Introduction 

The classification of Monera forms a part of the taxonomic system of hierarchy. This taxonomic classification has seen several modifications over the years. This system began with the two kingdom classification given by Linnaeus in 1735. The last mega taxonomic hierarchy that used the classification of Monera was the five-kingdom classification given by Whittaker in 1977. This comprises the kingdoms Monera, protista, fungi, plantae, animalia. Monera has been ranked as a kingdom in most taxonomic classifications.

The kingdom classification of monera, the prokaryotic organisms, are bacteria and archaea. These are collectively known as prokaryotes and are mostly single-celled. The term monera comes from the Greek word, which means solitary. 

Salient Features of Monera

1. The classification of Monera is the only kingdom in the five kingdoms (Monera, Protista, Fungi, Plantae, Animalia) that contains prokaryotic organisms.
2. The organisms in the classification of Monera do not have a nuclear membrane.
3. Monerans do not have well-defined cell organelles either because of a lack of cell membrane.
4. They are mostly single-celled organisms.
5. They are the more primitive life forms in evolutionary chronology.
6. Monerans generally reproduce by asexual reproduction (transverse binary fission).
7. Later, taxonomists divided monera based on the two subdivisions – eubacteria and archaea.
8. They have 70S Ribosomes ( eukaryotes have 80S) Ribosomes and lack membrane bound cell organelle like Golgi bodies, Mitochondria, ER etc.

Prokaryotes

Prokaryotes are single-celled organisms that do not have a nuclear membrane and therefore lack a true nucleus. Prokaryotes are more primitive in their evolutionary status than eukaryotes which have well developed nuclear membranes and cell organelles. Eubacteria and archaea are the main constituents of prokaryotes. 

Eubacteria

These are more commonly known as true bacteria. These are unicellular organisms and though some percentage of them cause diseases, they are made up of many more types of organisms and they work in many different roles in nature.

• Diversity: Eubacteria are ancient and have evolved into a group of vast diversity. They are mainly classified according to their modes of respiration and nutrition
• Nutrition: Bacteria take in nutrition mainly in two ways:

1.  Heterotrophic: These bacteria take in nutrition through an external source. The majority are saprophytes, bacteria that feed on the dead matter to help in putrefaction. The other kind of heterotrophic bacteria is parasitic.
2. Autotrophic: This group of bacteria are either photosynthetic or chemosynthetic. Cyanobacteria are the most populous members of photosynthetic bacteria. They are much larger than other bacteria and scientists categorised these as algae for some time. However, later they were correctly grouped with bacteria. Cyanobacteria possess chlorophyll like higher plants. However, in bacteria, chlorophyll is not contained in membrane-bound chloroplasts.

• Respiration: Bacteria respire anaerobically and aerobically. Anaerobic bacteria respire through fermentation

Cyanobacteria 

• These eubacteria classified as algae contain chlorophyll used to manufacture food from sunlight
• They exist in both fresh and seawater bodies in colonies, filamentous or unicellular forms
• Some cyanobacteria live in symbiotic relationships, with fungi providing the fungi with food. Such symbiotic relationships form lichens
• They come in many shapes and forms and have almost 2000 species
• Since some kinds of these bacteria can produce toxins, they are significant in water quality indices

Shape of Eubacteria

Sometimes eubacteria are distinguished based on their shapes. There are three main categories according to this criterion – 

1. Spherical bacteria or the cocci
2. Rod-shaped bacteria or the bacilli
3. Helically shaped bacteria or the spirilla

Eubacteria replicate by binary fission. Their genetic material is divided into two, and the two halves reproduce the original genetic material. That is, two sets of x shaped chromosomes are formed. Then the plasma membrane contracts in the middle, finally dividing into two identical cells.

Archaea

This group consists of archaebacteria. These organisms inhabit very highly hostile environments. While a few archaebacteria are heterotrophs, the larger share is chemoautotrophic. This means they make food by synthesising it from inorganic substances found in their environment. So they are classified according to their environment and the method of nutrition. 

Classification of Archaebacteria

Crenarchaeota

The Crenarchaeota are Archaea. They have a broad range of habitats. They can survive in extreme heat or high temperatures. They have special proteins which help to be tolerant  at temperatures as high as 2300C. They can be found in deep-sea vents and hot springs regions. This category has  thermophiles, hyperthermophiles and thermoacidophiles.

Euryarchaeota

They can survive in  extremely alkaline conditions. Bacteria of this category produce methane gas.  Methanogens and halophiles are included in this classification.

Korarchaeota

They possess the genes common with Crenarchaeota and Euryarchaeota. All three are believed to have descended from a common ancestor. Korarchaeota is supposed to be the oldest surviving organism on earth.  Hyperthermophiles are one example of this.

Thaumarchaeota

 Archaebacteria of this classification have capability to oxidize ammonia.

Nanoarchaeota

This is an obligate symbiont of archaea. Its  belongs to the genus Ignicoccus.

Kingdom Monera: Examples with Scientific Names

1. Eubacteria 

Clostridium kluyveri, Streptococcus pyogenes, Streptococcus pneumoniae, Escherichia coli and Staphylococcus aureus are some examples of eubacteria.

2. Archaebacteria 

Methanobrevibacter smithii, Methanobrevibacter ruminantium Halophiles – Halomonas elongata, Chromohalobacter, Thermoplasma picrophilus are some examples of archaebacteria.

Conclusion

In the 19th century, it was discovered that microorganisms cause diseases. More knowledge was gained about these living beings with subsequent advancements in science. These organisms inhabit a world of their own. A world that is rich in detail and full of things to discover. That is why scientists felt the need to add the classification of monera as part of the taxonomic hierarchy.