Monera Kingdom – Concept, types, characteristics and examples

We explain what the Monera kingdom is, the origin of the term, its characteristics and classification. How is your taxonomy and examples.

Monera kingdom - bacteria
The organisms of the monera kingdom are unicellular and prokaryotic.

What is the Monera kingdom?

The monera kingdom is one of the large groups in which biology classifies living beings, such as the animal, vegetable or fungi kingdom. Understands the simplest and most primitive life forms known, which can be very diverse in nature, although they have common cellular characteristics: they are unicellular and prokaryotic.

The monera kingdom is prior to all the others that exist But even today there are many questions regarding the appearance on Earth of eukaryotic cells, a key milestone in the development of multicellular beings. One of the most accepted ideas is the so-called Theory of serial endosymbiosis, developed by the American biologist Lynn Margulis in 1967. This theory proposes that a pair of unicellular organisms belonging to the monera kingdom would have developed a very close symbiosis, which led one to form part of the body of the other, taking care of some internal functions.

Today the term monera is in disuse. Other classification models are preferred, such as the one proposed by Carl Woese in the 1970s, which distinguishes three domains: Eukarya (which includes all eukaryotic organisms), Archaea (archaebacteria) and Bacterium (bacteria), the latter two being what previously formed the monera kingdom, and those that group together all prokaryotic organisms.

Origin of the term monera

Monera kingdom
Edóard Pierre Chatton discovered that bacteria do not have a cell nucleus.

The term “monera” has its roots in the Greek word coins (“Simple”), and it has been used over time, its specific meaning changed. At first It was proposed by the German naturalist and philosopher Ernst Haeckel in 1866, who was the first to propose a classification of life based on evolutionism. He distinguished three kingdoms: animal, plant and protist, bringing together all “simple” or “primitive” life forms, which did not seem to have a specific kinship with animals and plants, considered “superior”. Haeckel placed the microscopic moneras o coins at the base of the evolutionary tree and classified them within the protist kingdom.

Later, in the 1920s, the French naturalist Édouard Pierre Chatton discovered that bacteria do not have a cell nucleus and, based on this finding, he proposed the distinction and use of the terms prokaryotes and eukaryotes in the same sense that they are currently used, that is, organisms without and with a cell nucleus respectively. As a result, in 1939, Fred Alexander Barkley used the term monera to refer to a new kingdom formed by the set of all prokaryotic organisms, and which was divided between archaeophytes or Archeophyta (current cyanobacteria) and schizophytes or Schizophyta (bacteria).

Later in history, in 1956, American biologist Herbert Copeland reorganized the kingdoms of life into four categories: animals, plants, protoctists (where the unicellular eukaryotes and those of simple organization were) and moneras (where the prokaryotes were). To this classification, the American ecologist Robert Whittaker added the kingdom of fungi (Fungi) in 1969 and it is this last version of five kingdoms, revised in 2000, which today still appears in many texts and courses in Biology.

However, although this classification is still popularly used, the term monera has disappeared from the technical literature, after the changes proposed by the American microbiologist Carl Woese, known as the creator of the new molecular taxonomy. In the 1970s, this scientist discovered that prokaryotes fit into two different groups based on their structure, composition, and molecular genetics. Thus, Woese proposed the reorganization of this group of organisms in two different taxa, Archaea (or arches) and Bacterium (also called Eubacteria), with the new domain category. The other domain, Eukarya, It is made up of all eukaryotic organisms, divided into the four kingdoms that had already been described: Protista, Animals, Fungi and Plantae.

Characteristics of the Monera kingdom

The species of the monera kingdom do not possess organelles of any kind.

The species of the monera kingdom can be very diverse in their morphology and life habits, but they have minimal characteristics of evolutionary and biological simplicity that unify them, such as:

  • They measure 3 to 5 micrometers. They are the smallest living beings that exist on the planet.
  • They are unicellular and prokaryotic. They are unicellular organisms, that is, autonomous cells that do not form tissues, colonies, or more complex organisms, and also lack a cell nucleus where their DNA rests.
  • No organelles of any kind. Unlike eukaryotic cells, which are much larger and more complex internally, moneras are cells without “internal organs” such as mitochondria or plastids, although they do have simple structures that carry out internal processes.
  • Asexual reproduction. The reproductive processes of the moneras do not involve meiosis or the production of gametes (sex cells), but rather involve binary fusion and other processes in which one individual gives rise to two identical ones.
  • Circular DNA. The DNA of these species is found loose in the cytoplasm and has a circular rather than a double helix, being much simpler and smaller.
  • Nutrition. Many moneras are heterotrophic (saprophytic, parasitic or symbiotic), that is, they feed on organic matter from the environment; but they can also be autotrophic (photosynthetic or chemosynthetic), taking advantage of sunlight or chemical reactions in the environment to make their own food.

Monera kingdom classification

Monera kingdom
Cyanobacteria photosynthesize differently from plants.

The moneras comprise three large groups, which are:

  • Eubacteria. Their name means “true bacteria” and they represent the simplest forms of cellular life known on Earth. This group of prokaryotic organisms has a single chromosome and a rigid cell wall composed of peptidoglycan. Some bacteria are mobile and have flagella that they use to move around. They can have different shapes and based on this criterion they can be classified into bacilli (rod shape), cocci (round shape), spirilla (corkscrew shape) and vibrios (comma shape).
  • Archaebacteria. This group of prokaryotic microorganisms share some characteristics with bacteria, such as the lack of a nucleus, but their differences, such as the composition of their cell wall, place them in a separate taxon. Archaebacteria are characterized by living in conditions that make life impossible for other living beings. Thus, they are found in extreme habitats, where they survive by taking advantage of the chemical resources of the environment: salinity, heat, pH, etc. They are believed to be so numerous that they make up 20% of the terrestrial biomass.
  • Cyanobacteria Known as blue-green algae, they are the only prokaryotic organisms capable of oxygenic photosynthesis, although in a slightly different way than plants. They are the largest prokaryotic beings: they can reach dimensions of up to 60 micrometers. Their presence in most habitats allows them to form symbiotic relationships with other species.

Taxonomy of the Monera kingdom

This kingdom was initially subdivided into two: bacteria and archaea, each with its own subclassification. But after the discovery of ribosomal DNA in the 1980s, a new classification based on four different groups could be established:

  • Mendosicutes, Archaea or archaebacteria. Archeas means “ancient”, since initially they were supposed to be a species of protobacteria. It is difficult to classify them since they are really very small, but they present metabolic routes and internal processes much more similar to eukaryotes than to other traditional prokaryotes.
  • Mollicutes, Tenericutes or mycoplasmas. They are a type of mainly parasitic bacteria, whose notable feature is that they lack the cell wall present in most bacteria and that they have one of the simplest known forms and genetic code.
  • Gracilicutes or gram negative bacteria. They are a supergroup of bacteria that make up the groups Spirochaetes, Proteobacteria, Planctobacteria and Sphingobacteria. They are characterized by having a very thin cell wall (which is sometimes even absent) composed of murein and a double plasma membrane. The characteristics of their cell wall make them not susceptible to Gram staining, hence their name.
  • Firmicutes or gram positive bacteria. Called endobacteria, they have a very thick cell wall and are shaped like a bacillus or coconut. Gram positive bacteria are found in this group, which respond to the Gram stain by acquiring blue or purple color.

Importance of the Monera kingdom

The members of the Monera kingdom were the first organisms that existed when life originated in one way or another on the planet. Their simplicity allowed them to emerge and survive in the hostile initial conditions of the Earth, and even today it is possible to find representatives of this group in habitats where other forms of life cannot adapt or subsist. From this approach, to study the moneras is to try to deepen the knowledge about the very origin of life.

Today, a huge number of organisms belonging to the monera kingdom are known, including several that can be found in animals, humans, and plants. Within this group, many are human infectious agents, the understanding of which will better combat diseases and save lives. On the other hand, many microorganisms classified in this kingdom are used by humans in different industries, such as pharmaceuticals (to produce antibiotics) or the food (in the production of dairy products).

Finally, it is worth mentioning the importance of the Monera kingdom in the environment, in relation to its role in food chains and webs. Many bacteria are decomposers and therefore allow the carbon cycle to complete, returning this element to its inorganic form. There are also other nitrogen or oxygen fixers.

Examples of moneras

Monera kingdom
Escherichiacoli often parasitizes the human digestive tract.

Some examples of members of the monera kingdom are the various bacteria known to mankind, such as Escherichia coli, a gram-negative bacillus that often parasitizes the human digestive tract, or the Clostridium tetani, a very common bacterium in the soil and in the digestive tract of animals, which can cause tetanus, a fatal disease, in humans upon entering the bloodstream.