All organisms that are very small or microscopic in size, and cannot be seen with the naked eye are referred to as microorganisms. Microorganisms are visible under the microscope. Anton van Leeuwenhoek first observed microorganisms under the microscope. Microorganisms include bacteria, archaea, algae, fungi, protozoa, etc. Although viruses are not considered living organisms, sometimes they are also included in the microorganisms category.
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Classification is the process by which organisms are grouped into various categories based on morphological and physiological characteristics. There have been various attempts to classify organisms based on their morphological, physiological, cellular and molecular characteristics. Modern classification is also based on evolutionary relationships i.e. phylogenetic relationships.
Microorganisms are prokaryotic, such as bacteria, archaea,etc., as well as eukaryotic, such as protozoa, algae, fungi, etc. R.H. Whittaker elucidated the Five Kingdom Classification, which was based on the following characteristics:
- Cell type (prokaryotic and eukaryotic) and presence of nuclear membrane
- Presence of cell wall and its constituents
- Body organisation
- Mode of nutrition
- Mode of reproduction
- Phylogenetic relationships
Based on the following characteristics, R. H. Whittaker divided living organisms into five kingdoms. They are as follows:
- Monera – Unicellular prokaryotes
- Protista – Unicellular eukaryotes
- Fungi – Eukaryotic, heterotrophic (saprophytic/ parasitic) and with a cell wall (chitin)
- Plantae – Eukaryotic, autotrophic (photosynthetic) and with a cell wall (cellulose)
- Animalia – Eukaryotic, heterotrophic (holozoic/ saprophytic etc.) and without a cell wall
Let’s learn more in detail about the classification of microorganisms based on the Five Kingdom Classification.
Prokaryotic microorganisms include bacteria, cyanobacteria or blue-green algae, archaea, mycoplasma, etc. Eukaryotic microorganisms include protists, protozoans, slime moulds, algae, fungi, etc.
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Bacteria (Monera)
As per the Five Kingdom Classification, bacteria are classified in the kingdom Monera. It includes Eubacteria and Archaebacteria. They are all unicellular, have a prokaryotic cell which is devoid of a membrane-bound nucleus, and other organelles such as endoplasmic reticulum, mitochondria, Golgi bodies, etc.
Later, archaebacteria were placed into a different domain of the three-domain system, i.e. Bacteria, Archaea and Eukarya.
Bacteria are the most abundant microorganisms and are present almost everywhere. They are classified as Gram-positive and Gram-negative, based on the Gram’s staining pattern.
Based on the shape of the cell, bacteria are classified into four main groups that are as follows:
- Coccus or cocci (spherical)
- Bacillus or bacilli (rod-shaped)
- Spirillum or spirilla (spiral)
- Vibrium or vibrio (comma-shaped)
Further reading: Shapes Of Bacteria
Eubacteria
They are true bacteria. They have a rigid cell wall and may contain flagella. They are autotrophic as well as heterotrophic. Bacteria reproduce by binary fission and DNA transfer. Some bacteria produce spores under unfavourable conditions. Mycoplasma does not contain a cell wall.
- Photosynthetic autotrophs: This includes blue-green algae or cyanobacteria. They contain chlorophyll ‘a’ similar to green plants, and perform photosynthesis. Some cyanobacteria are also capable of fixing atmospheric nitrogen. E.g. Nostoc and Anabaena.
- Chemosynthetic autotrophs: These microorganisms utilise energy derived from the oxidation of inorganic substances such as nitrates, ammonia, sulphur, etc. and produce ATP. These organisms play an important role in nutrient recycling. E.g. purple sulphur bacteria.
- Heterotrophs: They are widely distributed and play a key role in the ecosystem as decomposers. They play a significant role in our lives. They are used for the industrial production of antibiotics, organic acids, etc. They also act as nitrogen-fixers, e.g. Rhizobium in the root nodules of legumes. Some bacteria are pathogenic to plants and animals causing various diseases, e.g. cholera, tuberculosis, typhoid, botulism, tetanus, citrus canker, fire blight of apple, etc.
Archaebacteria
They can thrive in extreme environmental conditions. They have different cell wall compositions, which enable them to survive in harsh conditions. The cell membrane of archaea is ether-linked as compared to ester-linked in bacteria. They are further classified into three main groups:
- Methanogens – They are found in marshy areas. They are found in the gut of many ruminating animals and are utilised for the commercial production of methane (biogas).
- Halophiles – They are found in extreme salty areas.
- Thermoacidophiles – They can tolerate extreme temperatures and low pH. They are found in hot springs.
Protozoa
In the Five Kingdom classification, protozoans are classified under the kingdom Protista, which includes unicellular eukaryotes. Protozoans are heterotrophs. They live as parasites or predators.
On the basis of their mode of locomotion, they are classified into four major groups. They are:
- Amoeboid – They have pseudopodia, which is used for movement and capturing prey. E.g. Amoeba, Entamoeba, etc.
- Flagellated – These protozoans are flagellated. They are either free-living or parasites. E.g. Trypanosoma, the causative organism of sleeping sickness, Leishmania, which causes kala-azar, etc.
- Ciliated – They are aquatic and have thousands of cilia present on the body surface, e.g. Paramoecium. Cilia also help in bringing water laden with food inside the cavity of the gullet.
- Sporozoans – They are non-motile. They produce spores. E.g. Plasmodium, the causative organism of malaria, Toxoplasma, etc.
Also Check: Protozoa
Slime Moulds
Slime moulds are also protists. They are saprophytes. They often form aggregates called plasmodium. They form spores that are extremely resistant to adverse conditions. Slime Moulds move along dead and decaying branches and leaves, and absorb organic compounds.
Fungi
Fungi are separated into a different kingdom. They are heterotrophic and have a rigid cell wall. They are parasites or saprotrophs. Fungi are microscopic, as well as quite big in size. They are cosmopolitan and grow in warm, humid places. A unicellular fungi – yeast, is used for the industrial production of bread and alcoholic beverages. Penicillium is used for the production of antibiotics. Some fungi cause diseases in plants and animals, e.g. wheat rust (Puccinia), Candida albicans causing fungal infection in humans.
Fungi are classified into four main classes based on their morphology and method of spore formation. They are:
- Phycomycetes – They are characterised by the presence of coenocytic mycelium. Spores are produced endogenously in the sporangium, e.g. Rhizopus, Mucor, etc.
- Ascomycetes – They are commonly known as sac-fungi. The mycelium is branched and septate. The asexual spores are produced exogenously on conidiophores and sexual spores are produced endogenously within asci. E.g. Penicillium, Saccharomyces (yeast), Aspergillus, Claviceps and Neurospora, etc.
- Basidiomycetes – Asexual spores are not formed. The basidiospores are exogenously produced. E.g. Puccinia (rust), mushrooms, Ustilago (smut), etc.
- Deuteromycetes – Commonly called imperfect fungi due to absence of sexual stage in the life cycle. Most fungi are decomposers and help in nutrient recycling. E.g. Colletotrichum, Alternaria and Trichoderma.
Algae
Unicellular photosynthetic organisms like diatoms, golden algae and dinoflagellates are placed under the kingdom Protista. They are microscopic and are plankton. They are photosynthetic and chief producers in the sea.
- Diatoms – They have a silicious cell wall which fits like a soap box. They are photosynthetic and ‘diatomaceous earth’ is used for polishing, filtration of oil, etc.
- Dinoflagellates – They are marine, and photosynthetic and appear in different colours. They have two unequal flagella, one longitudinal and one transverse. The cell wall has stiff cellulose plates. They are known to cause red tides due to rapid multiplication, e.g. Gonyaulax (a red dinoflagellate).
Read more: Dinoflagellates
Algae are placed in the kingdom Plantae. Algae vary in size and shape. They are unicellular or multicellular. Some green algae are microscopic in size, and many form colonies. Algae are classified based on the presence of pigment present in them:
- Chlorophyceae – Green algae. They have chlorophyll a and b pigments. Some green algae are microscopic. E.g. Chlamydomonas, Volvox, Ulothrix, Spirogyra. Chlamydomonas is a unicellular green alga. Volvox forms coenobium or a colony of thousands of cells.
- Phaeophyceae – Brown algae. They have chlorophyll a and c, and fucoxanthin pigments.
- Rhodophyceae – Red algae. They have chlorophyll a and d, and phycoerythrin pigments.
This was all about the Classification of Microorganisms. Learn more about other related concepts for NEET, only at BYJU’S.
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