![Auto-and heterotrophs](https://upload.wikimedia.org/wikipedia/commons/6/61/Auto-and_heterotrophs.png)
A heterotroph (; ) is an
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
that cannot produce its own food, instead taking nutrition from other sources of
organic carbon
Total organic carbon (TOC) is the amount of carbon found in an organic compound and is often used as a non-specific indicator of water quality or cleanliness of pharmaceutical manufacturing equipment. TOC may also refer to the amount of organic c ...
, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but not producers.
Living organisms that are heterotrophic include all
animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
s and
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
, some
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
and
protists,
and many
parasitic plant
A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are found in almost every biome. All parasitic plants develop a specialized organ called the ...
s. The term heterotroph arose in
microbiology in 1946 as part of a classification of
microorganisms based on their type of
nutrition
Nutrition is the biochemical and physiological process by which an organism uses food to support its life. It provides organisms with nutrients, which can be metabolized to create energy and chemical structures. Failure to obtain sufficient ...
. The term is now used in many fields, such as
ecology
Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overl ...
in describing the
food chain
A food chain is a linear network of links in a food web starting from producer organisms (such as grass or algae which produce their own food via photosynthesis) and ending at an apex predator species (like grizzly bears or killer whales), de ...
.
Heterotrophs may be subdivided according to their energy source. If the heterotroph uses chemical energy, it is a
chemoheterotroph
A Chemotroph is an organism that obtains 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 phototro ...
(e.g., humans and mushrooms). If it uses light for energy, then it is a
photoheterotroph
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. Con ...
(e.g.,
green non-sulfur bacteria
The Chloroflexia are a class of bacteria in the phylum Chloroflexota, known as filamentous green non-sulfur bacteria. They use light for energy and are named for their green pigment, usually found in photosynthetic bodies called chlorosomes.
Chl ...
).
Heterotrophs represent one of the two mechanisms of nutrition (
trophic level
The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it ...
s), the other being
autotroph
An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Wo ...
s (''auto'' = self, ''troph'' = nutrition). Autotrophs use energy from
sunlight (
photoautotroph Photoautotrophs are organisms that use light energy and inorganic carbon to produce organic materials. Eukaryotic photoautotrophs absorb energy through the chlorophyll molecules in their chloroplasts while prokaryotic photoautotrophs use chlorophyl ...
s) or oxidation of inorganic compounds (
lithoautotroph A lithoautotroph is an organism which derives energy from reactions of reduced compounds of mineral (inorganic) origin. Two types of lithoautotrophs are distinguished by their energy source; photolithoautotrophs derive their energy from light while ...
s) to convert inorganic
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
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).
Saprotroph
Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi ( ...
s (also called lysotrophs) are
chemoheterotroph
A Chemotroph is an organism that obtains 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 phototro ...
s that use
extracellular digestion
Extracellular phototropic digestion is a process in which saprobionts feed by secreting enzymes through the cell membrane onto the food. The enzymes catalyze the digestion of the food ie diffusion, transport, osmotrophy or phagocytosis. Since dige ...
in processing decayed organic matter. The process is most often facilitated through the
active transport
In cellular biology, ''active transport'' is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellul ...
of such materials through
endocytosis within the internal mycelium and its constituent
hyphae.
["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." ''Advanced Biology Principles'', p 296.]
Types
Heterotrophs can be
organotroph
An organotroph is an organism that obtains hydrogen or electrons from organic substrates. This term is used in microbiology to classify and describe organisms based on how they obtain electrons for their respiration processes. Some organotrophs su ...
s or
lithotroph
Lithotrophs are a diverse group of organisms using an inorganic substrate (usually of mineral origin) to obtain reducing equivalents for use in biosynthesis (e.g., carbon dioxide fixation) or energy conservation (i.e., ATP production) via aerobi ...
s. Organotrophs exploit reduced carbon compounds as electron sources, like
carbohydrates,
fat
In nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food.
The term often refers specifically to triglycerides (triple est ...
s, and
proteins from plants and animals. On the other hand, lithoheterotrophs use inorganic compounds, such as
ammonium,
nitrite, or
sulfur, to obtain electrons. Another way of classifying different heterotrophs is by assigning them as
chemotroph
A Chemotroph is an organism that obtains 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 phototr ...
s or
phototroph
Phototrophs () are organisms that carry out photon capture to produce complex organic compounds (e.g. carbohydrates) and acquire energy. They use the energy from light to carry out various cellular metabolic processes. It is a common misconcep ...
s. 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
Thiosulfate ( IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur with the chemical formula . Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, ...
, and molecular
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
.
Mixotroph A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs comp ...
s (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.
Origin and Diversification
The chemical
origin of life
In biology, abiogenesis (from a- 'not' + Greek bios 'life' + genesis 'origin') or the origin of life is the natural process by which life has arisen from non-living matter, such as simple organic compounds. The prevailing scientific hypothes ...
hypothesis suggests that life originated in a
prebiotic soup with heterotrophs.
The summary of this theory is as follows: early Earth had a highly
reducing atmosphere
A reducing atmosphere is an Atmosphere, atmospheric condition in which oxidation is prevented by removal of oxygen and other oxidizing gases or vapours, and which may contain actively Reducing agent, reducing gases such as hydrogen, carbon monoxi ...
and energy sources such as electrical energy in the form of lightning, which resulted in reactions that formed simple
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. T ...
s, which further reacted to form more complex compounds and eventually result in life. Alternative theories of an autotrophic origin of life contradict this theory.
The theory of a chemical origin of life beginning with heterotrophic life was first proposed in 1924 by
Alexander Ivanovich Oparin, and eventually published “The Origin of Life.” It was independently proposed for the first time in English in 1929 by
John Burdon Sanderson Haldane
John Burdon Sanderson Haldane (; 5 November 18921 December 1964), nicknamed "Jack" or "JBS", was a British-Indian scientist who worked in physiology, genetics, evolutionary biology, and mathematics. With innovative use of statistics in biolo ...
. While these authors agreed on the gasses present and the progression of events to a point, Oparin championed a progressive complexity of organic matter prior to the formation of cells, while Haldane had more considerations about the concept of genes as units of heredity and the possibility of light playing a role in chemical synthesis (
autotroph
An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Wo ...
y).
Evidence grew to support this theory in 1953, when
Stanley Miller
Stanley Lloyd Miller (March 7, 1930 – May 20, 2007) was an American chemist who made landmark experiments in the origin of life by demonstrating that a wide range of vital organic compounds can be synthesized by fairly simple chemical processe ...
’s conducted an experiment in which he added gasses that were thought to be present on
early Earth
The early Earth is loosely defined as Earth in its first one billion years, or gigayear (Ga, 109y). The “early Earth” encompasses approximately the first gigayear in the evolution of our planet, from its initial formation in the young Solar Sy ...
– water (H
2O), methane (CH
4), ammonia (NH
3), and hydrogen (H
2) – to a flask and stimulated them with electricity that resembled lightning present on early Earth. The experiment resulted in the discovery that early Earth conditions were supportive of the production of amino acids, with recent re-analyses of the data recognizing that over 40 different amino acids were produced, including several not currently used by life.
This experiment heralded the beginning of the field of synthetic prebiotic chemistry, and is now known as the
Miller–Urey experiment
The Miller–Urey experiment (or Miller experiment) is a famous chemistry experiment that simulated the conditions thought at the time (1952) to be present in the atmosphere of the early, prebiotic Earth, in order to test the hypothesis of the ...
.
On early Earth, oceans and shallow waters were rich with organic molecules that could have been used by primitive heterotrophs.
This method of obtaining energy was energetically favorable until organic carbon became more scarce than inorganic carbon, providing a potential evolutionary pressure to become autotrophic.
Following the evolution of autotrophs, heterotrophs were able to utilize them as a food source instead of relying on the limited nutrients found in their environment.
Eventually, autotrophic and heterotrophic cells were engulfed by these early heterotrophs and formed a
symbiotic relationship.
The
endosymbiosis
An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship.
(The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...
of autotrophic cells is suggested to have evolved into the
chloroplasts while the endosymbiosis of smaller heterotrophs developed into the
mitochondria, allowing the differentiation of tissues and development into multicellularity. This advancement allowed the further diversification of heterotrophs.
Today, many heterotrophs and autotrophs also utilize
mutualistic relationships that provide needed resources to both organisms. One example of this is the mutualism between corals and algae, where the former provides protection and necessary compounds for photosynthesis while the latter provides oxygen.
However this hypothesis is controversial as CO
2 was the main carbon source at the early Earth, suggesting that early cellular life were autotrophs that relied upon inorganic substrates as an energy source and lived at alkaline hydrothermal vents or acidic geothermal ponds. Simple biomolecules transported from space was considered to have been either too reduced to have been fermented or too heterogeneous to support microbial growth. Heterotrophic microbes likely originated at low H
2 partial pressures. Bases, amino acids, and ribose are considered to be the first fermentation substrates.
Heterotrophs are currently found in each domain of life:
Bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
,
Archaea, and
Eukarya
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bact ...
.
Domain Bacteria includes a variety of metabolic activity including photoheterotrophs, chemoheterotrophs, organotrophs, and heterolithotrophs.
Within Domain Eukarya, kingdoms
Fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
and
Animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
ia are entirely heterotrophic, though most fungi absorb nutrients through their environment.
Most organisms within Kingdom
Protist
A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
a are heterotrophic while Kingdom
Plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...
ae is almost entirely autotrophic, except for
myco-heterotrophic
Myco-heterotrophy (from Greek μύκης , "fungus", ἕτερος ', "another", "different" and τροφή ', "nutrition") is a symbiotic relationship between certain kinds of plants and fungi, in which the plant gets all or part of its food fro ...
plants.
Lastly, Domain Archaea varies immensely in metabolic functions and contains many methods of heterotrophy.
Flowchart
![AutoHeteroTrophs flowchart](https://upload.wikimedia.org/wikipedia/commons/5/55/AutoHeteroTrophs_flowchart.png)
*
Autotroph
An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Wo ...
**
Chemoautotroph
A Chemotroph is an organism that obtains 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 phototrop ...
**
Photoautotroph Photoautotrophs are organisms that use light energy and inorganic carbon to produce organic materials. Eukaryotic photoautotrophs absorb energy through the chlorophyll molecules in their chloroplasts while prokaryotic photoautotrophs use chlorophyl ...
*Heterotroph
**
Chemoheterotroph
A Chemotroph is an organism that obtains 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 phototro ...
**
Photoheterotroph
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. Con ...
Ecology
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 electron sources. Heterotrophs function as
consumers in food chain: they obtain these nutrients from
saprotrophic
Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi ( ...
,
parasitic
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...
, or
holozoic nutrients.
They break down complex organic compounds (e.g., carbohydrates, fats, and proteins) produced by autotrophs into simpler compounds (e.g., carbohydrates into
glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...
, fats into
fatty acids
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an B ...
and
glycerol
Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
, and proteins into
amino acids). They release the chemical energy of nutrient molecules by oxidizing carbon and hydrogen atoms from carbohydrates, lipids, and proteins to carbon dioxide and water, respectively.
They can catabolize organic compounds by respiration, fermentation, or both.
Fermenting
Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food p ...
heterotrophs are either facultative or obligate
anaerobes
An anaerobic organism or anaerobe is any organism that does not require molecular oxygen for growth. It may react negatively or even die if free oxygen is present. In contrast, an aerobic organism (aerobe) is an organism that requires an oxygenate ...
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, , sulfide).
These products can then serve as the substrates for other bacteria in the
anaerobic digest, and be converted into CO
2 and CH
4, which is an important step for the
carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere of the Earth. Carbon is the main component of biological compounds as well as ...
for removing organic fermentation products from anaerobic environments.
Heterotrophs can undergo
respiration
Respiration may refer to:
Biology
* Cellular respiration, the process in which nutrients are converted into useful energy in a cell
** Anaerobic respiration, cellular respiration without oxygen
** Maintenance respiration, the amount of cellul ...
, in which ATP production is coupled with
oxidative phosphorylation.
This leads to the release of oxidized carbon wastes such as CO
2 and reduced wastes like H
2O, H
2S, or N
2O into the atmosphere. Heterotrophic microbes’ respiration and fermentation account for a large portion of the release of CO
2 into the atmosphere, making it available for autotrophs as a source of nutrient and plants as a cellulose synthesis substrate.
Respiration in heterotrophs is often accompanied by
mineralization, the process of converting organic compounds to inorganic forms.
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.
S and N in organic carbon source are transformed into H
2S and NH
4+ through desulfurylation and
deamination
Deamination is the removal of an amino group from a molecule. Enzymes that catalyse this reaction are called deaminases.
In the human body, deamination takes place primarily in the liver, however it can also occur in the kidney. In situations of ...
, respectively.
Heterotrophs also allow for
dephosphorylation
In biochemistry, dephosphorylation is the removal of a phosphate (PO43−) group from an organic compound by hydrolysis. It is a reversible post-translational modification. Dephosphorylation and its counterpart, phosphorylation, activate and de ...
as part of
decomposition
Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is e ...
.
The conversion of N and S from organic form to inorganic form is a critical part of the
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
and
sulfur cycle. H
2S formed from desulfurylation is further oxidized by lithotrophs and phototrophs while NH
4+ formed from deamination is further oxidized by lithotrophs to the forms available to plants.
Heterotrophs’ ability to mineralize essential elements is critical to plant survival.
Most
opisthokont
The opisthokonts () are a broad group of eukaryotes, including both the animal and fungus kingdoms. The opisthokonts, previously called the "Fungi/Metazoa group", are generally recognized as a clade. Opisthokonts together with Apusomonadida and ...
s and
prokaryote
A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...
s are heterotrophic; in particular, all animals and fungi are heterotrophs.
Some animals, such as
coral
Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...
s, form
symbiotic relationships with autotrophs and obtain organic carbon in this way. Furthermore, some
parasitic plant
A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are found in almost every biome. All parasitic plants develop a specialized organ called the ...
s have also turned fully or partially heterotrophic, while
carnivorous plant
Carnivorous plants are plants that derive some or most of their nutrients from trapping and consuming animals or protozoans
Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryot ...
s consume animals to augment their nitrogen supply while remaining autotrophic.
Animals are classified as heterotrophs by ingestion, fungi are classified as heterotrophs by absorption.
References
{{reflist, 25em
Biology terminology
Trophic ecology