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Text Book of Microbiology (1)-274-285
Microbiology Part B (56Y)
Annamalai University
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GENERAL ACCOUNT OF CYANOBACTERIA The two divisions of algae contain prokaryotic algae which are now usually called bacteria in view of their cell structure being very similar to eubacteria. Thus cyanophyta (= Cyanophyceae) are called cyanobacteria and Prochlorophyta (= Prochlorophyceae) are termed Prochlorobacteria. The Division - Cyanophyta (Cyanobacteria) are the blue greens, the most primitive of algal organisms. They have existed on the surface of the earth since the early Precambrian. Blue green algae are kept under the class myxophycae or cyanophyceae of the algae division. They are commonly called as blue-green algae due to the presence of water soluble accessory photosynthetic pigments named C-phycocyanin and C- phycoerythrin which impart blue-green colour to the algae. Along with this certain characteristics Xanthophylls viz myxoxanthin, and myxoxanthophyll, oscillaxanthin, zeaxanthin are present in addition to chlorophyll -a and 13 - carotene etc. Fritsch (1945) classified the blue green algae into class myxophycae. He also called this algae as the members of class schizophyceae (Schizo = fission) and phycochromophyceae. An eminent Indian phycologist Prof. T. Desikachari (1959), Morris (1973) and Round (1973) kept this algae in Division Cyanophyta but Chapman (1962) gave them a separate name Myxophycophyta. But according to recent trend these members are kept under the members of prokaryota and called as cyanobacteria. GENERAL CHARACfERS OF CYANOBACTERIA (1) They have a prokaryotic cell organisation. (2) They lack the membrane bound cell organelles such as chloroplast, mitochondria, E., nucleus. They lack definite chromosome however DNA fibrils are present. (3) Photosynthetic pigments include chlorophyll- a, 13- carotene, xanthophylls
276 TEXT BOOK OF MICROBIOLOGY (lutein and zeaxanthin) and biliproteins (C-phycocyanin and phycoerythrin). No distinct plastid, the photosynthetic pigments are located on photosynthetic lamellae (thylakoid) generally scattered in the peripheral protoplast. (4) Thallus bears a specialized gliding movement. (5) Reserve food material is cyanophycean starch and cyanophycean granules. (6) According to Fischer (1897) the cytoplasm of cell is divided into two parts the outer or peripheral chromoplasm and the central colourless centroplasm. (7) Many members of class cyanophyta are involved in nitrogen fixation viz. Nostoc, Anabaena, Calotlzrix and Cylindrospermum. (8) All the members of this class screate mucilage and usually possess a mucilage sheath. This muciliage may be yellow, brown, red or purple in colour. (9) Many floating and planktonic algae member of this class bears gas vacuole. For example in Nostac, Calothrix, Phormidium etc. The gas vacuole provide buyoancy to the floating algae. (10) Vegetative reproduction is by means of hormogonia, endospore, and nanocytes like structure. (11) No asexual reproduction is reported. (12) No sexual reproduction. However genetic recombination has been observed in few species ego AnaCljstis nidulans. (13) No motile cells (even the reproductive cells are non motile) and lack flagella. OCCURRENCE Blue-green algae are found in all parts of the world from the tropics to the polar regions, and from the oceans to the top of the mountains. Majority of genera are found both in fresh as well as in brackish (saltish) water. High temperature, intense light and humid environment is required for the growth of thallus. According to Desikachari (1959) about 160 genera and 1500 species are reporated from the tropical areas. In India the members are represented by 85 genera and 750 species. The members of blue green algae are found in hot and humid environment, moist or alkaline land, along with this they require high intensity of light. The members of blue green algae grow extensively in the paddy fields of tropical and temperate climate region eg-Anabaena, Nostoc, Tolypothrix, and Cylindrospermum grow activily and fix atmospheric nitrogen. Some members of cyanobacteria are found in fresh water lakes, ponds and various types of water reservoirs. Many members grow extensively on the surface of fresh water and form water blooms. eg-MicrOCljstis and Arthospira. Such algae growth emit bad odour and render water obnoxious and undrinkable. Some blue green algae when abundant, may also impart their colour to water. For ego Trichodesmium erythraeum is responsible for red colour of the red sea. The red colour is due to the presence of red pigments in the cell, and the extensive growth of this algae impart the expression of red coloured water of the sea.
278 TEXT BOOK OF MICROBIOLOGY The trichomes of some members are 'homocystous' i. made up of uniform cells e. Oscillatoria but trichomes of majority of algae are 'heterocystous' i. it contain some special cells called 'heterocyst' at one or both ends or at regular intervals along their length ego Rivularia. Trichomes may be branched or unbranched: (a) Unbranched: Trichomes are unbranched (simple) in some members (e. Anabaena, Nostoc, Oscillatoria) simple trichomes may be either free floating (e. Spirulina) or aggregated into colonies (eg. Anabaena and Nostoc). Some time unbranched trichomes show false branching (eg. Scytonema). A cell of a trichome dies and one or both ends of the trichome at the point of the dead cell grow out as false branches. (b) Branched: Trichome are of the following three types:- Microcystis Aphanocapsa (8)
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GENERAL ACCOUNT OF CYANOBACTERIA Cyanophycin Nos toe (0)
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Microeolelts Trichomes Fig. 1 : (c) Thallus organization in cyanobacteria 279 (i) Simple branching : Such trichomes occur in some members ego 'Westiella. (ii) Heterotrichous : Thallus has well developed prostrate and erect branches. eg.- Mastigocoleus. (iii) Pseudoparenchymatous : A central row of cells is surrounded by a ring of pericentral cells which are connected to the former by pit connections. Siphonaceous and true parenchymatous forms are lacking in cyanophyta. CELL STRUcrURE Following structures are found in a prokaryotic cyanophyceae cell. (1) Sheath: Cell in some species possess an external diffluent delicate mucilaginous sheath. The sheath is usually thick and slimy but in a few forms such as Allacystis montana it is extremely delicate. The secretion of copious mucilage is an important characteristics of the blue green algae and earned for them the name myxophyceae which means slime algae. Sheath serves to hold the cell colonies together. The sheath may become lamellated or stratified and sometimes pigmented. In the later case it is usually yellowish or brownish in colour. Usually it is colourless. The slimy nature of the sheath endows the cell with great water absorbing and water retaining capacity favouring its survival under conditions of dessication. (2) Cell wall: Cell possess a distinct cell wall which is relatively thick and two layered i. outer and inner layers. The members of order stigonematales bear well developed pit connections between the cells.
GENERAL ACCOUNT OF CYANOBACTERIA 281 of material which is considered nuclear in nature. The nuclear material is arranged in a loose reticulum and includes fibrils in which DNA and RNA are present. In modern terminology genophore is the name suggested for such a primitive nucleus. The division is amitotic. Starch granUle Cell wall Pyrenoid re Fig. 2 : Cell structure PHOTOSYNTHETIC PIGMENTS AND CHROMATIC ADAPTATION Embedded in the cytoplasm within the plasma membrane are elongate flattened sacs named the lamellae or thylakoids. They are not seen under light microscope. The lamellae tend to be concentrated and organized in parallel arrays near the periphery of the protoplast. EM show that the thylakoid contain pigments and thus constitute the photosynthetic apparatus of the cyanophyte cells. Each thylakoid is bounded by two unit membranes with a small flattened area in between. The thylakoid bears the photosynthetic pigment like chI-a, p-carotene and lipid globules. Phycobilisomes particles are present on the surface of thylakoid. The particles contain the water soluble pigment C-phycocyanin and C-phycoerythrin. Bisalputra et al. (1969) suggest that the photo synthetic lamellae provides the sites for cellular respiration also and hence should be termed as photosynthetic respiratory membrane. These membrane can change their positions under certain conditions unlike other algae. In some blue green algae the visual colour of thallus depends upon the quality and type of light. For example in the presence of red light the thallus of Oscillatoria appears green in colour while in yellow light it
282 TEXT BOOK OF MICROBIOLOGY appears blue and green in the presence of red light. Thus this quality of thallus to change its colour according to the colour of light it receives is called as complementary chromatic adaptation or the Goidukov phenomenon called after the name of its russian discoverer, Goidukov (1902, 1906 to 1923). This chromatic adaptation helps the algae to maximum utilize the available light for photosynthesis. (5) Nucleoid: It is found in the centroplasm portion of the cell. The DNA fibres are spreaded irregularly in the nucleoid. The fibres lack histones and protamines. The genome is 2nm in width and 1 x 10 9 dalton in its molecular weight. According to Herdman (1974) the genome of cyanobacteria varies from 1 x 10 9 to 7 X 10 9. It means that about 2 to 6 replicas can be made of this genome. (6) Other Cell Inclusion: Irrespective of the above structures the cyanophycean cell contain other cell inclusion like cyanophycean granules, polyhedral RNA containing structure polyphosphate structures, alpha and beta particles. 70S ribosomes etc. Along with this polyglucoside granules are attached with the thylakoids. According to Lang (1972) these are combined forms of glycogen particles and polyphosphate structures along with this gas vacuoles or pseudovacuoles are also found. (I) Gas Vacuoles or Pseudovacuoles Recent observations have shown that the gas vacuoles or pseudo-vacuoles common in planktonic Oscillatoria and Anabaena species are of irregular shape and consist of cylindrical vesicles with conical ends stacked in arrays. These vesicles are bound by single membrane. The membrane is permeable to common gases. Fogg (1972) has shown that the gas-vacuoles are more commonly produced under low intensity of light and then suddenly collapse under high intensity of light. This results due to increased rate of photosynthesis which produces more quantity of sugar and increases the osmotic pressure so that there is a quick collapse of the gas- Cell wall vacuoles at the surface level of water in the Plasmalemma planktonic species. After collapse the filaments sink down at the bottom of water-reservoir. Thus gas vacuoles have a great ecological importance and serve to regulate the buoyancy of the planktonic forms. According to Pringsheim (1966) the gas vacuoles are produced under anaerobic conditions in Oscillatoria agardltii. (II) Heterocyst Certain members of the genera Nostocales and Stigonematales (which include filamentous forms) except Oscillatoriales produce enlarged, thick walled, pale yellowish specialized cells in addition to the vegetative Fig. 3 : Ultrastructure of heterocyst
284 TEXT BOOK OF MICROBIOLOGY (ii) The increase in the amount of phosphate in the medium leads to increase in heterocyst production. (iii) The concentration of nitrogen in the medium above a certain level results in complete inhibition of heterocyst production. FUNCTIONS OF HETEROCYST Various function suggested by different workers are:- (i) They are considered as a weak link of the filament or tricho·me. Thus trichome breaks down from the heterocyst portion and help in the vegetative reproduction or in hormogonia formation. (ii) Heterocyst also helps in the formation of endospore. For example in Nostoc COllll1lUne and Anabaena cycadacearum heterocyst helps in asexual reproduction. (iii) Some consider them to be the store houses of reserve food materials. (iv) In Nostoc commune and Gloetrichia raciborski they act as vestigial spores, which germinate and form new trichome on germination. (v) R. Singh (1961) and Fogg (1974) declared that the heterocysts are the site of nitrogen fixation in filamentous algae. (vi) According to Serpette (1948) heterocyst provide strength to the thallus. (vii) They promote the formation of akinete in the thallus. (viii) They are the site of oxyrespiration. (ix) In Calotlm·x weberi heterocyst act as a secondary reproductive structure. (x) According to Cannabacus (1929) there is a very close relationship between the heterocyst and gas vacuole formation. NUTRITION IN CYANOBACTERIA The cyanobacteria in general are obligate photoautotrophs because they can't grow in darkness even in the presence of organic nutrients in the substrate. The reserve food material is stored in the form of cyanophycean starch (a product of photosynthesis). Simon (1971) suggest that these cyanophycean starch granules are made up of polypeptide which contains only two amino acids viz. Arginine and Aspartic acid. The capacity of cyanobacteria to assimilate and metabolise exogenous organic compound is very limited and they can't use organic compounds as a source of energy. Movements: Despite the fact that blue green algae lack flagellated motile cells yet some genera like Oscillatoria, Spirulina and other show creeping or gliding movement in longitudinal direction of the longitudinal axis of a filament when in contact with a solid or semi-solid substrate. The movement is also accompanied by a clockwise or anti clock wise rotation of a trichome and is specific for a species. The filaments are usually in contact with a solid or semisolid substratum and their free ends show a slow or jerky but pendulum like oscillations. In Oscillatoriaceae the velocity of movement is usually well over 2 ~m sec- I and ranges up to 11 ~m sec-I. The path covered by filament is not straight but a curved one.
GENERAL ACCOUNT OF CYANOBACTERIA 285 Reproduction The cyanobacteria reproduce by simple and primitive methods of reproduction which are vegetative and asexual. Sexual reproduction is absent however genetic recombination has been reported by Kumar (1962) and Bazin (1968). Shestakov and Khyen (1970) have reported genetic recombination in Allacystis ridulalls. Vegetative Reproduction It is generally by fission, fragmentation and by the formation of hormogonia. (i) Fission: The unicellular cyanophyceae (ex. SYllecllOcystis) reproduce by this method called binary fission. This is the chief method of multiplication in the unicellular forms. The nuclear division (either by amitosis or mitosis) precedes the cell division. This is followed by cytokinesis. Under E. the cytokinesis reveals in filamentous algae as the involution of plasma membrane. This involution latter grows inward in the middle of the cell forming a_ cross plate between the two halves of the nuclear material. This divides the protoplast into two equal halves. With the centripetal growth of the cell membrane a ring-like septum arises from the inner layer of the cell wall and gradually grows inwards like a diphram with a decreasing aperture splitting the cross plate into two layers. Each daughter protoplast comes to possess a continuous plasma membrane. The completed septum or cross wall then thickens and finally divides into two layers, the ends of which are continuous with the inner layer of the parent cell wall. A constriction appears at the surface of the cross wall of the dividing cell exposing the fission between the daughter cells externally. As the daughter cells grows they set up the turgar pressure which pulls on their walls at the region of contact. The separation of daughter cells thus start at the periphery which proceeds towards the axis of the dividing cell gradually separating the two-daughter cells functioning as independent individuals. (2) Fragmentation: This plays an important role in non-filamentous and filamentous colonies. Reaching a certain size, the colony breaks up into small parts. Each part/ fragment is the beginning of a new colony which increases in size by repeated cell division. The fragmentation may also occur by mechanical means. It may result from the bites of the animals feeding on trichomes or stress caused by water currents or death of certain cells. (3) Hormogonia: The trichomes of filamentous genera breaks up within the gelatinous sheath into short sections or segments of one-many uniform living cells. These short length cells of trichome are called hormogonia or hormogones. The hormogones are delimited either by the formation of heterocyst or by the development of bi-concave separation disc or necridia (dead cell). In genera with terminal or no heterocysts the hormogonia are delimited by the formation from time to time of bi-concave, gelatinous separation discs (necridia) between some of the cells of the trichome. A living cell here and there in the trichome undergo lysis. Its protoplast breaks down to form viscous substance. The mucilage filled dead
Text Book of Microbiology (1)-274-285
Course: Microbiology Part B (56Y)
University: Annamalai University
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