Monera Students' Copy PDF

1 Monera  2 These are the prokaryotic, most primitive, simplest and microscopic organisms found in all habitats. The term monera was given by Stanier and Van Neil. It is one of the kingdom of five kingd...

1 Monera  2 These are the prokaryotic, most primitive, simplest and microscopic organisms found in all habitats. The term monera was given by Stanier and Van Neil. It is one of the kingdom of five kingdom system of classification of organisms proposed by Whitaker (1969). General characteristics of monera: The general characteristics of the kingdom Monera are as follows: 1. They are primitive, prokaryotes and unicellular organisms. 2. Membrane bound nucleus is absent. But, nucleoid having DNA, RNA and protein is present. 3. Double stranded circular DNA without histone protein is suspended in the cytoplasm. 4. A rigid cell wall is present. 5. Membrane bound cell organelles like mitochondria, plastid, golgi bodies, etc are absent. 6. Cosmopolitan in distribution. 7. Nutrition – both autotrophs and heterotrophs (saprophytes - feed on dead and decaying matter, parasitic - live on other host cells for survival and cause, symbiotic - in mutual relation with other organisms, commensalism - it is where one organism is benefited and the other is not affected). Monera  3 8. They are known as decomposers and mineralizers. 9. Respiration: May be: a) Obligate aerobes (Organisms must have oxygen for survival. Examples: Mycobacterium tuberculosis, Pseudomonas aeruginosa, Bacillus, Nocardia asteroids), b) Obligate anaerobes (Organisms cannot survive in the presence of oxygen. Examples: Peptostreptococcus, Clostridium, Actinomyces, Propionibacterium) and c) Facultative anaerobes (These organisms can survive with or without oxygen. Examples: Staphylococcus, Escherichia coli, Salmonella, Listeria, Shewanella oneidensis, Yersinia pestis). 10. Reserved food material is glycogen or fats or cyanophycean starch. 11. Circulation - is through diffusion. 12. Movement - is with the help of flagella. 13. Reproduction takes place by vegetative and asexual means. Sexual reproduction is absent but genetic recombination takes place by conjugation, transformation and transduction. Monera  4 Kingdom monera is classified into two major groups: Eubacteria and Archaebacteria. Archaebacteria: These are the ancient bacteria known to survive in extreme conditions. Eubacteria (*True monera): "Eu" means true and eubacteria are also known as true bacteria. Bacteria surviving in normal conditions. Based on the ribosomal RNA sequences a microbiologist, Carl Woese (1990) proposed three domain (six kingdoms) system of classification on organisms. Domain is defined as the highest rank of organisms. He grouped all the organisms into three domains; Eukarya (all eukaryotes), and kingdom Monera into Archae and Eubacteria. This way of putting organisms in three domains is based on a phylogenetic (evolutionary) approach. Three domains of organisms are: A.Archaea (Kingdom: Archaebacteria) B.Bacteria (Kingdom: Eubacteria) and C.Eukarya (Kingdoms: Protista, Plantae, Fungi and Animalia). Monera  5 Monera  6 Bacteria Bacteria are the microscopic organisms, which are often known as ‘germs’ and ‘microbes’. They are the simplest, smallest least differentiated and most primitive prokaryotes. A Dutchman, Anton van Leeuwenhoek discovered bacteria in 1676 and named as dierkens (animalcules). However the present name of bacteria was given by Ehrenberg in 1829. The branch of biology that deals with the study of bacteria is called bacteriology. Robert Koch is known as father and Louis Pasteur as founder of bacteriology. General Characteristics of Bacteria: 1) Bacteria are prokaryotic organisms that lack true nucleus (DNA is not enclosed in a nuclear membrane). 2) Bacteria are microscopic and unicellular organisms that may occur singly or in colonies. 3) They are cosmopolitan in distribution. 4) They have rigid cell wall made up of Mureins (peptidoglycan or mucopeptide) and Lipo polysaccharides. 5) 70S ribosomes are scattered in the cytoplasmic matrix. Monera 6. They lack membrane bound cell organelles. 7 7. Incipient nucleus (Nucleoid) is present i.e. nucleus without nuclear membrane, nucleoplasm, nucleolus and chromatin. Nucleiod consist DNA, RNA and protein. 8. They show both heterotrophic and autotrophic (possess bacteriochlorophyll, which is not in plastids) mode of nutrition. 9. Cyclosis is absent. 10. The reserved food material is glycogen or fats. 11. Motile bacteria possess one or more single stranded flagella. 12. Cell division takes place by amitosis. 13. The common method of multiplication is binary fission. 14. True sexual reproduction is absent, but genetic recombination occurs by conjugation, transformation and transduction. Systematic position (acc. Five kingdom system) Kingdom: Monera Division: Schizophytes Class: Schizomycetes Order: Pseudomonadales Family: Schizomycetaceae Genus: Vibrio Species: cholerae Monera  8 Plant like characteristics of bacteria: The plants like characteristics of bacteria are as follows: 1)They have cell wall. 2)They absorb food in solution form. 3)Like some algae and fungi, bacteria are also found in filamentous form. 4)They can prepare their food utilizing CO2. 5)They can synthesize some enzymes and vitamins which is absent in animals. Occurrence: Bacteria are found in soil, water, vegetables, fruits, foods and also in air. Bacteria can also survive in soil upto 5 metre depth, in hotsprings (78-80°C) or in ice bergs below -20°C. So, bacteria are cosmopolitan in nature. Morphology: Size: Bacteria are microscopic and unicellular organisms. Their size ranges from 0.2µm – 2µm in diameter. Smallest bacterium: Dialister pneumosintes Largest bacterium: Beggiatoa mirabilis Longest bacterium: Bacillus butschilli Monera  9 On the basis of shape of the cell, Cohn (1872) classified bacteria into four types: 1) Coccus (plural-cocci) bacteria: The cells of this type of bacteria are unicellular, oval, rounded or elliptical shape. They may either remain as a single cell or may aggregate together form various shapes. Their forms are as follows: a. Monococcus (micrococcus): This bacterium is single, oval and round cell. Example: Micrococcus flavus. b. Diplococcus: These oval or rounded bacteria remain in pairs. Example: Diplococcus pneumonia. c. Tetracoccus: These round bacteria are present in tetrad form i.e. four round cells, in two planes at right angles to one another. Example: Gaffkya tetragena. d. Streptococcus: These oval or rounded bacteria are present in chain of cells. Example: Streptococcus pyogenes. e. Staphylococcus: These oval or rounded bacteria are present like bunches of grapes giving and irregular cluster. Example: Staphylococcus aureus. f. Sarcina: These oval or rounded bacteria are present in regular and cubical box shape with eight or sixteen cells. Example: Sarcina lutea. Monera  10 Monera 2. Bacillus (plural-bacilli) bacteria: These types of bacteria are rod shaped or 11 cylindrical in shape. They are of following forms: a. Monobacillus: This type of bacterium is cylindrical or rod shaped present singly. b. Diplobacillus: This type of cylindrical or rod shaped bacteria are present in pairs. c. Streptobacillus: This type of cylindrical or rod shaped bacteria are present in chains. d. Pallisade: This type of cylindrical or rod shaped bacteria are present in stack like form. 3. Vibro (plural-vibrous): These bacteria are curved, comma shaped or C-shaped bacteria. They have flagellum at one end and are motile. Example: Vibro cholerae. 4. Spirilla (plural-spirilla): These bacteria are spiral, twisted or spring like with multiple curvature and terminal flagella. Example: Spirillum volutans. Monera  12 Other types of bacteria on the basis of shape: 1) Filamentous bacteria: These bacteria are thread or filament like in shape. Example: Streptomyces cholera. 2) Mycelia bacteria: These bacteria are asepted, branched, filamentous, fungal mycelium like. Example: Actinomycetes, Beggiatoa, Thiothrix, etc. 3) Stalked bacteria: These bacteria have stalked body. Example: Caulobacter, Gallionella, etc. 4) Budded bacteria: These bacteria have swollen, bud like body structure. Example: Rhodomicrobium. 5) Pleomorphic bacteria: These bacteria change their shape and size according to the changing environment. Example: Acetobacter, Rhizobium, etc. Monera  13 On the basis of number of flagella, bacteria are of following types: 1) Atrichos: These bacteria have no flagella. Example: Corynebacterium diptherae. 2) Trichous bacteria: These bacteria have flagella. They are of two types: a.Polar bacteria: These bacteria have flagella at one or both ends of the bacterial cell. They are of following types: Monotrichous: One flagellum is attached to one end of the bacteria cell. Example: Vibro cholera. Amphitrichous: One flagellum each is attached at both ends of the bacteria cell. Example: Nitrosomonas, Spirilla undulla, etc. Lophotrichous: A group of two or more flagella present at only one end of the bacterial cell. Example: Pseudomonas fluoroscenes. Cephalotrichous: Bunches (tuft) of flagella are attached to both ends of the bacteria cell. Example: Spirillum volutans. b. Non-polar bacteria (Peritrichous): The flagella in these bacteria are evenly distributed throughout the body surface of baceterial cell. Example: Escherichia coli, Clostridium, etc. Monera  14 Monera  15 Bacteria are basically unicellular and morphologically least complex. The detail structure of bacteria can only be seen under electron microscope. A typical ultrastructure of bacteria cell shows the following structural features: 1. Slime layer or capsule: Some bacterial cells are externally covered by a layer of gelatinous substance which is secreted by the protoplast called as slime layer. It is deposited on the cell wall. But in some virulent bacteria, the amino acids are also present in polysaccharides of slime layer. This makes slime layer thicker which is called capsule. The slime layer or capsule protects the bacterial cell from dessication, antibodies, pathogenic bacteria and viral attacks. 2. Cell wall: It is the outer thick, rigid and tough covering of a bacterial cell that lies just outside the plasma membrane. It is about 100-140 nm in thickness. It is composed of peptidoglycan, murein or mucopeptide. It provides shape and protect against mechanical injury, chemicals and pathogens. It also protects protoplasm. 3. Plasma / cell membrane: It is thin, elastic flexible and selectively permeable membrane that lies inner to the cell wall. It is about 30°A (angstorm) thick and is composed of phospholipids, proteins and some polysaccharides. The inner surface contains respiratory enzymes. It acts as a protective membrane which also helps to maintain osmotic regulation of cell. Monera  16 4. Mesosomes (Chondroids): The plasma membrane gives rise to finger like infolding in the cytoplasm called as mesosomes. These contain enzymes of electron transport system (ETS). They help in cell division and reproduction. 5. Cytoplasm: It is colourless viscous substance found between nuclear body and plasma membrane. It is granular due to presence of large number of ribosomes. It lacks membrane bound organelles like chloroplast, mitochondria, endoplasmic reticulum, golgi bodies, etc. Its major inclusions are gas vacuoles, ribosomes, thyllakoid or lamellae and reserved food material.  Gas vacuoles: These are found in aquatic bacteria that help in floatation.  Reserved food material: These are in the form of glycogen, starch, polyhydroxybutyric acid, lipids or proteins.  Thylakoids or lamellae: some photosynthetic bacteria consist of a number of thylakoids which are embedded in the cytoplasm called as chromatophores. The photosynthetic bacteria consists photosynthetic pigments in the membrane of thylakoids.  Ribosomes: Membraneless 70s ribosomes are homogenously distributed in the cytoplasm. They are called polyribosomes when found in small groups. These ribosomes are responsible for protein synthesis. Monera  17 6. Nuclear body: Incipient nucleus is present in bacteria cell that contain naked double stranded circular DNA. It is also known as bacterial chromosome and helps in transfer of genetic information and also in reproduction. Plasmids: In addition to bacterial chromosome, many bacteria contain extra circular chromosomal DNA called as plasmids. These are not necessary for the survival of the cell. 7. Flagella: These are long thread like structure made up of protein called flagellin. They are attached to the cell membrane with the help of basal body, the bluepharoplast. They help bacteria for locomotion. 8. Sex pilli or fimbriae: Some gram negative bacteria have minute, straight, hair-like protoplasmic outgrowths that come out from the cell wall known as sex pilli or fimbriae. They are rigid and composed of a pure protein pilin. They help in attachment and conjugation. Monera  18 Monera  19 Mode of nutrition: On the basis of nutrition bacteria are classified into two groups: 1. Autotropic bacteria: These bacteria are nonpathogenic, free living, self-sustaining in nature, which can prepare their own food by utilization of solar energy and inorganic components like carbon dioxide, nitrogen etc. They do not use water as raw material. Other compound’s Hydrogen is used and there is no evolution of Oxygen. They are of two types: i) Photoautotrophs: These bacteria can prepare their own food by the utilization of CO2, H2S, thiosulphate, Hydrogen or even some organic compounds in presence of sunlight and bacteriochlorophyll or Chlorobium chlorophyll. There is no evolution of oxygen. Example: Chlorobium, Rhodospirillium, Chromatium, etc. ii) Chemoautotrophs: These are the bacteria that can prepare their own food by the chemical reactions of the inorganic raw materials. They obtain chemical energy from Ammonia, Nitrate, Hydrogen sulphide and other inorganic compounds. Example: Nitrosomonas, Nitrobactor, etc. Monera  20 2. Heterotrophic bacteria: This type of bacteria cannot prepare their food themselves. They absorb or ingest organic carbon (rather than fixing carbon from inorganic sources such as carbon dioxide) in order to produce energy and synthesize compounds. They are of three types: a) Saprophytic bacteria: Bacteria which live and grow on the dead and decaying organic matter and get their food by digesting those matters are known as saprophytic bacteria. The most typical examples of saprophytic bacteria will be Rhizopus nigricans (or mushrooms from fungi). [These bacteria secret some digestive enzymes that convert the complex organic compounds of dead and decaying matters into simpler inorganic forms. Hence, they can decompose the dead organisms and help in nutrient recycling. Therefore, they are called nature’s scavengers.] [Note: The process of decomposition of carbohydrates by the saprophytic bacteria is called fermentation. The process of decomposition of proteins by the saprophytic bacteria is called putrefaction.] Monera  21 b) Symbiotic bacteria: Bacteria which live in obligatory mutual relationship with other living organisms in which both the partners are benefited are called symbiotic bacteria. Examples: Rhizobium bacteria present in the root nodule of leguminous plants helps in fixation of atmospheric nitrogen, Zoamastogopera found in the stomach of termites, enable them to digest cellulose. c) Parasitic bacteria: Bacteria which live on or within the host organisms for the food are called parasitic bacteria. In this association, the host is often harmed but the bacteria are always benefitted. These bacteria may be pathogenic or non- pathogenic. The parasitic bacteria that can cause disease are called pathogenic bacteria. For example: Vibrio cholera (Cholera), Clostridium tetani (Tetanus). The parasitic bacteria that do not cause any disease are called non-pathogenic bacteria. For example: Lactobacillus acidophilus, a part of normal intestinal organism (that ferments sugars into lactic acid), Escherichia coli forms part of the normal flora within the large and small intestines (that helps in digestion by breaking down undigested sugars. It also produces biotin and vitamin K, which are used in cellular processes). Monera  22 Gram’s stain in bacteria: The technique of staining of the bacterial cell is called Gram staining. This technique was developed by Danish bacteriologist Hans Christian Gram (1884). Process of staining: Crystal violet solution and iodine are the chemicals used in Gram staining. When crystal violet solution is applied in the bacteria fixed on slide, the bacterial cell looks deep violet or purple. After treating these stained bacterial cells with potassium iodide solution and then washed with absolute alcohol or acetone, some cells retain purple or deep violet colour while others lose it. Depending upon the Gram staining reactions, bacteria can be classified into two types, they are: 1. Gram positive bacteria: The bacteria that retain purple colour after Gram staining are called Gram positive bacteria. Example: Streptococcus. 2. Gram negative bacteria: The bacterial cells that do not retain the purple colour after Gram staining are called Gram negative bacteria. Example: Escherichia coli. The Gram negative bacteria retain red or pink colour when they are stained with safranin. Monera  23 Monera  24 Differences between Gram positive and Gram negative bacteria: S.N. Gram positive bacteria Gram negative bacteria They do not retain purple colour after Gram 1 They retain purple colour after Gram staining. staining. 2 Cell wall is thick and homogenous. Cell wall is thin and heterogenous. Cell wall is composed of Peptidoglycan, Cell wall is composed of Lipopolysaccharide, 3 Teichoic acid and Lipotechoic acid. Lipoproteins and Peptidoglycans 4 Porin proteins are absent. Porin proteins are present. 5 Mesosomes are more prominent. Mesosomes are less prominent. 6 Flagella have 2 rings in basal body. Flagella have 4 rings in basal body. Relatively complex nutrient materials are Relatively simple nutrient materials are 7 required. required. For examples: Staphylococcus, Streptococcus, For examples: Escherichia, Salmonella, Bacillus, Clostridium, Nocardia, Klebsiella, Proteus, Helicobacter, 8 Propionibacterium, Enterococcus, Hemophilus, Vibrio, Shigella, Neisseria, Corynebacterium, Listeria, Lactobacillus, Enterobacter, Pseudomonas, etc. Gardnerella, etc. Porins are present in the cell membrane, outer membrane of Gram negative bacteria, some Gram positive Mycobacteria, Outer Mitochondrial membrane and outer chloroplast membrane. Porins are involved in the exchange of nutrients over the outer membrane of Gram-negative bacteria but are also involved in pathogenesis. Monera  25 Growth of bacteria: Reproduction in bacteria takes place by binary fission. Binary fission leads to the formation of two daughter cells from a single bacterium called as bacterial growth. The process involves: 1) Cell growth and increase in its number of cellular components. 2) Replication of DNA, 3) Development of constriction in the cell until two daughter cells are formed by cytokinesis. 4) Each daughter cells receives a complete copy of the parental genome (organism's complete set of genetic instructions). Monera  26 The Bacterial Growth Curve In the laboratory, under favorable conditions, a growing bacterial population doubles at regular intervals as in following table. Number of generations Number of cells 0 1 1 2 2 4 3 8 Growth is by geometric progression: 1, 2, 4, 8, etc. or 20, 21, 22, 23.........2n (where n = the number of generations). This is called exponential growth. In reality, exponential growth is only part of the bacterial life cycle, and not representative of the normal pattern of growth of bacteria in Nature. When a fresh medium is inoculated with a given number of cells, and the population growth is monitored over a period of time, plotting the data will yield a typical bacterial growth curve (Figure below). Hence, the graphical representation of the number of living bacterial cells in a population over a period of time is called bacterial growth curve. A typical bacterial growth curve shows four characteristic phases of the growth: Lag Phase, Exponential (log) Phase, Stationary Phase and Death Phase. Monera  27 Describe the phases in your own words. Monera  28 Bacteria have both useful and harmful significances. So, they are considered both as the friends and enemies of human beings. Useful importances of bacteria: Bacteria have been used widely in the field of agriculture, industry and medicine. Some of their significances are: Importance of bacteria in agriculture: 1. Nitrogen fixation: Some bacteria convert atmospheric nitrogen into nitrogenous compounds and increase the fertility of the soil. They may be free living (Azobacter, Clostridium, etc) or symbiotic (Rhizobium bacteria in root nodule of leguminous plants). 2. Ammonification: Some saprophytic bacteria like Bacillus subtillis, Bacillus ramosus, etc convert the protein of dead and decaying bodies of animals and plants as well as excreta of animals first into amino acids and then into ammonia. This process is called ammonification. The converted ammonia is taken up by the plants from soil. 3. Nitrification: Some bacteria like Nitrosomonas and Nitrobacter convert ammonia into nitrites and nitrates respectively in the soil in presence of free oxygen. This process is called nitrification. 4. Decay of plant and animal bodies: Most of the bacteria along with the fungi help in the decay and decomposition of the dead bodies of plants and animals. 5. Sewage disposal: Some bacteria in towns and cities are used to convert complex substances into simple form which dissolves in water. The sewage is then filtered and filtrate is used as manure water in agriculture. Monera  29 Importance of bacteria in industry: 1. Dairy: Some bacteria are used in dairies for various purposes. They are:  Streptococcus lactis, Lactobacillus acidophilus and Lactobacillus lactis are used to prepare curd from pasteurized milk.  Streptococcus lactis is used in preparing cheese in cheese industry.  Lactobacillus bulgaricus, Streptococcus thermophillus etc are used to prepare yogurt. 1. Alcohol and acetone manufacture: Clostridium bacteria are used to manufacture Butyl alcohol and acetone from sugar molasses. 2. Tanning leather: Some bacteria are used to decompose fat and other compounds of skins by fermentation in leather tanning industries. 3. Retting of fibre: The process of separating fibres from the plant tissues is called retting. Clostridium and Pseudomonas species are used in this process. 4. Vinegar making industry: Acetobacter are used to convert alcohol into vinegar (used as preservative) in some industries. 5. Ensilage: Some bacteria are used to preserve the green fodder in silo (pits that can be used later. Silos are used in agriculture to store grain or fermented feed known as silage. 6. Production of enzymes: Enzymes like proteases, pectinase and streptokinase aare produced from Bacillus subtilis, Clostridium perfringens and Streptococcus pyrogens. Monera  30 Importance of bacteria in the field of medicine: 1. Antibiotics: Antibiotics are chemical substances produced from micro-organisms that inhibit the growth and development of bacteria, fungi, etc and even destroy them. S.N. Names of antibiotics Name of bacteria 1 Tetramycin Streptomyces ramous 2 Chloramphenical Streptomyces venezuelae 3 Bacitracin Bacillus licheniformis 4 Subtilin Bacillus subtilis 5 Polymyxin B Bacillus polymyxa 2. Antiserum: Antiserum is a blood serum containing antibodies against specific antigens, injected to treat or protect against specific diseases. Antiserum obtained from bacterial cells is used in treatment of meningitis, pneumonia, diphtheria, etc. 3. Vaccines: Vaccines are the living or dead micro-organisms that increase immunity after entering to the body produced by traditional method. Cholera vaccine is a killed vaccine used for the treatment of cholera, BCG (Bacillus Calmette-Guerin) is a living vaccine used for the treatment of Tuberculosis, TT vaccine used for tetanus, TAB (Typhoid and paratyphoid A and B vaccine) vaccine used for typhoid, etc. Monera  31 4. Production of vitamins: some vitamins are produced by using bacteria. S.N. Names of vitamins Name of bacteria 1 B2 Clostridium butylicum 2 B12 Bacillus megathermis 3 C Acetobacter aceti 4 B and K Escherichia coli 5. Protection of vagina: Lactobacillus acidophilous grown in vagina makes it acidic that protects vagina from micro-organisms. 6. Production of humulin: Human insulin (humulin) was produced by genetic engineering of E. coli. Monera  32 Harmful importances of bacteria: 1. Diseases: Bacteria cause several diseases like: S.N. Diseases Bacteria Plant diseases 1 Ring rot of potato Corynebacterium sepedonicum 2 Black rot of cabbage Xanthomonas campestris Animal diseases 1 Wound infection Proteus sp. 2 Black leg of cattle Clostridium chanvel Human diseases 1 Cholera Vibrio cholerae 2 Pneumonia Diplococcus pneumoniae 3 Tuberculosis Mycobacterium tuberculosis 4 Tetanus Clostridium tetani 2. Spoilage of food: Some saprophytic bacteria spoil several types of food stuffs: S.N. Names of food stuffs Name of bacteria 1 Curd Clostridium 2 Protein rich food Pseudomonas 3 Wine Acetobacter aceti 4 Meat Lactobacillus and Salmonella Monera  33 3. Food poisoning: Some bacteria causes food poisoning: S.N. Types of food poisoning Name of bacteria 1 Gas gangrene Clostridium perfringens 2 Staphylococcal food poisoning Staphylococcus aureus 4. Loss of fertility of soil: Bacteria like Bacillus denitrificans, Micrococcus denitrificans, Pseudomonas denitrificans, Bacillus subtilis, etc convert nitrites of the soil into atmospheric free nitrogen by the process of denitrification. This process reduces the fertility of the soil by reducing nitrogen in soil. 5. Damage of domestic articles: Leather and wood are damages by some bacteria like Spirochaete cytophage. Monera  34 Cyanobacteria: Cyanobacteria are also known as blue green algae (BGA). The characteristic features of Cyanobacteria are: 1) They are omnipresent, and occur in all possible kinds of habitats. 2) Their cellular structure is prokaryotic. 3) Filaments are called trichomes with sheath. 4) The pigments found in this group are: chl-a, P-carotene, Antheraxanthin, Aphanicin, Aphanizophyll, Flavacin, Lutein, Myxoxanthophyll, Oscilloxanthin, Zeaxanthin, Allophycocyanin, Phycocyanin, Phycoerythrin. 5) The storage products are cyanophycean starch and protein. 6) The flagella are absent. 7) They may be unicellular (Example: Chroococcus, Tetrapedia, Gloeocapsa), colonial (Example: Aphanocapsa, Nostoc, Aphanothece) and filamentous (Example: Oscillatoria). 8) Reproduce by vegetative and asexual methods only. Monera  35 Systematic position of Nostoc: Kingdom: Monera Division: Eubacteria Class: Cyanophyceae/myxophyceae Order: Nostocales Family: Nostocaceae Genus: Nostoc Common name: Moon spit / star jelly The term Nostoc was coined by Paracelsus. Nostoc is found in soil, on moist rocks, at the bottom of lakes and springs (both fresh- and saltwater), and rarely in marine habitats. It may also grow symbiotically within the tissues of plants, such as the evolutionarily ancient angiosperm Gunnera and the hornworts (a group of bryophytes), providing nitrogen to its host through the action of terminally differentiated cells known as heterocysts. These bacteria contain photosynthetic pigments in their cytoplasm to perform photosynthesis. Monera  36 Structure of colony: 1. Nostoc thallus is found in group or ball like colony. 2. The colony is made up of number of unbranched filaments of moniliform (Having a form resembling a string of beads, where the component parts or segments are more or less uniform in size and are spherical or rounded in shape) cells. 3. The colonies are greenish to bluish-green ball like structures within a gelatinous sheath. 4. Each colony contains thousands of straight or twisted filaments or trichomes. 5. Each trichome is surrounded by its individual sheath and called the filament. 6. A trichome consists of many cells arranged in a beaded manner. 7. Each cell is somewhat cylindrical or spherical in shape. 8. In filaments there are specialized thick walled, large, spherical or cylindrical, colourless empty cells with polar nodules called heterocysts. 9. Heterocysts are generally intercalary but in the young condition, they may be terminal. 10. Two polar nodules are present in each heterocyst. 11. Some cells of the filament become enlarged and filled with the food material. These thick-walled cells are called akinetes. Monera  37 Cell structure: The vegetative cell of Nostoc is oval, spherical or rounded in shape. Each cell have following parts: A. Cell wall: It is the outermost rigid and protective covering of cells. It is made up of peptidoglycan and mucopeptide. The cell wall is covered on the outer side by a gelatinous mucilaginous sheath. B. Cell membrane: Below the cell wall there is lipo-proteinous cell membrane. The cell membrane gives rise to an infolding in the cytoplasm called as lamellosome that helps in respiration, septum formation and separation of replicated DNA. C. Protoplasm: Protoplast of Nostoc shows the typical Myxophycean structure, i.e., inner colourless centroplasm and outer pigmented chromoplasm. In the chromoplasm, pigments, protein globules, fat droplets, gas vacuoles and cyanophycean starch granules are present. Photosynthetic lamellae and 70s ribosomes are also present in cytoplasm. While in the centroplasm, the incipient nucleus (genophore) having double stranded circular naked DNA is present. Monera  38 Reproduction in Nostoc: Nostoc reproduces only by vegetative and asexual methods. The sexual reproduction is completely absent. 1) Vegetative reproduction: Nostoc reproduces vegetatively by following ways: i. By fragmentation: In this method, the Nostoc colony breaks into several smaller fragments due to mechanical, physiological or other factors. Every fragment develops into new trichome of Nostoc. ii. By hormogones: Due to accidental means or death or decay of ordinary intercalary cells, the trichomes of Nostoc break at the joint where a heterocyst and vegetative cell adjoin. These short segments of trichome are called hormogonia. As soon as hormogonia come out of the colony, they grow rapidly into new filament and finally into new colony. 1) Asexual reproduction: Nostoc reproduces asexually by following methods: i. Akinetes: Akinetes are the thick walled resting spores formed under unfavourable conditions. The protoplasm of akinetes become active and breaks the outer thick wall as it return to favourable conditions. Hence, each akinete germinates to form a short hormogone which later grows to form a new colony. Monera  39 ii. Heterocysts: Heterocysts are the specialized cells for the nitrogen fixation formed in the filament of Nostoc. In Nostoc commune, heterocyst functions as resting spores. As the protoplast become functional, it germinates to form a new filament. During germination the protoplast of heterocyst divides and re-divides transversely to form four celled germling. Rupturing the wall of heterocyst, the germling develops into a new filament of Nostoc. Economic importance of Nostoc in agriculture:  Colonies of Nostoc are rich in nitrogenous substances and are edible in nature.  Nostoc increases the fertility of soil by fixing atmospheric nitrogen. Monera