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Learn more about biological classification

All organisms are classified into one of the five kingdoms namely monera, Protista, Fungi, Plantae and Animalia by R.H Whittaker. This classification is based on cell structure, thallus organization, mode of nutrition, reproduction and phylogenetic relationships.

Kingdom Monera:

The kingdom monera solely comprises of bacteria. Bacteria are found to be present in a wide range of extreme habitats like hot deserts, seas, and oceans, tropic zones, snowy mountains, soil etc.

Based on their shape, bacteria are further classified into four categories – spherical coccus (pl – cocci), rod-shaped bacillus (pl – bacilli), the comma-shaped Vibrium (pl – vibrio), and spiral shaped spirillum (pl – spirilla).

Characteristics of bacteria:

Structure of bacteria is simple, yet they have complex behavior.

Bacteria show extensive metabolic diversity

Mode of nutrition: either photosynthetic autotrophic, chemosynthetic autotrophic or heterotrophic

Bacteria are further classified as Archaebacteria and eubacteria.


Archaebacteria have different cell wall structure, which might be responsible for their existence in extreme conditions such as halophiles (salty areas), thermoacidophiles (hot springs) and marshy areas such as methanogens. These methanogens are present in the gut of ruminant animals aiding them in the digestion of food and are responsible for the production of biogas (methane) from the excretion of these animals.

Eubacteria (Latin, Eu – true, bacteria)

Presence of rigid cell wall, and/or flagellum for motility

Reproduction is seen mainly by fission. During unfavorable conditions, spore formation is also seen. Bacteria also reproduce by transfer of DNA from one bacterium into another.

Cyanobacteria are unicellular, photosynthetic autotrophic bacteria containing chlorophyll a pigment similar to that of green plants. Cyanobacteria live in colonies that generally surrounded by a gelatinous sheath. Some of these bacteria also help in fixing nitrogen. These specialized cells are known as heterocysts. Eg., Nostoc, and Anabaena

Chemosynthetic autotrophic bacteria are responsible for oxidizing nitrites, nitrates, and ammonia. The energy released in oxidization is used by the bacteria for the production of ATP. These bacteria also play a great role in recycling nutrients such as nitrogen, iron, sulfur, and phosphorus.

Heterotrophic bacteria: These bacteria are present everywhere and live on the dead, decaying and organic matter for nutrition. Many of these bacteria are helpful in the fermentation process, production of antibiotics, nitrogen fixation of soil etc. However, harmful bacteria also belong to the same class. Bacteria are responsible for causing various diseases such as cholera, typhoid, malaria, food poisoning, tetanus etc.

Mycoplasmas: Unicellular and lack cell wall and can live without oxygen. Many of the mycoplasma known is harmful to both plants and animals.

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