A heterotroph (/ˈhɛtərəˌtroʊf, -ˌtrɒf/;[1] Ancient Greek ἕτερος héteros = "other" plus trophe = "nutrition") is an organism that cannot produce its own food, relying instead on the intake of nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but not producers.[2][3] Living organisms that are heterotrophic include all animals and fungi, some bacteria and protists,[4] and parasitic plants. The term heterotroph arose in microbiology in 1946 as part of a classification of microorganisms based on their type of nutrition.[5] The term is now used in many fields, such as ecology in describing the food chain.

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., 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 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).[6] Saprotrophs (also called lysotrophs) are chemoheterotrophs that use extracellular digestion in processing decayed organic matter. It is a term most often associated with fungi. The process is most often facilitated through the active transport of such materials through endocytosis within the internal mycelium and its constituent hyphae.[7]

Auto-and heterotrophs
Cycle between autotrophs and heterotrophs. Autotrophs use light, carbon dioxide (CO2), and water to form oxygen and complex organic compounds, mainly through the process of photosynthesis (green arrow). Both types of organisms use such compounds via cellular respiration to both generate ATP and again form CO2 and water (two red arrows).


Heterotrophs can be organotrophs or lithotrophs. Organotrophs exploit reduced carbon compounds as electron sources, like carbohydrates, fats, and proteins from plants and animals. On the other hand, lithotrophs use inorganic compounds, such as ammonium, nitrite, and sulfur to obtain electron sources. 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.[8]

Photoorganoheterotrophs, such as Rhodospirillaceae and purple non-sulfur bacteria synthesize organic compounds using sunlight coupled with oxidation of organic substances, including hydrogen sulfide, elemental sulfur, thiosulfate, and molecular hydrogen. They use organic compounds to build structures. They do not fix carbon dioxide and apparently do not have the Calvin cycle.[9] Chemolithoheterotrophs like the Oceanithermus profundus[10] obtain energy from the oxidation of inorganic compounds. 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 methods as well as autotrophic methods.[11][12] Although mixotrophs have the ability to grow both under both heterotrophic and autotrophic conditions, C. vulgaris have higher biomass and lipid productivity when growing under heterotrophic conditions compared to autotrophic conditions.[13]

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.[9] 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.[14] This applies not only to animals and fungi but also to bacteria.[9]


AutoHeteroTrophs flowchart
Flowchart to determine if a species is autotroph, heterotroph, or a subtype


Many heterotrophs are chemoorganoheterotrophs that use organic carbon (e.g. glucose) as their carbon source, and organic chemicals (e.g. carbohydrates, lipids, proteins) as their energy and electron sources.[15] Heterotrophs function as consumers in food chain: they obtain these nutrients from saprotrophic, parasitic, or holozoic nutrients.[16] They break down complex organic compounds (e.g., carbohydrates, fats, and proteins) produced by autotrophs into simpler compounds (e.g., carbohydrates into glucose, fats into fatty acids and glycerol, and proteins into amino acids). They release energy by oxidizing carbon and hydrogen atoms present in carbohydrates, lipids, and proteins to carbon dioxide and water, respectively.

They can catabolize organic compounds by respiration, fermentation, or both. Fermenting heterotrophs are either facultative or obligate anaerobes that carry out fermentation in low oxygen environments, in which the production of ATP is commonly coupled with substrate-level phosphorylation and the production of end products (e.g. alcohol, CO2, sulfide).[17] These products can then serve as the substrates for other bacteria in the anaerobic digest, and be converted into CO2 and CH4, which is an important step for the carbon cycle for removing organic fermentation products from anaerobic environments.[17] Heterotrophs can undergo respiration, in which ATP production is coupled with oxidative phosphorylation.[17][18] This leads to the release of oxidized carbon wastes such as CO2 and reduced wastes like H2O, H2S, or N2O into the atmosphere. Heterotrophic microbes’ respiration and fermentation account for a large portion of the release of CO2 into the atmosphere, making it available for autotrophs as a source of nutrient and plants as a cellulose synthesis substrate.[19][18]

Respiration in heterotrophs is often accompanied by mineralization, the process of converting organic compounds to inorganic forms.[19] When the organic nutrient source taken in by the heterotroph contains essential elements such as N, S, P in addition to C, H, and O, they are often removed first to proceed with the oxidation of organic nutrient and production of ATP via respiration.[19] S and N in organic carbon source are transformed into H2S and NH4+ through desulfurylation and deamination, respectively.[19][18] Heterotrophs also allow for dephosphorylation as part of decomposition.[18] The conversion of N and S from organic form to inorganic form is a critical part of the nitrogen and sulfur cycle. H2S formed from desulfurylation is further oxidized by lithotrophs and phototrophs while NH4+ formed from deamination is further oxidized by lithotrophs to the forms available to plants.[19][18] Heterotrophs’ ability to mineralize essential elements is critical to plant survival.[18]

Most opisthokonts and prokaryotes are heterotrophic; in particular, all animals and fungi are heterotrophs.[4] Some animals, such as corals, form symbiotic relationships with autotrophs and obtain organic carbon in this way. Furthermore, some parasitic plants have also turned fully or partially heterotrophic, while carnivorous plants consume animals to augment their nitrogen supply while remaining autotrophic.

Animals are classified as heterotrophs by ingestion, fungi are classified as heterotrophs by absorption.


  1. ^ "Grc". Merriam-Webster Dictionary.
  2. ^ "Heterotroph Definition". Biology Dictionary. 15 December 2016.
  3. ^ Hogg, Stuart (2013). Essential Microbiology (2nd ed.). Wiley-Blackwell. p. 86. ISBN 978-1-119-97890-9.
  4. ^ a b "How Cells Harvest Energy". McGraw-Hill Higher Education.
  5. ^ Lwoff, A., C.B. van Niel, P.J. Ryan, and E.L. Tatum (1946). Nomenclature of nutritional types of microorganisms. Cold Spring Harbor Symposia on Quantitative Biology (5th edn.), Vol. XI, The Biological Laboratory, Cold Spring Harbor, N.Y., pp. 302–303, [1].
  6. ^ Wetzel, R. G. 2001. Limnology: Lake and River Ecosystems. Academic Press. 3rd. p.700.
  7. ^ Advanced biology principles, p 296—states the purpose of saprotrophs and their internal nutrition, as well as the main two types of fungi that are most often referred to, as well as describes, visually, the process of saprotrophic nutrition through a diagram of hyphae, referring to the Rhizobium on damp, stale whole-meal bread or rotting fruit.
  8. ^ Mills, A.L. (1997). The Environmental Geochemistry of Mineral Deposits: Part A: Processes, Techniques, and Health Issues Part B: Case Studies and Research Topics (PDF). Society of Economic Geologists. pp. 125–132. ISBN 978-1-62949-013-7. Retrieved 9 October 2017.
  9. ^ a b c Mauseth, James D. (2008). Botany: an introduction to plant biology (4th ed.). Jones & Bartlett Publishers. p. 252. ISBN 978-0-7637-5345-0.
  10. ^ M. L. Miroshnichenko et al., Oceanithermus profundus gen. nov., sp. nov., a thermophilic, microaerophilic, facultatively chemolithoheterotrophic bacterium from a deep-sea hydrothermal vent
  11. ^ Libes, Susan M. (2009). Introduction to marine biogeochemistry (2nd ed.). Academic Press. p. 192. ISBN 978-0-12-088530-5.
  12. ^ Dworkin, Martin (2006). The prokaryotes: ecophysiology and biochemistry (3rd ed.). Springer. p. 988. ISBN 978-0-387-25492-0.
  13. ^ Liang, Yanna (July 2009). "Biomass and lipid productivities of Chlorella vulgaris under autotrophic, heterotrophic and mixotrophic growth conditions". Biotechnology Letters. 31 (7): 1043–1049. doi:10.1007/s10529-009-9975-7. PMID 19322523.
  14. ^ Campbell and Reece (2002). Biology (7th ed.). Benjamin-Cummings Publishing Co. ISBN 978-0805371710.
  15. ^ Mills, A.L. "The role of bacteria in environmental geochemistry" (PDF). Retrieved 19 November 2017.
  16. ^ "Heterotrophic nutrition and control of bacterial density" (PDF). Retrieved 19 November 2017.
  17. ^ a b c Gottschalk, Gerhard (2012). Bacterial Metabolism. Springer Series in Microbiology (2 ed.). Springer. doi:10.1007/978-1-4612-1072-6. ISBN 978-0387961538.
  18. ^ a b c d e f Wade, Bingle (2016). MICB 201: Introductory Environmental Microbiology. pp. 236–250.
  19. ^ a b c d e Kirchman, David L. (2014). Processes in Microbial Ecology. OUP Oxford. pp. 79–98. ISBN 9780199586936.

2,4-Bis(4-hydroxybenzyl)phenol is a phenolic compound produced by the saprophytic orchid Gastrodia elata and by the myco-heterotroph orchid Galeola faberi.


Arachnitis uniflora, the sole species in the genus Arachnitis, is a non-photosynthetic species of plant. It is a myco-heterotroph which gets many of its nutrients from fungi of the genus Glomus which live in its roots.It is native to southern South America (Bolivia, Chile, Argentina) and the Falkland Islands.


An autotroph or primary producer, is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) from simple substances present in its surroundings, generally using energy from light (photosynthesis) or inorganic chemical reactions (chemosynthesis). They are the producers in a food chain, such as plants on land or algae in water (in contrast to heterotrophs as consumers of autotrophs). They do not need a living source of energy or organic carbon. Autotrophs can reduce carbon dioxide to make organic compounds for biosynthesis and also create a store of chemical energy. Most autotrophs use water as the reducing agent, but some can use other hydrogen compounds such as hydrogen sulfide. Some autotrophs, such as green plants and algae, are phototrophs, meaning that they convert electromagnetic energy from sunlight into chemical energy in the form of reduced carbon.

Autotrophs can be photoautotrophs or chemoautotrophs. Phototrophs use light as an energy source, while chemotrophs use electron donors as a source of energy, whether from organic or inorganic sources; however in the case of autotrophs, these electron donors come from inorganic chemical sources. Such chemotrophs are lithotrophs. Lithotrophs use inorganic compounds, such as hydrogen sulfide, elemental sulfur, ammonium and ferrous iron, as reducing agents for biosynthesis and chemical energy storage. Photoautotrophs and lithoautotrophs use a portion of the ATP produced during photosynthesis or the oxidation of inorganic compounds to reduce NADP+ to NADPH to form organic compounds.


Chemotrophs are organisms that obtain energy by the oxidation of electron donors in their environments. These molecules can be organic (chemoorganotrophs) or inorganic (chemolithotrophs). The chemotroph designation is in contrast to phototrophs, which use solar energy. Chemotrophs can be either autotrophic or heterotrophic. Chemotrophs are found in ocean floors where sunlight cannot reach them because they are not dependent on solar energy. Ocean floors often contain underwater volcanos that can provide heat as a substitute for sunlight's warmth.


Chilomonas is a genus of cryptophytes, including the species Chilomonas paramecium. Chilomonas is a protozoa (heterotroph). Chilomonas is golden brown and has two flagella.

Generalist and specialist species

A generalist species is able to thrive in a wide variety of environmental conditions and can make use of a variety of different resources (for example, a heterotroph with a varied diet). A specialist species can thrive only in a narrow range of environmental conditions or has a limited diet. Most organisms do not all fit neatly into either group, however. Some species are highly specialized (the most extreme case being monophagy), others less so, and some can tolerate many different environments. In other words, there is a continuum from highly-specialized to broadly-generalist species.

Omnivores are usually generalists. Herbivores are often specialists, but those that eat a variety of plants may be considered generalists. A well-known example of a specialist animal is the koala, which subsists almost entirely on eucalyptus leaves. The raccoon is a generalist because it has a natural range that includes most of North and Central America, and it is omnivorous, eating berries, insects, butterflies (Hackberry Emperor, for example), eggs and small animals. Monophagous organisms feed exclusively, or nearly so, on a single other species.

The distinction between generalists and specialists is not limited to animals. For example, some plants require a narrow range of temperatures, soil conditions and precipitation to survive while others can tolerate a broader range of conditions. A cactus could be considered a specialist species. It will die during winters at high latitudes or if it receives too much water.

When body weight is controlled for, specialist feeders such as insectivores and frugivores have larger home ranges than generalists like some folivores (leaf eaters). Because their food source is less abundant, they need a bigger area for foraging. An example comes from the research of Tim Clutton-Brock, who found that the black and white colobus, a folivore generalist, needs a home range of only 15ha. On the other hand, the more specialized red colobus monkey has a home range of 70 ha, which it requires to find patchy shoots, flowers and fruit.When environmental conditions change, generalists are able to adapt, but specialists tend to fall victim to extinction much more easily. For example, if a species of fish were to go extinct, any specialist parasites would also face extinction. On the other hand, a species with a highly specialized ecological niche is more effective at competing with other organisms. For example, a fish and its parasites are in an evolutionary arms race, a form of co-evolution, in which the fish constantly develops defenses against the parasite, while the parasite in turn evolves adaptations to cope with the specific defenses of its host. This tends to drive the speciation of more specialized species provided conditions remain relatively stable. This involves niche partitioning as new species are formed, and biodiversity is increased.


Geosiris is a genus in the Iridaceae family of flowering plants, first described as a genus in 1894. It was thought for many years to contain only one species, Geosiris aphylla, endemic to Madagascar. But then in 2010, a second species was described, Geosiris albiflora, from Mayotte Island in the Indian Ocean northwest of Madagascar.Geosiris aphylla is sometimes called the "earth-iris." It is a small myco-heterotroph lacking chlorophyll and obtaining its nutrients from fungi in the soil. The genus name is derived from the Greek words geos, meaning "earth", and iris, referring to the Iris family of plants.Its rhizomes are slender and scaly, and stems are simple or branched. The leaves are alternate, but having no use, are reduced and scale-like. The flowers are light purple.

In 1939, F. P. Jonker assigned Geosiris to its own family Geosiridaceae in Orchidales, and this was adopted in the Cronquist system, with a note that the family was closely related to Iridaceae or Burmanniaceae. The Angiosperm Phylogeny Group has since subsumed the family into Iridaceae, it is within the Nivenioideae subfamily.

Mesodinium chamaeleon

Mesodinium chamaeleon is a ciliate of the genus Mesodinium. It is known for being able to consume and maintain algae endosymbiotically for days before digesting the algae. It has the ability to eat red and green algae, and afterwards using the chlorophyll granules from the algae to generate energy, turning itself from being a heterotroph into an autotroph. The species was discovered in January 2012 outside the coast of Nivå, Denmark by professor Øjvind Moestrup.

In contrast to certain other species of the genus, Mesodinium chamaeleon can be maintained in culture for short periods only. It captures and ingests flagellates including cryptomonads. The prey is ingested very rapidly into a food vacuole without the cryptomonad flagella being shed and the trichocysts being discharged. The individual food vacuoles subsequently serve as photosynthetic units, each containing the cryptomonad chloroplast, a nucleus, and some mitochondria. The ingested cells are eventually digested. This type of symbiosis differs from other plastid-bearing Mesodinium spp. in retaining ingested cryptomonad cells almost intact. The food strategy of the new species appears to be intermediate between heterotrophic species, such as Mesodinium pulex and Mesodinium pupula, and species with red cryptomonad endosymbionts, such as Mesodinium rubrum.


Monotropsis is a monotypic genus of plants containing the single species Monotropsis odorata, also known as sweet pinesap or pygmy pipes. It is a member of the subfamily Monotropoideae of the heath family, Ericaceae. It is native to the Appalachian Mountains in the south-eastern United States, and is viewed as being uncommon throughout its range.

Like all members of the subfamily, Monotropsis odorata does not contain chlorophyll; it is a myco-heterotroph, getting its food through parasitism upon fungi rather than photosynthesis. These fungi form a mycorrhiza with nearby tree species.

M. oderata has a sweet smell which has been likened to nutmeg, cinnamon or violets.


Myco-heterotrophy (from Greek μύκης mykes, "fungus", ἕτερος heteros, "another", "different" and τροφή trophe, "nutrition") is a symbiotic relationship between certain kinds of plants and fungi, in which the plant gets all or part of its food from parasitism upon fungi rather than from photosynthesis. A myco-heterotroph is the parasitic plant partner in this relationship. Myco-heterotrophy is considered a kind of cheating relationship and myco-heterotrophs are sometimes informally referred to as "mycorrhizal cheaters". This relationship is sometimes referred to as mycotrophy, though this term is also used for plants that engage in mutualistic mycorrhizal relationships.


Parasitaxus usta is a rare species of conifer of the family Podocarpaceae, and the sole species of the genus Parasitaxus. It is a woody shrub up to 1.8 m found only in the remote, densely forested areas of New Caledonia, first discovered and described by Vieillard in 1861.

It is generally mentioned that Parasitaxus usta is the only known parasitic gymnosperm. The species remarkably lacks roots and is always found attached to the roots of Falcatifolium taxoides (another member of the Podocarpaceae). However, the question is still left open, as the plant is in any case not a haustorial parasite, which is usually the case with angiosperms. Certain experts therefore consider the plant as a myco-heterotroph.

Molecular phylogenetic analysis also suggest affinities between Parasitaxus and the genera Manoao (New Zealand) and Lagarostrobos (Tasmania).The species was first described as Dacrydium ustum Vieill.; other synonyms include Podocarpus ustus (Vieill.) Brongn. & Gris, and Nageia usta (Vieill.) Kuntze. The name is often cited as Parasitaxus ustus, but this is grammatically incorrect, as, according to Latin, the genus name Parasitaxus is (like Taxus) gender-feminine, with which the species name's gender must agree (Nickrent 2006). The scientific name translates as "Burnt Parasitic Yew."

Pholisma arenarium

Pholisma arenarium is a species of flowering plant in the borage family known by several common names, including desert Christmas tree, scaly-stemmed sand plant, and purple sand food. As the name implies, the loaf-like part of the root is edible. It is native to northwestern Mexico, Arizona and southern California, where it grows in many habitat types, including desert, chaparral, and sandy coastal dunes. It is a fleshy perennial herb taking a compact cylindrical or ovate shape up to 20 or 30 centimeters tall above ground, often with part of the stem below the sandy surface. It is a parasitic plant growing on the roots or of various shrubs such as burrobush, Yerba Santa, California croton, rabbitbrush, and ragweeds. As a heterotroph which derives its nutrients from other plants, it lacks chlorophyll and is brownish-gray or whitish in color. There are hairy, glandular, pointed leaves along the surface of the plant. Flowers emerge between them, each roughly one centimeter wide, the rounded corolla lavender to deep or bright purple with a white margin.


Photoheterotrophs (Gk: photo = light, hetero = (an)other, troph = nourishment) are heterotrophic phototrophs—that is, they are organisms that use light for energy, but cannot use carbon dioxide as their sole carbon source. Consequently, they use organic compounds from the environment to satisfy their carbon requirements; these compounds include carbohydrates, fatty acids, and alcohols. Examples of photoheterotrophic organisms include purple non-sulfur bacteria, green non-sulfur bacteria, and heliobacteria. Recent research has indicated that the oriental hornet and some aphids may be able to use light to supplement their energy supply.


Phototrophs (Gr: φῶς, φωτός = light, τροϕή = nourishment) are the organisms that carry out photon capture to acquire energy. They use the energy from light to carry out various cellular metabolic processes. It is a common misconception that phototrophs are obligatorily photosynthetic. Many, but not all, phototrophs often photosynthesize: they anabolically convert carbon dioxide into organic material to be utilized structurally, functionally, or as a source for later catabolic processes (e.g. in the form of starches, sugars and fats). All phototrophs either use electron transport chains or direct proton pumping to establish an electro-chemical gradient which is utilized by ATP synthase, to provide the molecular energy currency for the cell. Phototrophs can be either autotrophs or heterotrophs. As their electron and hydrogen donors are inorganic compounds [Na2S2O3 (PSB) and H2S (GSB)] they can be also called as lithotrophs, and so, some photoautotrophs are also called photolithoautotrophs. Examples of phototroph organisms: Rhodobacter capsulatus, Chromatium, Chlorobium etc.

Primary nutritional groups

Primary nutritional groups are groups of organisms, 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 and organic or inorganic compounds; the sources of carbon can be of organic or inorganic origin.

The terms aerobic respiration, anaerobic respiration and fermentation do not refer to primary nutritional groups, but simply reflect the different use of possible electron acceptors in particular organisms, such as O2 in aerobic respiration, or nitrate (NO3−), sulfate (SO42−) or fumarate in anaerobic respiration, or various metabolic intermediates in fermentation. Because all ATP-generating steps in fermentation involve modifications of metabolic intermediates instead of the use of an electron transport chain fermentation is often referred to as substrate-level phosphorylation.


Profundimonas is a Gram-negative, facultatively anaerobic and heterotroph bacteria genus. It is in the family of Oceanospirillaceae. It has one known species, Profundimonas piezophila. The species was isolated from deep seawater from Puerto Rico.

Thermotoga petrophila

Thermotoga petrophila is a hyperthermophilic, anaerobic, non-spore-forming, rod-shaped fermentative heterotroph, with type strain RKU-1T.


Thiobacillus is a genus of Gram-negative Betaproteobacteria. Thiobacilus thioparus is the type species of the genus, and the type strain thereof is the StarkeyT strain, isolated by Robert Starkey in the 1930s from a field at Rutgers University in the United States of America. While over 30 "species" have been named in this genus since it was defined by Martinus Beijerinck in 1904, (the first strain was observed by the biological oceanographer Alexander Nathansohn in 1902 - likely what we would now call Halothiobacillus neapolitanus), most names were never validly or effectively published. The remainder were either reclassified into Paracoccus, Starkeya (both in the Alphaproteobacteria); Sulfuriferula, Annwoodia, Thiomonas (in the Betaproteobacteria); Halothiobacillus, Guyparkeria (in the Gammaproteobacteria), or Thermithiobacillus or Acidithiobacillus (in the Acidithiobacillia). The very loosely defined "species" Thiobacillus trautweinii was where sulfur oxidising heterotrophs and chemolithoheterotrophs were assigned in the 1910-1960s era, most of which were probably Pseudomonas species. Many species named in this genus were never deposited in service collections and have been lost.All species are obligate autotrophs (using the transaldolase form of the Calvin-Benson-Bassham cycle) using elementary sulfur, thiosulfate, or polythionates as energy sources - the former Thiobacillus aquaesulis can grow weakly on complex media as a heterotroph, but has been reclassified to Annwoodia aquaesulis. Some strains (E6 and Tk-m) of the type species Thiobacillus thioparus can use the sulfur from dimethylsulfide, dimethyldisulfide, or carbon disulfide to support autotrophic growth - they oxidise the carbon from these species into carbon dioxide and assimilate it. Sulfur oxidation is achieved via the Kelly-Trudinger pathway.

Trophic mutualism

Trophic mutualism is a key type of ecological mutualism. Specifically, "trophic mutualism" refers to the transfer of energy and nutrients between two species. This is also sometimes known as resource-to-resource mutualism. Trophic mutualism often occurs between an autotroph and a heterotroph. Although there are many examples of trophic mutualisms, the heterotroph is generally a fungus or bacteria. This mutualism can be both obligate and opportunistic.

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