Primary nutritional groups

An organism may be placed into one of the four major nutritional goups according to their carbon, energy, and electron sources.

• Carbon source refers to the source of carbon used by an organism for growth and development. An organism is defined has heterotrophic when using organic substrates to get its carbon for growth and development, whereas it is autotrophic, when its source of carbon is of mineral origin (CO₂).

• Energy source refers to the nutrients that are used by the organism to produce ATP, required for the synthesis of cells. An organism is defined phototrophic when using light as a source of energy, whereas it is chemotrophic when using a mineral source.

• The basis for energy metabolism of all living organisms is the oxidation-reduction reaction in which electrons move from an electron donor to an electron acceptor. Energy is released during the reaction. Nutrients are used as sources of electrons, required for the synthesis of cells. An organism is defined organotrophic when using an organic compound as a source of electrons, whereas it is called lithotrophic when using a mineral source.

A final factor is the type of compound being used as an electron acceptor. It can be mineral, such as O₂ (aerobic respiration) or under other forms, such as NO₃^-, SO₄^2- (anaerobic respiration), or it can be organic (fermentation).

Examples

All sort of combinations may exist in nature. For example a cyanobacteria is photolithotrophic, and fungi are chemo-organo-heterotrophic. Eucaryotess are generally easy to categorise. All animals are heterotrophic, as are fungi. Plants are photoautotrophic. Each group of eucaryotic microorganisms is limited to one nutritional mode.

On the contrary, procaryotess show a great diversity of nutritional categories. For example, purple S bacteria, or cyanobacteria are photolithoautotrophic, purple non-S bacterias are photoorganoheterotrophic. Some bacteria limit themselves to only one nutritional group, whereas others are facultative and switch from one mode to the another depending on the nutrient sources available. For example, archeobacteria are chemo-organo and/or chemo-litho autotrophic.