Primary nutritional groups

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

The carbon source refers to the type of carbon used by the organism for the growth and development of cells. An organism is defined heterotrophic when using organic substrates to get its carbon from organic sources for growth and development, whereas it is autotrophic, when its source is of mineral origin (CO2).

The 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 of chemical source.

The basis for energy metabolism of all living organisms is the oxidation-reduction reaction in which electrons move from an electron donnor to an electron acceptor. Energy is released during the reaction.

Nutrients are also 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 electron, whereas it is called lithotrophic when using a mineral source.

Finally, the last 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-, SO42- (anaerobic respiration), or it can be organic (fermentation).

Examples

All sort of combinations may exist; for example a cyanobacteria is photolithotrophic, and fungi are chemoorganoheterotrophic.

Eucaryotes are usually pretty easy to categorise. All animals are heterotrophic, as well as fungi. Plants are autotrophic. Just as well, each group of eucaryotic microorganisms is limited to one nutritional mode.

On the opposite, procaryotes show a great diversity of nutritional categories. For example, purple S bacteria, or cyanobacteria are photolithoautotrophic, purple non-S bacterias are photoorganoheterotrophic.
Besides, if some bacterial species limit themselves to only one nutritional group, others are facultative and will switch from one mode to the other depending on which nutrients are available. For exemple, archeobacteria are chemo organo and/or chemo-litho autotrophic.