The main determinants of a microorganism nutritional type are it sources of carbon and energy. Microorganisms  have two sources of energy available. Microbes that oxidize chemical compounds (either organic or inorganic) for energy are chemotrophs; those that use light (photosynthesis) as their energy sources are called phototrophs. The terms for carbon and energy source are often merged into a single world (Table 1). 

     

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Autotrophs 

These microbes derived energy from one or two possible non-living sources: sunlight (photoautotrophs) and chemical reactions (chemoautotrophs).

• Photoautotrophs: ​Use inorganic chemical substance as sources of energy and carbon dioxide as the main source of carbon.

• Chemoautotrophs: Microorganisms that oxidize inorganic chemical substance as sources of energy and carbon dioxide as the main source of carbon.

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Heterotrophs

The majority of these organisms are chemoheterotrophs that derive both carbon and energy from organic compounds. They process organic molecules by respiration or fermentation and release energy in the form of ATP.

• Chemoheterotrophs:  Microbes that use organic chemical substances as sour es of energy and organic compounds as the main source of carbon. Microorganisms belong to one of two main categories that differ in how they obtain their organic nutrients. Saprobes (bacteria and fungi) are free-living microorganisms that feed primarily on organic detritus from dead organisms and  parasites ordinarily derive nutrients from the cells or tissues of a living host.

• Photoheterotrophs: Use the light as source of energy and organic compounds as the main source of carbon.

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Microorganisms also have only two sources for electrons, the lithotrophs that used reduced inorganic; and those  extract electrons form organic compounds called organotrophs. A combination of the above terms describes four nutritional types of microorganisms (Table 2).

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