Category : 11th Class
Cyanobacteria have both beneficial and harmful effects in human affairs.
(1) Growth of cyanobacteria in hard water is most probably responsible for the deposit of limestones.
(2) Since they grow, photosynthesis, multiply and ultimately die, thus adding organic matter to the soil and increasing its fertility.
(3) Whereas some cyanobacteria act to breakdown rock, the species that live in hot springs actually build rocks. This they accomplish by depositing salts of calcium and silica within the gelatinous sheath of the algal cell wall.
(4) Balls of Nostoc commune are collected, boiled and consumed as food by the Chinese and South Americans. The prepared food is called 'Yoyucho'.
(5) Some cyanobacteria, such as Anabaena, Lyngbya etc. help in conservation of soil, thus checking soil erosion.
(6) Few species of Anabaena and Aulosira are inoculated in ponds to check the development of mosquito larvae.
(7) Certain cyanobacteria like Nostoc commune, Scytonema ocellantum, Aulosira fertissima are used for reclamation of usar (sterile alkaline) soil.
(1) Their most harmful effect is undoubtedly the formation of blooms in bodies of water.
(2) They choke the intake of water supply systems and give the water a disagreeable odour giving a fishy taste to drinking water.
(3) Many cyanobacteria produce toxins. They are directly or indirectly harmful for human. e.g., Nostoc, Anabaena, Microcystes etc.
Habitat : Nostoc is found in aquatic and terrestrial habitat. The alga forms a jelly like mass in which numerous filaments are embedded. When young, they are more or less spherical, solid and microscopic in size. With advance in age, the colony grows and becomes macroscopic. In species like N. amplissimum it attains the diameter of 30cm or almost equals to the size of hen's egg in N. punctiforme. A number of species of Nostoc on soil. They often swell up and glisten after rains and therefore called fallen stars.
Morphology : The plant is filamentous and trichome are unbranched and appear moniliform..
All the cells of the trichome are similar in structure but at intervals are found slightly larger rounded, light yellowish, thick walled cells called as heterocysts. Trichome mostly breaks near heterocyst and forms harmogonia and thus they help in its multiplication.
The heterocysts are intercalary and possess a very thick outer wall. Each heterocyst is connected with vegetative cells on two sides through prominent pores in the wall which later are occupied by a refractive cyanophycean granule called polar nodule.
Each cell trichome in Nostoc has primitive nucleus and chromoplasm and very much resembles in all details to a cyanophycean cell. Vacuoles and definite chromatophores are absent. The cellwall is differentiated into two layers. Outside the cellwall there is a mucilaginous sheath. Due to confluence of various mucilaginous sheaths of filaments, a mucilaginous colony is formed. The cell is prokaryotic.
Reproduction in Nostoc
There is no sexual reproduction in Nostoc but it reproduces asexually by following methods :
(1) Hormogonia : The filaments break at number of places into smaller pieces called as hormogonia by death and decay of an ordinary cell. They slip out of the mucilage sheath and grow into new plant. Frequently trichomes break near heterocysts.
(2) Resting spores or akinetes : Under certain conditions some of the vegetative cells enlarge and accumulate food material and develop thick walls. These are called akinetes and may be arranged on either side of the heterocysts or in between two heterocysts. In mature akinete the outer wall may be 2-3 layered and its protoplasm becomes highly granular. The akinetes germinate after a period of rest and the contents are liberated out through a pore. The protoplast by further division forms the filament.
(3) Heterocysts : In exceptional case like N. commune the heterocyst may be come functional and on germination produces a new colony.
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