Practical: Study of Nostoc

 Practical: Study of Nostoc

Nostoc is a filamentous, photosynthetic cyanobacterium. It commonly occurs in freshwater, moist soil, wet rocks and other damp habitats. Colonies are usually gelatinous because numerous filaments remain embedded in a mucilaginous matrix.

Materials Required

Fresh or preserved Nostoc material, clean glass slide, cover slip, dropper, dissecting needle, water, blotting paper and compound microscope.

Procedure

Take a small quantity of Nostoc colony with the help of a dissecting needle and place it on a clean glass slide. Add one or two drops of water and gently tease the material so that the filaments become separated. Place a cover slip carefully over the material without trapping air bubbles. Remove excess water with blotting paper. First observe the preparation under low power and then under high power of the compound microscope.

Observations

The plant body of Nostoc is thalloid and usually forms a soft, jelly-like colony. A colony contains numerous unbranched filaments embedded in a common gelatinous or mucilaginous matrix.

Each filament is called a trichome. The trichome consists of a chain of rounded or bead-like vegetative cells, giving it a characteristic necklace-like appearance. The cells contain photosynthetic pigments but lack a true membrane-bound nucleus and chloroplast because Nostoc is a prokaryotic cyanobacterium.

At intervals in the trichome, comparatively larger and pale cells called heterocysts can be observed. Heterocysts are specialized cells associated with fixation of atmospheric nitrogen. Their presence is an important identifying feature of Nostoc.

Another type of enlarged, thick-walled cell called an akinete may also be observed, particularly in mature filaments. Akinetes are resistant resting cells that help the organism survive unfavorable environmental conditions.

Identifying Features

1. Colony is gelatinous and usually green to blue-green in appearance.
2. Numerous unbranched trichomes are embedded in a common mucilaginous matrix.
3. Trichomes consist of bead-like vegetative cells.
4. Large, pale heterocysts occur at intervals in the filament.
5. Akinetes may occur in mature filaments.
6. Heterocysts are associated with atmospheric nitrogen fixation.
7. Reproduction occurs mainly through fragmentation, hormogonia and akinetes.

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