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Primary nutritional groups are groups oforganisms, divided in relation to the nutrition mode according to the sources of energy and carbon, needed for living, growth and reproduction. The sources of energy can be light or chemical compounds; the sources of carbon can be of organic or inorganic origin.^[1]

The termsaerobic respiration,anaerobic respiration andfermentation (substrate-level phosphorylation) do not refer to primary nutritional groups, but simply reflect the different use of possible electron acceptors in particular organisms, such asO₂ in aerobic respiration, ornitrate (NO⁻
₃),sulfate (SO²⁻
₄) orfumarate in anaerobic respiration, or various metabolic intermediates in fermentation.

Phototrophs absorb light inphotoreceptors and transform it into chemical energy.
Chemotrophs release chemical energy.

The freed energy is stored aspotential energy inATP,carbohydrates, orproteins. Eventually, the energy is used for life processes such as moving, growth and reproduction.

Plants and some bacteria can alternate between phototrophy and chemotrophy, depending on the availability of light.

Organotrophs use organic compounds aselectron/hydrogen donors.
Lithotrophs use inorganic compounds as electron/hydrogen donors.

Theelectrons or hydrogen atoms fromreducing equivalents (electron donors) are needed by both phototrophs and chemotrophs inreduction-oxidation reactions that transfer energy in the anabolic processes ofATP synthesis (in heterotrophs) orbiosynthesis (in autotrophs). The electron or hydrogen donors are taken up from the environment.

Organotrophic organisms are often also heterotrophic, using organic compounds as sources of both electrons and carbon. Similarly, lithotrophic organisms are often also autotrophic, using inorganic sources of electrons and CO₂ as their inorganic carbon source.

Some lithotrophic bacteria can utilize diverse sources of electrons, depending on the availability of possible donors.

The organic or inorganic substances (e.g., oxygen) used as electron acceptors needed in the catabolic processes of aerobic or anaerobicrespiration andfermentation are not taken into account here.

For example, plants are lithotrophs because they use water as their electron donor for the electron transport chain across the thylakoid membrane. Animals are organotrophs because they use organic compounds as electron donors to synthesize ATP (plants also do this, but this is not taken into account). Both use oxygen in respiration as electron acceptor, but this character is not used to define them as lithotrophs.

Heterotrophsmetabolize organic compounds to obtain carbon for growth and development.
Autotrophs usecarbon dioxide (CO₂) as their source of carbon.

Achemoorganoheterotrophicorganism is one that requires organicsubstrates to get itscarbon for growth and development, and that obtains its energy from the decomposition of an organic compound. This group of organisms may be further subdivided according to what kind of organic substrate and compound they use.Decomposers are examples of chemoorganoheterotrophs which obtain carbon and electrons or hydrogen from dead organic matter.Herbivores andcarnivores are examples of organisms that obtain carbon and electrons or hydrogen from living organic matter.

Chemoorganotrophs areorganisms which use the chemical energy inorganic compounds as their energy source and obtain electrons or hydrogen from the organic compounds, including sugars (i.e.glucose), fats and proteins.^[2] Chemoheterotrophs also obtain the carbon atoms that they need for cellular function from these organic compounds.

Allanimals are chemoheterotrophs (meaning they oxidize chemical compounds as a source of energy and carbon), as arefungi,protozoa, and somebacteria. The important differentiation amongst this group is that chemoorganotrophs oxidize only organic compounds whilechemolithotrophs instead use oxidation ofinorganic compounds as a source of energy.^[3]

The following table gives some examples for each nutritional group:^[4][5][6][7]

*Some authors use-hydro- when the source is water.

Look up-troph in Wiktionary, the free dictionary.

The common final part-troph is from Ancient Greekτροφήtrophḗ "nutrition".

Some, usually unicellular, organisms can switch between different metabolic modes, for example between photoautotrophy, photoheterotrophy, and chemoheterotrophy inChroococcales.^[13]Rhodopseudomonas palustris – another example – can grow with or withoutoxygen, use either light, inorganic or organic compounds for energy.^[14] Suchmixotrophic organisms may dominate theirhabitat, due to their capability to use more resources than either photoautotrophic or organoheterotrophic organisms.^[15]

All sorts of combinations may exist in nature, but some are more common than others. For example, most plants arephotolithoautotrophic, since they use light as an energy source, water as electron donor, and CO₂ as a carbon source. All animals and fungi arechemoorganoheterotrophic, since they use organic substances both as chemical energy sources and as electron/hydrogen donors and carbon sources. Someeukaryotic microorganisms, however, are not limited to just one nutritional mode. For example, some algae live photoautotrophically in the light, but shift to chemoorganoheterotrophy in the dark. Even higher plants retained their ability to respire heterotrophically on starch at night which had been synthesised phototrophically during the day.

Prokaryotes show a great diversity ofnutritional categories.^[16] For example,cyanobacteria and manypurple sulfur bacteria can bephotolithoautotrophic, using light for energy,H₂O or sulfide as electron/hydrogen donors, and CO₂ as carbon source, whereasgreen non-sulfur bacteria can bephotoorganoheterotrophic, using organic molecules as both electron/hydrogen donors and carbon sources.^[8][16] Many bacteria arechemoorganoheterotrophic, using organic molecules as energy, electron/hydrogen and carbon sources.^[8] Some bacteria are limited to only one nutritional group, whereas others are facultative and switch from one mode to the other, depending on the nutrient sources available.^[16]Sulfur-oxidizing,iron, andanammox bacteria as well asmethanogens arechemolithoautotrophs, using inorganic energy, electron, and carbon sources.Chemolithoheterotrophs are rare because heterotrophy implies the availability of organic substrates, which can also serve as easy electron sources, making lithotrophy unnecessary.Photoorganoautotrophs are uncommon since their organic source of electrons/hydrogens would provide an easy carbon source, resulting in heterotrophy.

Synthetic biology efforts enabled the transformation of the trophic mode of twomodel microorganisms from heterotrophy to chemoorganoautotrophy:

• Escherichia coli wasgenetically engineered and thenevolved in the laboratory to use CO₂ as the sole carbon source while using the one-carbon moleculeformate as the source of electrons.^[10]
• ThemethylotrophicPichia pastoris yeast was genetically engineered to use CO₂ as the carbon source instead ofmethanol, while the latter remained the source of electrons for the cells.^[11]

• Trophic ecology
• Physiology

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