A heterotroph (; from Ancient Greek ἕτερος (héteros), meaning "other", and τροφή (trophḗ), meaning "nourishment") is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly matter from other organisms. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but not producers. Living organisms that are heterotrophic include most animals, all fungi, some bacteria and protists, and many parasitic plants. The term heterotroph arose in microbiology in 1946 as part of a classification of microorganisms based on their type of nutrition. The term is now used in many fields, such as ecology, in describing the food chain. Heterotrophs occupy the second and third trophic levels of the food chain while autotrophs occupy the first trophic level. Heterotrophs may be subdivided according to their energy source. If the heterotroph uses chemical energy, it is a chemoheterotroph (e.g., humans and mushrooms). If it uses light for energy, then it is a photoheterotroph (e.g., haloquadratum walsbyi and green non-sulfur bacteria). Heterotrophs represent one of the two mechanisms of nutrition (trophic levels), the other being autotrophs (auto = self, troph = nutrition). Autotrophs use energy from sunlight (photoautotrophs) or oxidation of inorganic compounds (lithoautotrophs) to convert inorganic carbon dioxide to organic carbon compounds and energy to sustain their life. Comparing the two in basic terms, heterotrophs (such as animals) eat either autotrophs (such as plants) or other heterotrophs, or both. Detritivores are heterotrophs which obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces). Saprotrophs (also called lysotrophs) are chemoheterotrophs that use extracellular digestion in processing decayed organic matter. The process is most often facilitated through the active transport of such materials through endocytosis within the internal mycelium and its constituent hyphae.
Types Heterotrophs can be organotrophs or lithotrophs.
Organoheterotrophs exploit reduced carbon compounds (organics) as electron sources, such as carbohydrates, fats, and proteins from plants and animals. Lithoheterotrophs, on the other hand, use inorganic compounds such as ammonium, nitrite, or sulfur, to obtain electrons. Another way of classifying different heterotrophs is by assigning them as chemotrophs or phototrophs. Phototrophs utilize light to obtain energy and carry out metabolic processes, whereas chemotrophs use the energy obtained by the oxidation of chemicals from their environment.
Photoorganoheterotrophs, such as Rhodospirillaceae and purple non-sulfur bacteria synthesize organic compounds using sunlight coupled with oxidation of organic substances. They use organic compounds to build structures. They do not fix carbon dioxide and apparently do not have the Calvin cycle. Chemolithoheterotrophs like Oceanithermus profundus obtain energy from the oxidation of inorganic compounds, including hydrogen sulfide, elemental sulfur, thiosulfate, and molecular hydrogen. Mixotrophs (or facultative chemolithotroph) can use either carbon dioxide or organic carbon as the carbon source, meaning that mixotrophs have the ability to use both heterotrophic and autotrophic methods. Although mixotrophs have the ability to grow under both heterotrophic and autotrophic conditions, C. vulgaris have higher biomass and lipid productivity when growing under heterotrophic compared to autotrophic conditions. Heterotrophs, by consuming reduced carbon compounds, are able to use all the energy that they obtain from food for growth and reproduction, unlike autotrophs, which must use some of their energy for carbon fixation. Both heterotrophs and autotrophs alike are usually dependent on the metabolic activities of other organisms for nutrients other than carbon, including nitrogen, phosphorus, and sulfur, and can die from lack of food that supplies these nutrients. This applies not only to animals and fungi but also to bacteria.
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