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Kingdom Monera: Archaebacteria, Cyanobacteria, Mycoplasma, Rickettsia and Actinomycetes

Kingdom Monera: Archaebacteria, Cyanobacteria, Mycoplasma, Rickettsia and Actinomycetes


You know that all organisms in this world differ in their characteristics. It is applicable even to unicellular organisms like bacteria. 

Scientists found that some bacteria are found in extreme conditions such as high temperatures, high acidity and high salinity. Examples include hydrothermal vents.

hydrothermal vents

Do these bacteria have characteristics similar to normal bacteria which are found in and around us? Is it part of eubacteria? 


These bacteria show differences from Eubacteria. They are separated from them and are placed in archaebacteria. Similarly, there are some bacteria who lack cell walls and are pathogenic. 

Let’s take a deep dive into the details of special members of kingdom Monera. 

Table of Contents


They are a group of ancient bacteria. Most archaebacteria are obligate anaerobes but they can also be aerobic or facultative anaerobic. Their cell walls do not possess peptidoglycan like that of eubacteria. It is made up of complex polysaccharides and complex polypeptides. Cell membrane of archaebacteria is highly complex. It is composed of polysaccharides and glycolipids. In archaebacteria, the sequence of nucleotides in 16s rRNA is different from bacterial rRNA.


Classification of Archaebacteria

On the basis of habitat, archaebacteria are classified as methanogens, halophiles and thermoacidophiles.


These are methane producing bacteria and can produce methane from carbon dioxide and acetic acid present in sewage. Methanogens are obligate anaerobes. They can also ferment cellulose present in cow dung to produce methane and biogas. They are found in the rumen of cattle, in marshes, swamps and bogs. Examples include Methanobacterium and Methanococcus.



These archaebacteria are found in extremely salty areas. They are obligate anaerobes. Halophiles possess a pigment, bacteriorhodopsin which can harness solar energy and is responsible for the purple colour of halophiles. This pigment absorbs the bright light and directly forms ATP. Halophiles are chemoautotrophic. Examples include Halobacterium and Halococcus

salty marshes


These Archaebacteria are found at those places where temperature is approximately 80℃ and medium is acidic (pH = 2). They are normally found in hydrothermal vents and hot sulphur springs. They can also survive at 100℃ temperature. They are facultative anaerobes.They are chemoautotrophs. Examples include Thermus aquaticus

hydrothermal vents

Blue-green algae (Cyanobacteria)

They are photosynthetic prokaryotes. They were the first organism to produce oxygen on Earth. Cyanobacteria are found in freshwater, marine water and terrestrial habitats. Flagella is absent. They show slow gliding movement. 

Forms of Cyanobacteria

Cyanobacteria can exist in three forms as follows:

  • Unicellular - Single cells perform all the activities. Examples include Spirulina

  • Colonial - They exist in groups. Individual cells are held together in a gelatinous matrix in them. Examples include Microcystis

  • Filamentous - They are seen as ribbon-like multicellular strands. They often form blooms in water bodies. Examples include Anabaena and Nostoc.










Structure of Cyanobacteria

The cell has an envelope made up of peptidoglycan. They possess naked DNA and 70S ribosomes. They possess membrane bound structures like plastids, mitochondria, Golgi bodies, lysosomes, endoplasmic reticulum and vacuoles. They are surrounded by a mucilaginous sheath made up of mucopolysaccharides. They contain photosynthetic pigments chlorophyll a, phycocyanin (blue coloured) and phycoerythrin (red coloured). They are photoautotrophic in nature. They possess plasmids and transposons


Filamentous cyanobacteria such as Anabaena and Nostoc have a special type of cell responsible for nitrogen fixation, known as heterocyst. These cells fix atmospheric nitrogen in the form of ammonia. They lack mucilage covering. It possesses an enzyme named nitrogenase to fix atmospheric nitrogen.


Akinetes are enveloped, non-motile, thick-walled, dormant cells seen in filamentous, heterocyst-forming cyanobacteria like Nostoc and Anabaena. They are resistant to desiccation and cold climates.



They are also known as Mycoplasma Like Organisms (MLO) or Pleuropneumonia Like Organism (PPLO). They are the simplest and smallest known free-living organisms with size ranges from 0.1-0.15 μ. 


Structure of Mycoplasma

They lack a cell wall and can hence change their shape, i.e, they are pleomorphic. They possess a three layered cell membrane. They have a naked DNA lies coiled in the cytoplasm. They possess 70S ribosomes. They lack flagella. These are mostly parasitic while some are saprophytic. They are pathogenic to both plants and animals. Common diseases include potato purple top disease, little leaf disease of brinjal, bunchy top of papaya etc.

Structure of Mycoplasma


Rickettsia are rod shaped, Gram negative bacteria. These are obligate intracellular parasites mostly found in insects and animals. They are smaller than bacteria and have smaller genomes too. They are non motile coccus or rod shaped. They reproduce by binary fission. They were discovered by H.T. Ricketts. They do not form spores. They cause Rocky mountain spotted fever, typhus fever etc.



They are gram positive prokaryotes that are generally anaerobic. They possess mycelial structure similar to that present in fungi. These are non-motile due to the absence of flagella. They synthesise different antibiotics. For example, Streptomyces griseus produces streptomycin. They are found in terrestrial as well as aquatic habitats and can help in decomposition of organic matter. 


Practice Problems of Kingdom Monera

  1. Read the following statements about cyanobacteria and choose the incorrect statement. 
    1. Form blooms in water bodies
    2. Have specialised nitrogen fixing cell called heterocyst
    3. Colonial, unicellular, filamentous forms
  • Pathogenic to both plants and animals

Solution: Cyanobacteria are not pathogenic to plants and animals. Hence, option d is correct. 

FAQs of Kingdom Monera

Question1. Why are blue green algae called cyanobacteria ?

Answer. Blue green algae like other bacteria do not possess a well defined nucleus with nuclear membranes. For this reason, they are called cyanobacteria.

Question2. Which bacteria is used for producing biogas?

Answer. Methanogens are archaebacteria having the ability to digest cow dung and produce biogas.

Question3. Name the prokaryote who lack a cell wall and are pleomorphic?

Answer. Mycoplasma are smallest pathogenic prokaryotes who lack cell walls and have the ability to change their shape, that is, they are pleomorphic. 

Question4. Mention the characteristic of Rickettsia?

Answer. Rickettsia are rod shaped, Gram negative bacteria. These are obligate intracellular parasites mostly found in insects and animals. They are smaller than bacteria and have smaller genomes too. They are non motile coccus or rod shaped. They reproduce by binary fission. They do not form the spores and cause diseases such as typhus fever, Rocky mountain spotted fever etc.

Question5. You found three bacteria X, Y and Z. X is found in hydrothermal vents and the places at pH 2. Y has the pigment bacteriorhodopsin and can directly synthesise ATP using sunlight and Z has the ability to digest cellulose and is used to produce biogas. Identify X, Y and Z?

Answer. X is Thermoacidophiles as these bacteria are found at higher temperature regions such as hydrothermal vents, hot sulphur springs etc. Y are Halophiles as these bacteria have the pigment bacteriorhodopsin which harness sunlight to synthesise ATP and provide purple colour to them. Z is Methanogens because they have the ability to digest cellulose and produce methane gas. For this reason, they are used to digest cow dung and produce biogas.

Question6. You found a bacteria which has the following characteristics - they are gram positive bacteria, they synthesise different antibiotics and help in the decomposition of organic matter. Identify the bacteria?

Answer. Actinomycetes are gram positive bacteria and synthesise antibiotics such as streptomycin and decompose organic matter. They possess mycelial structure similar to that present in fungi. These are non-motile due to the absence of flagella. 

Question7. A teacher gave students two bacterial cultures ‘A’ and ‘B’. One culture ‘A’ has a cell wall made up of peptidoglycan while ‘B’ has a cell wall made up of complex polysaccharides and polypeptides. Identify A and B.

Answer. Eubacteria have cell walls made up of peptidoglycan. Thus, A is eubacteria.

Archaebacteria have their cell walls made up of complex polypeptides and polysaccharides. Thus, B is archaebacteria.

Other Related Topics

The Living World Biological Classification Plant Kingdom
Animal Kingdom Morphology of Flowering Plants Anatomy of Flowering Plants
Structural Organization in Animals Cells: The Unit of Life Biomolecules
Cell Cycle and Division Transport in Plants Mineral Nutrition
Photosynthesis in Higher Plants Respiration in Plants Plant Growth and Development
Digestion and Absorption Breathing and Exchange of Gases Body Fluids and Circulation
Excretory Products and their Elimination Locomotion and Movement Neural Control and Coordination
Chemical Coordination and Integration

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