Lesson 4: Biological Classification- Monera

Kingdom Monera:

Characteristic feature of the kingdom Monera: (Monos – single; Dougherty and Allen,1960) imcludes all prokaryotes.

a)It is the most primitive form of life having originated from more ancient living stock termed as progenote. 

b)The kingdom includes Eubacteria, Archaebacteria, Actinomycetes, Mycoplasma, Spirochaetes, Rickettsiae, Chlamydiae,

Cyanobacteria etc.  

c)These are unicellular/ colonial/ multicellular/prokaryotic organisms without nuclear membrane, nucleolus, chromatin and histone  proteins.

d)Membrane bound organelles are absent.

e) Cyclosis is absent and the ribosomes are of 70S type.

f) Cell wall is made  up of peptidoglycan  ( exceptions are Archaebacteria and Mycoplasmas).

g) Nucleoid, or genotype or incipient nucleus or prochromosome is composed of naked DNA, RNA, and non-histone proteins.

h) Respiratory enzymes are found associated with plasma membrane.

i)    Reproduction by asexual method.

(j) Cell division is amitotic type and lacks spindle formation.

Some of the Important Monerans are:

1.Archaebacteria:

They form the primitive group of bacteria and are the oldest of living fossils.  They are prokaryotes and unicellular. They are found in very harsh conditions( e.g the bottom of the sea or in volcanic vents) This is thought to be because the early Earth’s atmosphere was filled with poisonous gases and was very hot – nothing could survive, except the archaebacteria. These slowly gave way to modern organisms when the Earth’s conditions settled down and oxygen was introduced to the atmosphere. The Archaebacteria are of three types:

a) Methanogens:

They are the obligate anaerobic form  of Gram negative bacteria that produces methane gas from CO2 or formic acid. These bacteria are found in rumen ( first part of the stomach) of cattle. It is also found in the marshy areas. In the biogas plant they produce methane, thereby exhibiting the fact that they cannot live in the ppresence of oxygen. Cell wall contains protein ( Methanospirillium, Methanococcus) or non-cellulosic polysaccharides (e.g., Methanosarcina) or psuedomurein (e.g methanobacterium)  in which N- acetylalosamiuronic acid is present instead of NAM.

 

b) Halophiles:

Halophilic bacteria are Gram negative obligate anaerobic forms associated with coccoid forms of bacteria. They are so called because they are salt loving and are found in places where the salt content are very high.Their habitats are tidal pools, salt ponds, brines, salted fish,salted hides etc. Halobacteria can grow well in culture medium containing as high as 25-35% of NaCl.In the presence of sunlight they develop a purple pigment (bacteriohodopsin) in membrane to use sunlight. They trap sunlight with help of the purple pigment and syntesize ATP but do not use the latter for synthesizing organic food.

c) Thermoacidophiles: 

They are aerobic forms of Gram negative bacteria found in hot sulphursprings i.e they are found in extremely acidic conditions with very high temperatures. At a temperature of about 800 they oxidize sulphur to sulphur dioxide. They are facultative anaerobes and are chaemosynthetic in nature e.g. Thermoplasma, Thermoproteus..They can survive in areas with temperatures as high as 230 degrees Fahrenheit and with pHs below 2 (hydrochloric acid, which is incredibly strong, Hcl  has a pH of 1). 

The third phylum is the thermoacidophiles. These bacteria are found in extremely acidic conditions and in areas with very high temperatures. These locations include volcanic vents and hydrothermal vents (cracks in the ocean floor where scalding water leaks out).

2. Eubacteria:

(A)History of Eubacteria; 

Anton Von Leeuwenhoeck (1675) a dutch naturalist discovered bacteria from stored rain water and tartar of teeth and termed as wild animalcules.  He discovered the microbial world and coined the term 'dierkens'.

(i)Ehrenberg coined the term Bacteria.

(ii)Nageli placed the bacteria in Schizomycetes, called fission fungi.

(iii)Louis Pasteur proposed the 'germ theory of disease' . He discovered bacteria causing chicken cholera, invented antirabies vaccine.

He coined the term microorganism. He is considered as the father of modern Biology and also Father of sterilization techniques.:    

 (iv)Robert Koch gave Koh postulates ie

(a)The microorganism must be found in abundance in  all organisms suffering from the disease, but should not be found in healthy animals.

(b)The microorganism must be isolated from a diseased organism and grown in pure culture. 

(c)The cultured microorganism should cause disease when introduced into a healthy organism.

(d)The microorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

(e)Joseph Lister developed the technique of aseptic culture.

(f)D.A. Bergey gave classification of bacteria in the 'Manual of Determinative Bacteriology'.

(g)Se'dillot used the term microbe instead of animalcules.

(B) Habitat: 

Cosmopolitan in distribution. They are present in air, water, soil, plant and animal.

(C)    Size:

(i)It ranges from 0.1-1.5 µm in diameter and 2-10  µm in length.

  

(ii) Smallest -Rod shaped Dialister pneumosintes (0.15-0.3  µm long) present in the nasopharynx of man during the early stages of influenza.

(iii) Spirillum laidlaw: Epulopscium fishelsoni (600  µm x 80µm ) and Thiomargarita ramibiensis (750 µm ) are among the largest of unicellular bacteria.

(iv) Filamentous bacterium Beggiatoa mirabilis is the largest bacterium (16-45 µm diameter and upto several cms long.)

(D) Shape: 

Cohn classified eubacteria into 4 types based upon their shapes:

(1) coccus ( pl. Cocci) Spherical or nearly spherical ,  small and always non-flagellated.

(i)Micrococcus: Occurs singly e.g. Micrococcus luteus, Micrococcus roseus.

(ii)    Diplococci : Found in pairs. e.g. Diplococcus pneumoniae.

(iii)Streptococci: Cells remain attached to form a chain. e.g. Streptococcus lactis, Leptotricha buccalis.

(iv)Staphylococci: Irregular bundles of cells or grape like cluster eg. Staphylococcus aureus. (v)Sarcinae: Three dimensional geometrical figures like cubes e.g. Sarcina.

2.Bacillus  (Pl. Bacilli) : Rod shaped / cigarette like with rounded or blunt ends. The shape is most common.Motile/ non-Motile. They may occur in various forms:

(i) Monobacillus: single

(ii) Diplobacillus: Occurs in groups of two.

(iii) Streptobacilli: Found in a chain e.g streptobacillus

(iv) when th cells of the chain have a much larger area of contact with each other  then  these are said to have formed trichomes e.g. Beggiatoa.

(v) If the cells are lined side by side like matchsticks and  at angles to one another. This arrangement is said to be palisade e.g. Corynebacterium diptheriae.

(vi)In many bacteria e.g streptomyces cells are arranged to form unicellular, long, branched filaments called hyphae.

3. Vibrio: (Singular vibrion) Bacteria with less than one complete twist or turn , resembling a comma in appearance . e.g Vibrio cholerae.

4.Spirilla:  (singular Spirillum) : Coiled forms of bacteria exhibiting twists with one or more turns giving a spiral appearance e.g. Spirillum minus.

Other common shapes:

i.Stalked bacterium: e.g. Caulobacter

ii.Budding Bacterium: e.g. Rhodomicrobium

iii. Pleomorphic: Occurs in more than one form.e.g. Rhizobium, Corynebacterium, Azotobacter, Mycobacterium.

    

e) Flagella:

 (i)  Structure:  presence of microtubules. Presence of a single filament of globular protein called flagellin.

    

(ii) Parts of a flagellum:

    

a) Basal Body: Most complex portion of a flagellum and has 4 rings (L,P,,S,and M) in Gram Negative and 2 rings in Gram positive bacteria.

    

b) Hook: It is made up of different protein subunits.

    

c) Filament: Longest and obvious portion of the flagellum. Protein molecules are arranged in a spiral manner. It is 20nm wide and 1-70 nm long and consists of 8 vertical rows of flagellin.

   

(iii) Flagellar arrangement:

  

(a) Atrichous: Flagella absent e.g. Lactobacillus , Pasteurella.

   

Single flagellum on either of the poles of the bacterial cell.

   

(b) Monotrichous: Single flagellum on either of the poles of the bacterial cell. e.g. Vibrio, Thiobacillus 

Bacterial Flagellation----A. Atrichous    B.Monotrichous    C. Flagellation    D. Amphitrichous    E.Lophotrichous     F.Peritrichous

 (c)Amphitrichous - One flagellum or more on each pole of the bacterial cell e.g. Nitrosomonas.

(d)Lophotrichous - Flagella in groups present on one pole of the bacterium. e.g. Lophotrichous

(e)Peritrichous - Flagella present allround the body of the bacterial cell.e.g Clostridium tetani.

(f)Cephalotrichous-  Bacteria with many flagella attaches at one ende.g Pseudomonas fluoroscens.

g) Pilus and fimbriae:

(i)sharing conjugation.

(h) Gram staining Technique: 

(i) It was introduced by Christian Gram in 1884.

The Gram negative bacteria becomes colourless on treating with destaining agent due to the presence of a thin cel wall and presence of lipids in the cell wall.

Difference between Gram Positive and Gram Negative Bacteria

h) Glycocalyx :

(i)The glycocalyx is a carbohydrate-enriched coating that covers the outside of many eukaryotic cells and prokaryotic cells, particularly bacteria. When on eukaryotic cells the glycocalyx can be a factor used for the recognition of the cell. On bacterial cells, the glycocalyx provides a protective coat from host factors. The possession of a glycocalyx on bacteria is associated with the ability of the bacteria to establish an infection.

(ii)    It protects the cells and also helps in adhesion.

(iii)    It is represented by a slime layer or  capsule.

(iv) The slime layer is composed of Dextran, Dextrin amd Levan while capsule is made up of polysaccharids and D- glutamic acid.

(v) The slime layer protects the cells from loss of water and nutrients.

(vi) Capsules provides gummy and sticky characters to the cell.

(vii)There are two prominent functions of the glycocalyx. The first function is to enable bacteria to become harder for the immune cells called phagocytes so surround and engulf. This is because the presence of a glycocalyx increases the effective diameter of a bacterium and also covers up components of the bacterium that the immune system would detect and be stimulated by. Thus, in a sense, a bacterium with a glycocalyx becomes more invisible to the immune system of a host.

The bacterial cell is covered by mucilage. The cell is differntiated into cell wall, plasma membrane, cytoplasm, nucleoid, plasmids, flagella, pili, and fimbriae. Th mucilage, cell wall and plasma membrane are collectively known as the cell envelope.

i) Cell Wall:

(i)The vast majority of bacteria have a cell wall containing a special polymer called peptidoglycan. The cell wall lies outside the cell membrane, and the rigid peptidoglycan is important in defining the shape of the cell, and giving the cell mechanical strength.

The bacterial cell wall is a unique biopolymer in that it contains both D- and L-amino acids. Its basic structure is a carbohydrate backbone of alternating units of N-acetyl glucosamine and N-acetyl muramic acid. The NAM residues are cross-linked with oligopeptides. The terminal peptide is D-alanine although other amino acids are present as D- isomers. This is the only biological molecule that contains D-amino acids and it is the target of numerous antibacterial antibiotics. The cell wall of Gram-positive bacteria lies beyond the cell membrane and is largely made up of pepidoglycan. There may be up to 40 layers of this polymer, conferring enormous mechanical strength on the cell wall. Other polymers including teichoic and teichuronic acids also lie in the cell walls of Gram-positive bacteria. These act as surface antigens.

(ii)  It is made up of peptidoglycans, muriens or mucoprotein.

 

(iii)Glycan portion forms the backbone of peptidoglycan and is composed of NAM  (N-acetyl muramic acid) and NAG'( N-acetylglucoseamine) joined together by ß -1,4-linkage.

(iv) tetrapedite chain is attached with NAM.

(v) teichoic acid are acidic polymers consisting of a carbohydrate (e.g. Glucose) phosphate and an alcohol.  It performs several functions as binding metals, acting as receptor sites for some viruses, and maintaining cells at low pH to prevent the degradation of cell wall s by production of enzymes.

(vi) porins functions as channels for the entry and exit of hydrpohilic low molecular weight substances.

(vii) Outer layer of cell wall in Gram negative bacteria contains lipopolysaccharides (LPS) that act as main surface antigen in cell wall.

(j) Mesosome: (Fitz James)

(i)In some locations, the cell membrane forms internal folds in the cytoplasm called mesosomes. Mesosomes presumably increase the internal surface area available for membrane activities. It has been proposed that mesosomes function in cell wall synthesis and to guide the duplicated bacterial chromosomes into the two daughter cells during cell division.

(ii)Predominantly found in Gram positive bacteria

Type:

Central mesosome: It hold the nucleoid and separates the nucleoid and the septal position.

Peripheral mesosome: Helps in storing Respiratory enzymes like succine dehydrogenase , cytochrome oxidase.

(k) Plasmid (Lederbergh and Hayes).

A plasmid is aDNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double stranded and in many cases, circular supercoiled, double stranded naked DNA. Plasmids usually occur naturally in bacteria, but are sometimes found in eukaryotic organisms (e.g., the 2-micrometre-ring in  Saccharomyces cerevisiae.

Plasmid size varies from 1 to over 1,000 kilobase pairs The number of identical plasmids within a single celll can range anywhere from one to even thousands under some circumstances. Plasmids can be considered to be part of the mobilome, since they are often associated with conjugation, a mechanism of horizontal gene transfer.

Types of Pasmids:

(i)Sex Plasmid: It carries the sex fertile factor responsible for transfer of genetic material during conjugation.

(ii)R-Plasmid: Confers resistance to antibiotics , having resistance transfer factor (RTF)

(iii)Col- Plasmid: Produces special proteins colicins (bactericin) to kill other bacteria.

(iv)Degradative Plasma: Decompose hydrocarbonsinto petroleum.

(v) Ti Plasmid and Ri Plasmid: Tumor inducing and rhizogene plasmids respectively It is large in size of about 200kbp.

Structure of a Bacterial Cell

(i)    Nutrition in bacteria

(m) Respiration
  
Depending upon the mod of respiration and their capability to perform alternate mode of respiration bacteria are of the following types:

(i)    Obligate Aerobes: Obligate aerobes need oxygen to oxidize substrates (for example sugars and fats) in order to obtain energy.

They use oxygen as the terminal electron acceptor during aerobic respiration. They have the advantage of yielding more energy than the obligate anaerobes. However, they also have to face high levels of oxidative Examples of obligate aerobic bacteria are: Nocardia ( Gram Positive) Pseudomonas aeruginosa ( Gram Negative), Mycobacterium tuberculosis (acid fast), and Bacillus ( Gram Positive).

(ii)     Obligate Anaerobes: Obligate anaerobes that fail to grow in the presence of oxygen. eg.Clostridium tetani, C. botulinum, and C.

Perfringens.

(iii)     Facultative aerobes:  These are anaerobic forms predominantly but can live in the presence of Oxygen. e.g Chlorobium.

(iv)      Facultative Anaerobes: A facultative anaerobes  make ATP  by aerobic respiration if oxygen is present but is also capable of switching to fermentation. In contrast, obligate anaerobes die in the presence of oxygen.

(v)    Some examples of facultative anaerobic bacteria are  Staphylococcus ( Gram Positive), Escherichia coli ( Gram negative), and Listeria (Gram positive).

(vi)    Aerotolerant Aerobes:An aerotolerant is considered an anaerobe since it does not use oxygen to oxidize molecules during energy production. They  are strictly fermentative. e.g. Lactobacillus.

(vii)    Anaerotolerant aerobes: Bacteria continue to perform aerobic respiration even in the absence  of oxygen by using oxygen of oxidized salts e.g. Denitrifying bacteria.

n)  Reproduction:

(a)    Binary Fission.  Most bacteria reproduce by a relatively simple asexual process called binary fission: each cell increases in size and divides into two cells. During this process there is an orderly increase in cellular structures and components, replication and segregation of the bacterial DNA, and formation of a septum or cross wall which divides the cell into two progeny cells The process is coordinated by the bacterial membrane perhaps by means of mesosomes. The DNA molecule is believed to be attached to a point on the membrane where it is replicated. The two DNA molecules remain attached at points side-by-side on the membrane while new membrane material is synthesized between the two points.

This draws the DNA molecules in opposite directions while new cell wall and membrane are laid down as a septum between the two chromosomal compartments. When septum formation is complete the cell splits into two progeny cells. The time interval required for a bacterial cell to divide or for a population of bacterial cells to double is called the generation time. Generation times for bacterial species growing in nature may be as short as 15 minutes or as long as several days. This mechanism of replication was suggested by Cairn (1963) and is known as Cairn's model.

(b) by endospore formation: An endospore is a  dormant , tough, and non-reproductive structure produced by certain bacteria from the  Firmicute phylum. The name "endospore" is suggestive of a spore or seedlike form (endo means within), but it's not a true  spore (not an offspring). It's a kind of stripped-down, dormancy form that the bacterium can reduce itself to. The endospore becomes important when the bacterium is experiencing an environment that is deleterious to the usual vegetative state of the bacterium, notably including when the bacterium is getting dried out (desiccated). Endospores enable the survival of a bacterium through periods of environmental stress. When the environment returns to favorable, the endospore can reactivate itself to the vegetative state. Not all, nor even most, types of bacteria can change to the endospore form. Examples that can include Bacillus  and  Clostridium.

The endospore consists of the bacterium's DNA and part of its cytoplasm, surrounded by a very tough outer coating.Endospores can survive without nutrients. They are resistant to ultraviolet radiation, desiccation, high temperature, and chemical disinfectants. Common anti-bacterial agents that work by destroying vegetative cell walls don't work on endospores. Endospores are commonly found in soil and water, where they may survive for long periods of time.

o) Genetic Recombination or Parasexuality:

 Three methods are known by which genetic recombination is achieved by bacteria. These are called as parasexual because they do not involve syngamy and meiosis. ( i.e true sexual reproduction) In this order of their discovery they are transformation, conjugation and transformation.

a) Transformation: The donor and the recipient do not come in contact. In this process the bacteria picks up a small DNA fragments of its dead relatives from the surrounding medium with the help of membrane rceptors. The competence of a bacterium to pick up a DNA and get transformed is present usually at the end of its active growth period. The process was discovered by Griffith in 1928 while working on the bacteria Diplococus ( Pneumococus) pneumoniae    which causes pneumonia.

b) Conjugation:Involves DNA transfer between the cells in direct contact and larger fraction of the donor. DNA may be exchanged as compared to other means of genetic recombination. The process was first described by Lederberg and Tatum ( 1946) in Escherichia coli.

It may involve the replication and transfer of the F plasmid ( Fertility Factor) from donor (F+) to the recipient (F-) thus making the former also a donor. At times the foreign plasmid integrate with bacterial genome and the bacteria comes to be known as supermale. When F- is crossed with supermale the frequency of recombination is increased by a 1000 times and that is why the super ale is called as Hfr or high frequency recombination.

c)    Transduction: A small double stranded piece of DNA is transferred fron donor to recipient by bacteriophage.This mode of genetic recombination in bacteria was first demonstrated by Zinder and Lederberg in1952 in Salmonella typhimurium.

Types of Transduction:

(i)    Generalised Transduction: Transducing bacteriophage can transfer any gene of the donor bacterium e.g. T4 bacteriophage.

(ii)    Restricted ( specialized) transduction: Transducing bacteriophages can carry only a special region of the bacterial DNA to a recipient e.g. Bacteriophage.

(iii)    Abortive transductiion: Dna fragment from donor bacterium is not integrated in the genome of the recipient bacterium and is lost after one or few generations.

p) Economic importance :

Beneficial activities:

1.    Sewage Disposal: E.coli, Clostridium, Psudomonas, Streptococcus.
2.    Free living Nitrogen fixers: Beijerinckia, Clostridium
3.    Symbiotic nitrogen fixers: Rhizobium, Frankia, Xanthomonas.
4.    Ammonifying bacteria: Bacillus mycoides, Bacillus ramosus
5.    Lactic Acid Production: Lactobacillus bulgaicus or Lactobacillus delbrueckii convert ammoniated sugar solution into lactic acid.
6.    Vinegar production: Acetobacter aceti, Acetobacter scizenbachi.
7.    Retting of Fibres: Clostridium perfringens ,Psudomonas fluoroscence.
8.    Curing of leaves:  Curing of tea leaves by Micrococcus candisans and tobacco leaves by Bacillus megatherium.
9.    Vitamins: Riboflavin is prepared from Clostridium butylicum , Cobalamin(B12) produced from Bacillus megatherium.
10. Single cell Protein: Rhodopseudomonas capsulata , Methylophilus methylotropus ( Source of protein).
11. Pollution Control: Pseudomonas putida degrades petroleum wastes. Flavobacterium can decompose 2,4-D. DDT can be decomposed by Acetobacter aerogens . Ganga water contain Bdellovibrio bacteriovirus that maintains the purity of the water.
12. Poly-β- hydroxybutyrate is used to produce biodegradable plastic.
13. Antibiotics: ( term as coined by Waksman;1942)

 B:  Harmful Activity:

1.    Spoilage of domestic articles: Attack on domestic articles and resulting in their dacay Spirochaete cytophaga and Cellulomonas.
2.    Denitrification:Thiobacillus denitrificans, Pseudomonas aerugenosa, Micrococcus denitroficans.
3.    Desulphurification: Desulphovibrio desulphuricans.
4.    Spoilage of alcoholic beverages: Acetobacter aceti.
5.    Diseases: About 90% of human diseases are bacterial.
      
See table:
                         Table: 1

 Animal diseases: See table 2:
                            Table 2:

 Plant Diseases:  See Table 3:

                    Table 3:

3.  Cyanobacteria (BGA)


Cyanobacteria are aquatic and  photosynthetic, that is, they live in the water, and can manufacture their own food. Because they are bacteria, they are quite small and usually unicellular, though they often grow in colonies large enough to see. They have the distinction of being the oldest known fossils, more than 3.5 billion years old, in fact! It may surprise you then to know that the cyanobacteria are still around; they are one of the largest and most important groups of bacteria on earth.
Many Proterozoic oil deposits are attributed to the activity of cyanobacteria. They are also important providers of nitrogen fertilizer in the cultivation of rice and beans. The cyanobacteria have also been tremendously important in shaping the course of evolution and ecological change throughout earth's history. The oxygen atmosphere that we depend on was generated by numerous cyanobacteria during the Archaean and proterozoic Eras. Before that time, the atmosphere had a very different chemistry, unsuitable for life as we know it today.

The other great contribution of the cyanobacteria is the origin of plants. The chloroplast with which plants make food for themselves is actually a cyanobacterium living within the plant's cells. Sometime in the late Proterozoic, or in the early Cambrian, cyanobacteria began to take up residence within certain eukaryote cells, making food for the eukaryote host in return for a home. This event is known as endosymbiosis, and is also the origin of the eukaryotic mitochondrion.
Because they are photosynthetic and aquatic, cyanobacteria are often called "blue-green algae". This name is convenient for talking about organisms in the water that make their own food, but does not reflect any relationship between the cyanobacteria and other organisms called algae. Cyanobacteria are relatives of the bacteria, not eukaryotes, and it is only the chloroplast in eukaryotic algae to which the cyanobacteria are related.

Structure of a  Cyanobacterial cell

They are gram positive in nature.  They contain chlorophyll, besides that α,β Carotene, bit of myxoxanthin and phycobilin or phycobilliproteins. There are three types phycobilliproteins: c- phycocyanin, c- phycoerythrin, and allophycocyanin. Cell wall is 4 layered and the ell membrane lacks sterol with 2: 1 protein andphospholipd ratio. Lammesome connects nucleoid to  cell membrane. The reserve food material is cyanophycin protein, cynophycean starch or α granules and β granules ( fat drplets). These are non- flagellate and movement  f it occurs by special gliding motion. Sometimes the same species , when grown under different wavelenghts of light, exhibits variations in pigment composition.thus the alga is able to absorb maximum available light for photosynthesis. The capacity to change color with complementary effect towards light is known  as Gaidukov Phenomenon ( given by Gaidukov) or complementary chromatic adaptation. e.g. Trichodesmium erythreum causes red sea.


Blue Green Algae:
They are the only monerans which are capable of performing oxygenic photosynthesis and this respect are similar to the higher plants. Many of them are important nitrogen fixers. The existence of processes of oxygenic photosynthesis and nitrogen fixation in some organism is very surprising. The nitrogen fixation is an anaerobic process and the enzyme nitrogenase is inactivated by the presence of oxygen , evnthough in low concentration.  The nirogen fixing cyanobacteria are filamentous that produce a specialized type of cell, called heterocycst within which nitrogen fixation occurs.

The heterocysts are distinct and thick walled cells with pale yellw homogeneous contents.These may be terminal or intercalary.Under the light microscope it appaers to be surrounded by a thick two layered wall. The outer thick layer is persistent  made up of pectin or cellulose only. One or two pores also perforate the heterocyst wall. The wall is more thickened towards the pores. A promonent granule called the polar granule is present at the pore of heterocyst. Through the pores of heterocyst protoplasmic connections areestablished with the neighbouring cells the heterocysts lack photosystem II and therefore do not evolve oxygen.

Some Blue green algae like Anabaena , Nostoc form a thick layer on soil surface during rainy season. These can be used for reclaiming user soils. Many species of blue-green algae belonging to genera, Anabaena, Aulosira, Nostoc, Scytonema  etc are known to fix atmospheric nitrogen.Anabaena, Tolypothrix, and Aulosira play an important role in enriching (upto 20 %) rice fields with nitrogen.

Mycoplasma:

Prokaryotic microorganisms lacking cell walls and therefore resistant to many antibiotics. Formerly known as pleuro pneumonia like organisms (PPLO). A causative agent of pneumonia in humans and some domestic animals, is Mycoplasma pneumoniae. Troublesome contaminants of animal cell cultures, in which they may grow attached or close to cell surfaces, subtly altering properties of the cells, but escaping detection unless specifically monitored. Similar organisms, spiroplasms cause various diseases in plants.

These bacteria were first isolated and described in the late 1800s, although early researchers were unable to specifically identify the bacteria in their isolates.  The name Mycoplasma was given by Nowak (1929).They have no cell walls. Their lack of cell walls causes them to have a very elastic shape which can vary at any given time. These bacteria are also less susceptible to many commonly used drugs, since  antibiotics often target the cell wall, and Mycoplasma have no cell walls to grab on to.

These gram negative bacteria often contaminate cell cultures in the laboratory, creating  colonies with a distinctive fried-egg appearance caused by a concentration of bacteria in the middle of the colony, and a scattering around the edges.

One Mycoplasma species, M. pneumoniae, causes atypical pneumonia also known as walking pneumonia. Other species have been linked with pelvic inflammatory disease, more general respiratory infections, and several chronic diseases. In people with conditions like fibromyalgia and chronic fatigue syndrome, unusually high numbers of Mycoplasma bacteria have been noted, suggesting that the bacteria may be playing a role in the condition. Some research has also implicated the bacteria in autoimmune disorders. 

Structure of Mycoplasma