Kingdom Monera – Classification, Characteristics, Importance, Examples

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What is Monera?

  • Monera, derived from the Greek term “μονήρης” (monḗrēs), signifying “single” or “solitary,” historically represents a biological kingdom encompassing prokaryotes. These organisms are characterized by their unicellular nature and the absence of a distinct nucleus. The taxonomic classification of Monera has undergone significant evolution over time.
  • Ernst Haeckel, in 1866, initially introduced Monera as a phylum. This classification was later elevated to kingdom status by Édouard Chatton in 1925. The most widely recognized classification incorporating Monera was the five-kingdom system, formulated by Robert Whittaker in 1969.
  • However, with the advent of the three-domain system, proposed by Carl Woese in 1977, the organisms previously grouped under Monera were reclassified into two distinct domains: Archaea and Bacteria, with Eukarya forming the third domain.
  • This reclassification was based on the evolutionary lineage of life forms. It’s noteworthy that the term “Monera” is paraphyletic, meaning it doesn’t encompass all descendants from a shared common ancestor. Current scientific consensus posits that Archaea and Eukarya share a closer evolutionary relationship than either does with Bacteria. Despite its historical significance, the term “moneran” remains an informal descriptor for members of this group, alongside the term “prokaryote.”
  • Historically, the majority of bacteria were categorized under Monera. Interestingly, certain Cyanobacteria, colloquially termed blue-green algae, were initially classified within Plantae due to their photosynthetic capabilities.
  • Monera epitomizes the earliest and most abundant group of organisms on Earth. These prokaryotic entities possess unenclosed DNA, forming a structure known as the nucleoid. Contrasting with eukaryotic organisms, monerans lack membrane-bound organelles.
  • Their microscopic stature and preference for moist habitats mean they can be found in diverse environments, from animal and plant bodies to geothermal springs. It’s imperative to note that the designation “Monera” has become obsolete for many researchers, given the realization that archaea and bacteria, the two primary constituents of this kingdom, aren’t as genetically akin as previously believed. In fact, archaea exhibit a closer genetic affinity to eukaryotes than to bacteria.
  • In summation, the Monera kingdom represents the earliest known microorganisms on our planet, characterized by their unicellular nature, absence of a true nucleus, and lack of membrane-bound organelles. These organisms predominantly thrive in moist environments, from the depths of oceans to the interiors of other organisms.
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Definition of Monera

Monera is a historical biological kingdom encompassing prokaryotes, which are unicellular organisms lacking a distinct nucleus. This kingdom primarily included bacteria and archaea, and its members are characterized by the absence of membrane-bound organelles. Over time, taxonomic classifications have evolved, and the term “Monera” is less commonly used today.

Characteristics of Monera 

Organisms classified under the kingdom Monera exhibit a distinct set of characteristics that differentiate them from other life forms. These characteristics are as follows:

  1. Cellular Structure: Monerans are unicellular entities. Their cellular organization is simple, and they lack a defined nucleus. Instead, their DNA is unenclosed, existing as a nucleoid within the cell.
  2. Cell Wall Composition: The cell wall of Monerans is rigid, primarily composed of peptidoglycan. This structure provides them with stability and protection.
  3. Ribosomal Type: These organisms possess 70S ribosomes, which play a crucial role in protein synthesis.
  4. Absence of Membrane-Bound Organelles: Monerans lack several cellular organelles found in eukaryotic cells. Notably, they do not have mitochondria, lysosomes, plastids, Golgi apparatus, endoplasmic reticulum, or centrosomes.
  5. Reproductive Mechanisms: Monerans predominantly reproduce asexually. The primary modes of reproduction are binary fission and budding, where the organism divides to produce identical offspring.
  6. Locomotion: The flagellum is the primary locomotory organ in many Monerans. It facilitates movement and navigation in their environment.
  7. Ecological Role: Monerans play a pivotal role in ecosystems as decomposers. They break down organic matter, facilitating nutrient cycling and mineralization.
  8. Nutritional Modes: Monerans exhibit diverse nutritional strategies. They can be autotrophic, producing their own food through processes like photosynthesis. Alternatively, they can be heterotrophic, deriving nutrients from organic sources. Some Monerans are parasitic, relying on host organisms for sustenance, while others are saprophytic, feeding on decaying organic matter.

In summary, the kingdom Monera encompasses organisms with a unique set of characteristics, primarily defined by their simple cellular structure, absence of membrane-bound organelles, and diverse nutritional strategies. These attributes underscore their evolutionary significance and ecological importance.

Classification of Monera – Subkingdoms of Monera

The Kingdom Monera, representing the vast realm of prokaryotic organisms, is systematically classified into three primary sub-kingdoms: Archaebacteria, Eubacteria, and Cyanobacteria. Each of these sub-kingdoms is distinguished by unique structural, functional, and ecological attributes.

  1. Archaebacteria:
    • Habitat: Archaebacteria are ancient bacteria that thrive in extreme environments. Examples include halophiles in saline regions, thermoacidophiles in hot springs, and methanogens in marshy areas.
    • Cell Wall Structure: Their cell wall’s unique composition differentiates them from other bacteria, enabling survival in harsh conditions.
    • Nutrition: They primarily exhibit autotrophic nutrition.
    • Genetic Distinctiveness: The nucleotide sequences of their t-RNA and r-RNA are distinct, setting them apart from other bacterial groups.
  2. Eubacteria:
    • General Description: Often referred to as “true bacteria,” eubacteria represent the conventional bacteria familiar to most, such as E. coli.
    • Cell Wall Composition: Their cell wall is rigid, predominantly composed of peptidoglycans.
    • Locomotion: Movement is facilitated by flagella.
    • Reproductive Structures: Some eubacteria possess pili, short appendages aiding in sexual reproduction and facilitating attachment to hosts.
    • Classification: Based on cell wall composition and staining properties, eubacteria are categorized as either gram-positive or gram-negative.
    • Examples: Notable eubacteria include Rhizobium and Clostridium.
  3. Cyanobacteria:
    • Alternative Name: Commonly known as blue-green algae.
    • Photosynthesis: Being photosynthetic, they contain pigments such as chlorophyll, carotenoids, and phycobilins, enabling them to produce oxygen.
    • Habitat: Predominantly aquatic.
    • Nitrogen Fixation: Certain cyanobacteria have the capability to fix atmospheric nitrogen, playing a pivotal role in the nitrogen cycle.
    • Examples: Cyanobacteria like Nostoc, Anabaena, and Spirulina are representative of this group.

In addition to their individual characteristics, Monerans play crucial roles in various ecological processes. They contribute significantly to soil enrichment and are integral to the nitrogen cycle. Furthermore, they have applications in food production, antibiotic synthesis, and sewage treatment, underscoring their importance in both natural and human-made environments.

In summary, the classification of Monera into Archaebacteria, Eubacteria, and Cyanobacteria reflects the diversity and evolutionary significance of prokaryotic organisms, each group having its distinct set of attributes and ecological roles.


  • Bacteria are microscopic entities characterized by their adaptability to inhabit diverse environments. These organisms, while minute, play pivotal roles in various ecological and biological processes, ranging from beneficial symbiotic relationships to causing diseases. Structurally, bacteria exhibit a simplistic cellular organization.
  • Unlike eukaryotic cells, they lack a defined nucleus and contain limited cell organelles. Their cellular architecture is primarily defined by two protective layers: the external cell wall and the internal cell membrane.
  • These layers safeguard the bacterium from external threats and maintain cellular integrity. Additionally, certain bacteria possess an external capsule, providing an added layer of protection. It’s noteworthy that some bacteria, specifically those belonging to the genus Mycoplasma, are devoid of a cell wall, making them unique within the bacterial domain.
  • The bacterial surface is often adorned with specific structures that aid in various functions. Pili, short hair-like projections, envelop the bacterial surface and play roles in adhesion and genetic exchange. In contrast, flagella are elongated, whip-like appendages that impart motility, allowing bacteria to navigate through their environment. Nutritionally, bacteria exhibit diverse strategies.
  • Autotrophic bacteria harness energy from inorganic compounds, utilizing atmospheric constituents like carbon dioxide, hydrogen, hydrogen sulfide, and ammonia to derive essential carbon and hydrogen. This mode of nutrition enables them to thrive in environments where organic nutrients are scarce.
  • On the other hand, heterotrophic bacteria rely on external organic substances for sustenance. Depending on their relationship with other organisms and their environment, they can function as saprotrophs (feeding on decaying matter), parasites (deriving nutrients at the host’s expense), or symbionts (engaging in mutually beneficial relationships).
  • In summation, bacteria, with their simple yet versatile cellular design, are integral components of the biosphere. Their diverse nutritional strategies and adaptability enable them to colonize a myriad of habitats, underscoring their evolutionary success and ecological significance.

Bacterial Shape

Bacteria exhibit a diverse array of morphological forms, each with its unique structural attributes. The shape of a bacterium often provides insights into its ecological niche, mode of movement, and potential interactions with its environment. Here, we delve into the primary shapes that bacteria can assume:

  1. Cocci: These bacteria are characterized by their spherical or oval morphology. Depending on their arrangement, cocci can be further classified into:
    • Micrococcus: Singular, individual cells.
    • Diplococcus: Paired cells.
    • Tetracoccus: Groups of four cells.
    • Streptococcus: Linear chains of cells.
    • Staphylococcus: Clusters resembling grape bunches.
  2. Bacilli: These are rod-shaped bacteria. They can be solitary or may form chains. Some bacilli possess flagella, which are whip-like structures aiding in movement.
  3. Vibrios: These bacteria exhibit a curved, comma-like or kidney-shaped morphology. Typically, they have a flagellum at one end, facilitating locomotion.
  4. Spirillum: These are rigid, spiral or helical-shaped bacteria. Their coiled structure is often accompanied by flagella at one or both ends, allowing them to move in a corkscrew manner.
  5. Filamentous: Resembling fungal mycelia, these bacteria are composed of elongated filaments. This structure can sometimes be mistaken for fungal growth due to its thread-like appearance.
  6. Stalked: These bacteria are equipped with a stalk-like extension. The stalk can serve various functions, including attachment to surfaces or aiding in nutrient acquisition.
  7. Budded: In these bacteria, certain regions of the cell body exhibit swellings or protrusions. These swellings can be involved in processes like reproduction or nutrient storage.

In conclusion, the diverse shapes of bacteria not only reflect their evolutionary adaptability but also influence their ecological roles, interactions with other organisms, and survival strategies. Understanding bacterial morphology is fundamental to the study of microbiology, as it often provides clues about the bacterium’s lifestyle, pathogenicity, and ecological significance.

Structure of Bacteria

Bacteria, as prokaryotic organisms, possess a distinct cellular architecture that differentiates them from eukaryotic cells. Here, we elucidate the primary structural components of bacterial cells:

  1. Capsule: This is an external, viscous layer present in many bacteria. Composed predominantly of polysaccharides and occasionally amino acids, the capsule offers protection against desiccation and phagocytosis. In pathogenic bacteria, such as Bacillus anthracis and Diplococcus pneumoniae, the capsule serves as a significant virulence factor.
  2. Cell Wall: Encasing the bacterial cell, the cell wall provides structural integrity and protection. It is primarily composed of peptidoglycan, a macromolecular complex of polysaccharides and proteins. The cell wall’s composition and thickness vary among bacteria, leading to classifications like gram-positive and gram-negative based on staining properties.
  3. Plasma Membrane: Situated beneath the cell wall, the plasma membrane is a selectively permeable barrier composed of lipids and proteins. It regulates the transport of substances in and out of the cell and houses various cellular processes.
  4. Cytoplasm: This gelatinous matrix, rich in water, enzymes, nutrients, and waste products, is where cellular metabolism and replication occur. It encompasses ribosomes, chromosomes, and plasmids, among other cellular components.
  5. Nucleoid: Unlike eukaryotes, bacteria lack a membrane-bound nucleus. Instead, their genetic material, in the form of circular DNA, resides in a region termed the nucleoid.
  6. Plasmids: These are extrachromosomal, small circular DNA fragments found in many bacteria. Plasmids often carry genes conferring specific traits, such as antibiotic resistance, and play a pivotal role in genetic engineering.
  7. Flagella: These are long, whip-like appendages that bestow motility upon bacteria. Depending on their number and arrangement, flagella allow bacteria to move towards favorable conditions or away from adverse environments.
  8. Pili and Fimbriae: These are short, hair-like structures protruding from the bacterial surface. While pili facilitate genetic exchange during conjugation, fimbriae aid in adhesion to surfaces and host tissues.
  9. Ribosomes: These are RNA-rich granules responsible for protein synthesis. Though functionally similar to eukaryotic ribosomes, bacterial ribosomes are smaller and have a distinct molecular composition.

In essence, the bacterial cell, with its array of structural components, is a marvel of biological engineering. Each component plays a specific role, ensuring the bacterium’s survival, reproduction, and interaction with its environment. Understanding the intricacies of bacterial structure is fundamental to microbiology, aiding in areas ranging from antibiotic development to biotechnological applications.

Nutrition in Bacteria

Bacteria, with their vast diversity, have evolved various nutritional strategies to adapt to a wide range of environments. Based on their nutritional requirements and mechanisms, bacteria can be broadly categorized into the following groups:

  1. Archaebacteria:
    • Habitat: These ancient bacteria are extremophiles, thriving in harsh environments such as saline regions (halophiles), high-temperature zones like hot springs (thermoacidophiles), and swampy areas where they produce methane (methanogens).
    • Cell Wall Composition: Their unique cell wall structure, distinct from other bacterial groups, equips them to survive in extreme conditions.
    • Nutritional Mode: They predominantly follow an autotrophic mode of nutrition.
    • Genetic Distinctiveness: Their tRNA and rRNA nucleotide sequences set them apart from other bacterial groups.
  2. Eubacteria:
    • General Characteristics: Often termed “true bacteria,” eubacteria represent a vast group of bacteria commonly found in various environments.
    • Cell Wall: Their rigid cell wall is primarily composed of peptidoglycans.
    • Locomotion: Many eubacteria possess flagella, aiding in their movement.
    • Reproductive Structures: Some eubacteria have pili, short projections that facilitate genetic exchange and attachment to host tissues.
    • Classification: Based on the composition of their cell wall and staining properties, they can be categorized as gram-positive or gram-negative. Notable examples include Rhizobium and Clostridium.
  3. Cyanobacteria:
    • Alternate Name: Often referred to as blue-green algae.
    • Photosynthesis: Being photosynthetic, they harness solar energy, utilizing pigments like chlorophyll, carotenoids, and phycobilins.
    • Habitat: They predominantly inhabit aquatic ecosystems.
    • Nitrogen Fixation: Some cyanobacteria have the unique ability to fix atmospheric nitrogen, playing a crucial role in the nitrogen cycle. Examples of cyanobacteria include Nostoc, Anabaena, and Spirulina.

In summary, bacteria play multifaceted roles in various ecosystems. From enriching soils and facilitating the nitrogen cycle to aiding in sewage treatment and serving as a food source for certain organisms, their nutritional strategies underscore their adaptability and ecological significance. Understanding these strategies is pivotal for both environmental studies and biotechnological applications.

Importance of Monera

Monera, encompassing prokaryotic organisms like bacteria and cyanobacteria, plays a pivotal role in various ecological and biological processes. Here’s an overview of the importance of Monera:

  1. Decomposition and Recycling: Bacteria are primary decomposers in many ecosystems. They break down organic matter, dead organisms, and waste products, converting them into simpler substances. This process recycles nutrients, making them available for uptake by plants and other organisms.
  2. Nitrogen Fixation: Certain bacteria, especially some cyanobacteria and those living in symbiotic relationships with plants, can convert atmospheric nitrogen into a form that plants can use. This process, known as nitrogen fixation, is vital for the growth of many plants that cannot directly utilize atmospheric nitrogen.
  3. Oxygen Production: Cyanobacteria, through photosynthesis, were responsible for the initial oxygenation of the Earth’s atmosphere billions of years ago. They continue to contribute to the planet’s oxygen levels today.
  4. Digestion: Some bacteria live in the intestines of animals, including humans, and aid in digestion. They help break down complex food substances and produce essential vitamins.
  5. Disease Control: While some bacteria cause diseases, others can prevent them. For instance, certain bacteria produce antibiotics that kill or inhibit the growth of harmful bacteria.
  6. Biotechnological Applications: Bacteria are extensively used in biotechnology for the production of medicines, enzymes, and other bioactive compounds. Genetic engineering often employs bacteria to produce proteins or to study gene functions.
  7. Bioremediation: Certain bacteria can break down pollutants and toxic waste, a process known as bioremediation. They help clean up oil spills, degrade synthetic materials, and detoxify areas contaminated with heavy metals.
  8. Food Production: Bacteria play a crucial role in the production of various foods and beverages. For instance, lactic acid bacteria are used in making yogurt, cheese, and fermented sausages, while other bacteria aid in brewing beer and fermenting wine.
  9. Scientific Research: Bacteria, due to their simple structure and rapid growth, serve as model organisms in scientific research. Studies on bacteria have provided insights into fundamental biological processes and principles.
  10. Ecological Balance: Bacteria are an integral part of the food web in many ecosystems. They serve as food for many microorganisms and play a role in regulating ecosystem functions.

In summary, Monera, particularly bacteria, are indispensable for the functioning of our planet. They maintain ecological balance, drive essential biogeochemical cycles, and have vast applications in medicine, agriculture, and industry.

Examples of Monera

Monera is a historical kingdom that encompassed prokaryotic organisms, which are primarily bacteria and cyanobacteria (formerly known as blue-green algae). Here are some examples of organisms that were classified under Monera:

  1. Escherichia coli (E. coli): A common bacterium found in the intestines of humans and other animals. While many strains are harmless, some can cause food poisoning.
  2. Staphylococcus aureus: A bacterium that can be found on the skin and in the nasal passages of humans. Some strains can cause skin infections and other more severe conditions.
  3. Bacillus anthracis: The causative agent of anthrax, a disease that affects both humans and animals.
  4. Mycobacterium tuberculosis: The bacterium responsible for tuberculosis, a serious respiratory disease.
  5. Clostridium botulinum: Produces the botulinum toxin, which causes the disease botulism. This toxin is also used in small doses for medical and cosmetic procedures, known as Botox.
  6. Nostoc: A genus of cyanobacteria that forms colonies composed of filaments. They are known for their ability to fix atmospheric nitrogen.
  7. Anabaena: Another genus of cyanobacteria that can fix atmospheric nitrogen. They form symbiotic relationships with certain plants, providing them with nitrogen.
  8. Spirulina: A genus of cyanobacteria that is used as a dietary supplement due to its high protein content and other beneficial nutrients.
  9. Methanogens: A group of archaebacteria that produce methane as a metabolic byproduct. They are found in anaerobic environments, such as the guts of ruminant animals and in marshes.
  10. Halophiles: Archaebacteria that thrive in highly saline environments, like salt flats or salt mines.
  11. Thermoacidophiles: Archaebacteria that live in extremely hot and acidic environments, such as hot springs or hydrothermal vents.

These examples showcase the diversity of the Monera kingdom, with organisms adapted to a wide range of environments and possessing varied metabolic capabilities.


Which of the following is NOT a characteristic of Monera?
a) Unicellular
b) Prokaryotic
c) Presence of a nucleus
d) Lack of membrane-bound organelles

Which domain(s) were derived from the kingdom Monera in the three-domain system of classification?
a) Eukarya
b) Archaea
c) Bacteria
d) Both b and c

Which type of bacteria is found in extreme salty environments?
a) Thermophiles
b) Halophiles
c) Methanogens
d) Cyanobacteria

Which bacterial structure is responsible for motility?
a) Pili
b) Ribosomes
c) Flagella
d) Capsule

Which of the following bacteria lack a cell wall?
a) Cyanobacteria
b) Eubacteria
c) Mycoplasma
d) Archaebacteria

Which bacteria are known as “blue-green algae”?
a) Eubacteria
b) Mycoplasma
c) Cyanobacteria
d) Archaebacteria

Which of the following bacteria can fix atmospheric nitrogen?
a) Halophiles
b) Thermophiles
c) Cyanobacteria
d) Methanogens

The taxon Monera was first proposed as a phylum by:
a) Carl Woese
b) Robert Whittaker
c) Édouard Chatton
d) Ernst Haeckel

Which of the following is an informal name for members of Monera?
a) Eukaryote
b) Moneran
c) Protist
d) Fungi

Which bacterial structure aids in genetic exchange during conjugation?
a) Flagella
b) Ribosomes
c) Pili
d) Capsule


What is Monera?

Monera is a biological kingdom that historically encompassed prokaryotic, single-celled organisms lacking a nucleus. It has since been replaced by the three-domain system of classification.

What organisms were classified under Monera?

Monera included bacteria and cyanobacteria, which were once known as blue-green algae.

Why is the classification of Monera no longer widely used?

The classification of Monera has been superseded by the three-domain system, which divides organisms based on evolutionary history into Bacteria, Archaea, and Eukarya.

What are the primary differences between Archaea and Bacteria?

While both are prokaryotic, Archaea are often found in extreme environments and have distinct genetic and biochemical differences from Bacteria.

How do Monerans reproduce?

Most Monerans reproduce asexually through binary fission, where a single cell divides into two identical daughter cells.

What is the significance of cyanobacteria in the environment?

Cyanobacteria are photosynthetic organisms that play a crucial role in producing oxygen and fixing atmospheric nitrogen, making it available for other organisms.

Are all Monerans harmful?

No, while some Monerans can cause diseases, many play beneficial roles such as decomposing organic matter, aiding in digestion, and producing antibiotics.

What are the primary structures found in Moneran cells?

Moneran cells typically have a cell wall, plasma membrane, cytoplasm, ribosomes, and a region containing DNA called the nucleoid. Some may also have structures like flagella, pili, and capsules.

How do Monerans obtain their nutrition?

Monerans exhibit diverse nutritional strategies, including autotrophy (making their own food) and heterotrophy (obtaining food from the environment). Some can also derive energy from inorganic compounds.

What role do Monerans play in the nitrogen cycle?

Certain Monerans, especially some cyanobacteria, can fix atmospheric nitrogen, converting it into a form that plants can use for growth.

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