled, granular resting spores, formed to survive unfavorable conditions.
Chlorophyceae (Green Algae)
Chlamydomonas
[Image of Chlamydomonas cell structure]
- Thallus: Unicellular, oval or spherical.
- Motility: Motile, with two anterior flagella of equal length.
- Key Features:
- Possesses a single, large, cup-shaped chloroplast.
- Contains one or more pyrenoids (for starch storage) within the chloroplast.
- An eyepot (stigma) is present, which detects light.
Chlorella
[Image of Chlorella cells]
- Thallus: Unicellular, spherical.
- Motility: Non-motile (lacks flagella).
- Key Features: Has a single, cup-shaped or C-shaped chloroplast.
- Reproduction: Asexual, by forming autospores (miniature versions of the parent cell) within the parent cell wall.
Volvox
- Thallus: Colonial, coenobium (a colony with a fixed number of cells).
- Motility: Motile. The entire spherical colony rolls through the water using the coordinated beating of the flagella of its outer cells.
- Key Features:
- A large, hollow sphere made of 500 to 50,000 cells embedded in a gelatinous matrix.
- Cells are biflagellate, similar to Chlamydomonas.
- Reproduction: Often shows daughter colonies (asexual reproduction) visible inside the parent colony.
Chara
- Thallus: Macroscopic (visible to naked eye), branched, and differentiated into nodes and internodes.
- Key Features:
- Often called "stonewort" because the thallus is calcified and feels rough.
- Whorls of short branches (called "branchlets" or "leaves") arise from the nodes.
- Reproductive Structures: Macroscopic and complex, found at the nodes.
- Nucule: The female sex organ (oogonium). Oval-shaped and surrounded by a spiral of tube cells.
- Globule: The male sex organ (antheridium). Spherical and reddish-orange.
Xanthophyceae: Vaucheria
Class: Xanthophyceae (Yellow-Green Algae)
- Thallus: Filamentous, branched, and coenocytic (aseptate), meaning it's one long, tube-like cell with many nuclei and no cross-walls.
- Key Features: Chloroplasts are small and discoid, lacking pyrenoids. Appears yellow-green.
- Reproductive Structures:
- Asexual: Forms a large, multiflagellate synzoospore at the tip of a filament.
- Sexual: Oogamous. The oogonium (female, large and spherical) and antheridium (male, curved and horn-shaped) are often found close together on the same filament.
Bacillariophyceae: Pinnularia
Class: Bacillariophyceae (Diatoms)
- Thallus: Unicellular, golden-brown.
- Motility: Some are motile, gliding via a raphe.
- Key Features:
- Cell wall is a silica "box" called a frustule, made of two overlapping halves (thecae).
- Pinnularia is boat-shaped (pennate diatom) and bilaterally symmetrical.
- A central slit called a raphe is visible, which is involved in its gliding movement.
Phaeophyceae (Brown Algae)
Ectocarpus
- Thallus: Simple, filamentous, and branched. Looks like a small, fuzzy tuft.
- Key Features: Shows an isomorphic alternation of generations (haploid and diploid plants look identical).
- Reproductive Structures:
- Plurilocular Sporangia: Cone-shaped, multicellular, with many small chambers. Produce diploid (on 2n plant) or haploid (on 1n plant) zoospores by mitosis.
- Unilocular Sporangia: Globe-shaped, single-celled. Only found on diploid (2n) plants. Undergoes meiosis to produce haploid zoospores (meiospores).
Sargassum
- Thallus: Macroscopic, complex, and differentiated into:
- Holdfast: Attaches to a rock.
- Stipe: A stem-like main axis.
- "Blades": Leaf-like, flattened photosynthetic structures.
- Key Features: Presence of spherical air bladders that help the thallus float.
- Reproductive Structures: Sex organs are located in specialized, swollen branch tips called receptacles. The receptacles are covered in small pores leading to chambers called conceptacles, which contain the antheridia and oogonia.
Rhodophyceae (Red Algae)
Polysiphonia
- Thallus: Highly branched, filamentous, and reddish.
- Key Features: The thallus is polysiphonous, meaning it is composed of a central axial filament (siphon) surrounded by a set of pericentral cells (siphons).
- Reproductive Structures: (Part of a very complex life cycle)
- Tetrasporophyte (2n): Bears tetrasporangia, which divide by meiosis to form 4 haploid tetraspores.
- Gametophytes (1n): Male plant has spermatangia (antheridia); Female plant has carpogonia (oogonia).
- Cystocarp: A prominent, urn-shaped structure that forms on the female plant after fertilization. It is a diploid structure that releases carpospores.
Batrachospermum
- Thallus: A freshwater red alga. The thallus is gelatinous and looks like a beaded string (resembles frog spawn).
- Key Features: Complex, filamentous construction with whorls of branches arising from a central axis, giving it its beaded appearance.
- Reproduction: Involves complex carpogonia and spermatangia.