Six Kingdom Classification: There were only two kingdoms, Plants and Animals when Linnaeus developed his system of classification. Later, the use of microscopes led to the discovery of new organisms and the identification of differences in cells. A two-kingdom system was no longer useful. Today, the system has Six Kingdom Classifications – Plants, Animals, Protists, Fungi, Archaebacteria, and Eubacteria. The organisms are classified into their kingdoms by cell type (complex/simple), their ability to make food and the number of cells in their body.

We all, by now, must have understood the need for the classification of living organisms. Many classification schemes have been proposed since ancient times. The classification schemes improved as the knowledge of fields like Microscopy, Biochemistry, and Genetics. In this article, we look at the brief history and details of the six kingdom classification scheme.

History of Classification Systems

Many schemes of classifying the living organisms were proposed and were improved from time to time. Kingdom is the second-highest rank below the rank domain. Kingdoms are divided into smaller groups called phyla. The hierarchy of classification goes as:

• Domain
  • Kingdom
    • Phylum
      • Class
        • Order
          • Family
            • Genus
              • Species

This is called a taxonomic rank and the last two names, genus and species, form the biological nomenclature.

What is the Six Kingdom Classification?

Six kingdom system of classification is introduced by Carl Woese et. The six kingdoms are:

• Animalia,
• Plantae,
• Fungi,
• Protista,
• Archaea/Archaebacteria, and
• Bacteria/Eubacteria.

Fig: Six Kingdom Classification scheme Proposed by Carl Woose in \(1977\)

We get you the six kingdom classification with examples below:

Two Kingdom Classification Scheme

Living organisms have been classified into plants and animals since ancient times. Aristotle classified animals while his pupil Theophrastus classified plants.

Two kingdom classification was given by Carl Linnaeus. He classified living beings into plants and animals, but he also included minerals in his scheme of classification.

Fig: Two Kingdom Classification Scheme of Carl Linnaeus

Three Kingdom Classification Scheme

After the invention of the microscope by Antonie van Leeuwenhoek and his descriptions of microscopic organisms, it became very clear that classifying microorganisms or the lower creatures in either plant or animal kingdoms were blurring their boundaries. Hence, in \(1860\), John Hogg extended the classification scheme of Carl Linnaeus by proposing the third kingdom called Protoctista. But, he retained Regnum Lapideum as the fourth kingdom of minerals.
3 kingdom classification was given by Ernst Haeckel. In \(1866\), he extended this three kingdom classification scheme and revised the contents of the kingdom Protista many times. But, he did not retain the kingdom of minerals (Regnum Lapideum).

Fig: Three kingdom Classification Scheme initially proposed by John Hogg

Four Kingdom Classification Scheme

The invention and development of the microscope led to the differentiation between organisms with no distinct nucleus (prokaryotes) and those with a well-defined nucleus (eukaryotes). Based on this distinction, Robert Copeland, in \(1938\), proposed a four kingdom classification with the kingdom Monera to include the organisms with the indistinct nucleus.

In the \(1960{\rm{s}}\), a concept of superkingdom or empire was proposed by Roger Stanier and C. B. van Niel. Two empires such as Prokaryota and Eukaryota were proposed by them, and this laid the basis for three domain schemes of classification.

Fig: Four Kingdom Classification Scheme proposed by Roger Stanier and C. B. van Niel

Five Kingdom Classification Scheme

Robert Whittaker recognised the need for an additional kingdom Fungi and in \(1969\), proposed a new \(5\) kingdoms scheme of classification. This system of classification of organisms is based on differences in nutrition by organisms and is widely accepted and used in many works. In this scheme, fungi are saprotrophs, plants are photosynthesizing multicellular autotrophs whereas animals are multicellular heterotrophs.

Fig: Five Kingdom Classification Scheme by Robert Whittaker

Six Kingdom Classification Scheme

6 kingdom classification was given by Carl Woese in \(1977\).  He essentially separated the monera into archaebacteria and eubacteria based on the ribosomal RNA structure. This led to the proposal of three domains of life as ‘Bacteria, Archaea and Eukarya’. This model, combined with five kingdom classification, put forth six kingdoms clustered into three domains. He believed that all three domains originated from a common ancestor called ‘progenote’. In this scheme, kingdom monera is replaced with kingdom Archaebacteria and kingdom Eubacteria.

Fig: Six Kingdom Classification Scheme proposed by Carl Woese

Difference Between Archaebacteria and Eubacteria

Carl Woese proposed the scheme of six kingdom classification by separating archaebacteria from eubacteria, based on the sequence of \(16\;{\rm{S}}\) ribosomal RNA genes. Archaebacteria differ from eubacteria in some important ways such as the composition of the cell wall.

	Archaebacteria	Eubacteria
1	Capable of living and thriving in extreme conditions	Capable of living and thriving in normal conditions
2	Archaea thrive in extreme conditions such as hot springs, ocean depths, geothermal vents etc	Eubacteria are found in air, soil, water and in and on other organisms
3	Cell wall is made up of pseudopeptidoglycan	Cell wall is made up of peptidoglycan with muramic acid or sometimes lipopolysaccharides
4	Asexual reproduction with binary fission, fragmentation or budding	Sexual as well as asexual reproduction
5	Archaebacteria are non-pathogenic	Eubacteria are pathogenic

Features of 6 Kingdom Classification

This six kingdom scheme of classification is based on similarities or common characteristics. The basic characteristics used for classification in this scheme are cell type, nutrition and reproduction. Two cell types are prokaryotic and eukaryotic cells, modes of nutrition are photosynthetic, saprotrophic and heterotrophic while reproduction is sexual and asexual type.

We will look into the salient features of each of these kingdoms:

A. Kingdom Archaebacteria

These are the most primitive organisms on earth. They are single-celled prokaryotes with unique ribosomal RNA types. The nucleus is primitive and their cell wall composition makes them suitable to live in some extreme places on the earth like hot springs and geothermal vents. They are part of the microbiota of all organisms. They can be found in the guts of animals and humans. They exhibit a great variety of chemical reactions in their metabolism
1. Domain: Archaebacteria
2. Organisms: Methanogens, halophiles, thermophiles, and psychrophiles
3. Cell type: Prokaryotic
4. Metabolism: Use hydrogen, oxygen, carbon dioxide, methane, hydrogen sulfide or elemental sulfur, and ammonia depending on the species.
5. Nutrition: Phototrophs (sunlight), lithotrophs (inorganic compounds), organotrophs (organic compounds)
6. Reproduction: Asexual reproduction by binary or multiple fission, budding and fragmentation.

Fig: Archaebacteria

B. Kingdom Eubacteria

These are considered to be true bacteria. They are found in diverse places on earth and are part of the microbiota of animals and humans. Many of them cause diseases. Bacteria show variety in their cell shapes like round, spiral, comma shape, rod-shaped, etc.
1. Domain: Eubacteria
2. Organisms: Bacteria, cyanobacteria
3. Cell type: Prokaryotic
4. Metabolism: Aerobes need oxygen while anaerobic bacteria do not use oxygen
5. Nutrition: Absorption, photosynthesis or chemosynthesis
6. Reproduction: Asexual by binary fission, high growth rate

Fig: Gram-positive bacteria belong to Kingdom Eubacteria

C. Kingdom Protista

It is a diverse group of organisms with some resembling animals (protozoa) while others showing characteristics of plants (algae) and fungi (slime mould). Protists mark the evolution from the indistinct nucleus to a well-defined nucleus and are thus eukaryotes. Some may have animal-like cell organelles (mitochondria) while some may have plant cell organelles (chloroplast). These organisms can be found in equally diverse places.
1. Domain: Eukarya
2. Organisms: Amoeba, dinoflagellates, diatoms, euglena, plasmodium, giardia, etc.
3. Cell type: Eukaryotic
4. Metabolism: Aerobic (oxygen is needed for metabolism)
5. Nutrition: Phototrophic and heterotrophic (ingestion or absorption)
6. Reproduction: Mostly asexual reproduction by binary or multiple fission, sexual reproduction by gamete formation is also observed.

Fig: Paramecium as an example of Protista

D. Kingdom Fungi

The fungi may be microscopic as yeast or moulds or most familiar multicellular forms such as mushrooms. The characteristic feature of fungi is the presence of chitin in their cell walls. They do not photosynthesize but absorb the food by secreting digestive enzymes on the food substrates. The term typically used to describe this mode of nutrition is the saprotrophic mode of nutrition. Many of the fungal species cause diseases but they are important as well for recycling the nutrients back to the environment.
1. Domain: Eukarya
2. Organisms: mushrooms, yeast, moulds
3. Cell Type: eukaryotic, unicellular or multicellular
4. Metabolism: aerobes (need oxygen), some are facultative anaerobes ( do not need oxygen)
5. Nutrition: saprotrophic (secrete digestive enzymes onto the substrates)
6. Reproduction: sexual or asexual through spores

Fig: Fungi as an example of Kingdom Fungi

E. Kingdom Plantae

Plants are extremely important for all life on earth and are useful for getting oxygen, shelter, food, clothing, timber and medicines. Plants can be vascular or non-vascular, flowering or non-flowering and seed-bearing or non-seed bearing. They are the producers of almost all food chains in all major biomes.
1. Domain: Eukarya
2. Organisms: trees, herbs, shrubs, mosses, ferns etc
3. Cell type: eukaryotic, multicellular
4. Metabolism: strictly aerobes (need oxygen)
5. Nutrition: photosynthesis
6. Reproduction: asexual reproduction by vegetative propagation and sexual reproduction by gamete formation

Fig: Trees belong to Kingdom Plantae

F. Kingdom Animalia

This is a most diverse group of organisms. They are eukaryotic, and multicellular and depend on plants and other animals for food. Animalia is further classified broadly into non-chordates and chordates.
1. Domain: Eukarya
2. Cell type: Eukaryotic, multicellular
3. Organisms: Worms, insects, arthropods, spiders, fish, birds, mammals etc
4. Metabolism: Strictly aerobes (need oxygen)
5. Nutrition: Heterotrophs (depend on plants or other animals)
6. Reproduction: Mostly sexual through gamete formation, some forms show asexual reproduction by budding and fission.

Fig: Animals belong to Kingdom Animalia

Summary

Humans, owing to their curious nature, always tried to classify the living world. As the scope of our knowledge widened due to new inventions and discoveries, the distinctions between organisms became stark and boundaries of already made groups and categories blurred. Criteria for classifying organisms became finer and thus new classification schemes were proposed and accepted.  

Inventions in the field of biotechnology are playing their roles too and these tools are more powerful such that the characterization of any living organism can be done at DNA and protein levels. A single nucleotide difference between the species can be detected. However, as of now, the five-kingdom classification scheme is still used in many standard books but the six kingdom classification scheme is a widely accepted one.

FAQs on 6 Kingdom Classification

Listed here are some frequently asked questions on the topic 6 kingdom classification:

Q.1: What are the major characteristics of the six kingdom classification?
A: Major characteristics of the six kingdom classification are:
a. Bifurcation of kingdom Monera into two kingdoms as Archaebacteria and Eubacteria.
b. All living organisms are classified into six kingdoms as Archaebacteria, Eubacteria, Protista, Fungi, Plantae and Animalia.
c. The three kingdoms are clustered together into three domains as Bacteria, Archaea and Eukarya.

Q.2: On what basis six kingdom classification was proposed?
A: Key differences between archaebacteria and eubacteria such as the composition of the cell wall and \(16\;{\rm{S}}\) RNA genes were the basis six kingdom classification scheme.

Q.3: What does Six Kingdom Classification say?
A: Six kingdom Classification classifies the living organisms into six kingdoms clustered into three domains.

Q.4: Who proposed two kingdom classification system?
A: Carl Linnaeus proposed the two kingdom classification.

Q.5: Explain the six kingdom classification with an example.
A:

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