Types of microorganisms on the basis of their nutrition source

Carbon, hydrogen, oxygen, energy and electrons- all are required for the growth of microorganisms. On the basis of the source from which microorganism fulfills their requirement, they are classified into different classes.

On the basis of carbon source:

Autotrophs: They obtain carbon from inorganic sources like carbon dioxide (CO₂).

Heterotroph: They uses organic compound as a carbon source.

On the basis of energy source:

Phototrophs: They uses energy from sunlight.

Chemotrophs: They obtain energy from the oxidation of either organic (chemoorganotrophs) or inorganic (chemolithotrophs) chemical compounds by a process called chemosynthesis.

On the basis of electron source:

Lithotrophs: The word 'lithos' means rock and 'troph' means consumer; lithotrophs means “eaters of rock”. They uses inorganic substances to obtain electrons.

Organotrophs: They obtain electrons from organic substances.

Now, on the basis of the primary source of carbon, electron and energy; microorganisms are classified into four nutritional classes. They are mentioned below:

Photoautotrophs: They are also called as photolithotrophic autotroph or photolithoautotroph. As the name implies, their source of energy is light, carbon source is CO₂ and they uses inorganic substances as a source of electron. For instance, Purple and green sulfur bacteria extracts electron from inorganic donors like hydrogen, elemental sulfur and hydrogen sulfide. Algae and cyanobacteria also belongs to this class.

Photoorganotrophs: They are also known as photoorganotrophic heterotrophs or photoorganoheterotrophs. They are generally found in polluted lakes and streams. They use energy from sunlight, electron from organic substances and carbon from organic source. Purple nonsulfur bacteria and green nonsulfur bacteria are the examples of photoorganotrophs.

Chemolithotrophs: They are also known as chemolithotrophic autotrophs or chemolithoautotrophs. They basically oxidize reduced inorganic compounds like iron, nitrogen or sulfur to get energy and electron. As they are autotrophs, they use CO₂ as carbon source. They are involved in the chemical transformation of elements like conversion of ammonia to nitrate or sulfur to sulfate etc. Few examples of this includes sulfur-oxidizing bacteria, iron-oxidizing bacteria like Acidithiobacillus ferrooxidans, nitrifying bacteria like Nitrobacter etc.

Chemoorganotrophs: They are also known as chemo- heterotrophs or chemoorganoheterotrophs. As the name implies, they uses organic matter as electron and carbon source. Energy is obtained from either organic or inorganic compound. For this group of microorganism, a same organic source can satisfy all the requirements. Protozoa, fungi and non-photosynthetic bacteria belongs to this class.

Note: Some microbial species are present in nature that has metabolic flexibility i.e. they can change their metabolic patterns in response to environmental changes. Such microbes are known as mixotrophic. In other words, they can mix different source of carbon and energy depending upon the environmental condition. For instance, purple nonsulfur bacteria are photoorganoheterotrophs in the absence of oxygen but at normal oxygen level, they become chemotrophs and oxidize organic molecules to meet their energy requirement.