Write short notes on the following:

Cyanobacteria are blue-green algae bacteria. They are prokaryotic and their nuclear material is deoxyribonucleic acid (DNA) which is present in the protoplasm in dispersed form without any nuclear membrane throughout the cell. The membrane bounded plastids and large vacuoles are absent from the cell of cyanobacteria. The cell walls of cyanobacterial show some chemical similarity to those of bacteria. Certain cyanobacteria may be infected with viruses which resemble bacteriophages which also show similarity between cyanobacteria and bacteria.

Following are the important characteristic features found in the members of cyanobacteria.

(i) The members of cyanobacteria occur in all possible habitats.

(ii) They lack flagella.

(iii) The members of cyanobacteria have ability to fix atmospheric nitrogen in soil.

(iv) Some cyanobacteria live in symbiotic association with other organisms.

(v) Some members are found as symbionts with protozoans and called ‘cyanellae ‘,

(vi) They may be unicellular (e.g. Chroococcus. Tetrapedia, Gloeocapsa), Colonial (e.g. Aphanocapsa, Nostoc, Aphanothece) and filamentous (e.g. oscillatoria).

(vii) The filaments are called trichomes which are generally surrounded by a sheath.

(viii) The trichomes of oscillatoria show oscillating movement.

(ix) In some other cases the trichomes move forward and back within the sheath. Such movement is called gliding movement.

(x) In some cases the cells of the filamentous genera accumulate much food, become thick walled and called akinetes which face the adverse condition.

(xi) In many cases thick walled, spherical heterocyst are found.

(xii) The storage products of cyanobacteria are the cyanophycean starch and protein.

(xiii) They form a thick stratum on the surface of saline usar soils during rainy season and can be of use in reclamation of usar (barren) lands.

(xiv) They may reproduce asexually by endospore, (e.g. Dermocarpa) and exospores (e.g. Chamaesiphon).

(xv) The common examples of cyanobacteria are: Nostoc, Oscillatoria, Anabaena, Gloeocapsa, Chroococcus, Cylindrospermum, Rivularia etc.

The molecular nitrogen (N₂) is found in the atmosphere about 78% as a relatively inert gas. This nitrogen cannot be used directly by higher plants. The plants assimilate only fixed forms of nitrogen e.g., nitrate and ammonium ions. The nitrogen (N₂) reacts with certain elements or compounds under suitable conditions to form fixed form of nitrogen, the process referred to as nitrogen fixation. Thus, the phenomenon of conversion of free nitrogen into nitrogenous salts to make it available for absorption by plants. The nitrogen fixation is divided into following two types :

I. Physical Nitrogen Fixation: Atmosphere free nitrogen combines with the oxygen due to electric discharge and thunder and forms nitric oxide. The nitric oxides are again oxidized with oxygen to form nitrogen peroxide.

The nitrogen peroxide combines with water during rains to form nitrous acid and nitric acid. The acid falls on the ground alongwith rain water and reacts with alkaline radicals to produce water soluble nitrates and nitrites.

The nitrates are soluble in water and are directly absorbed by the plants.

II. Biological Nitrogen Fixation: The process is carried by two types of microorganism those which are free living or asymbiotic and those which live in close symbiotic association with other plants. However, a third category of microorganisms which fix nitrogen in association with roots of grasses and cereals has been reported. It is called associative symbiotic nitrogen fixation.