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Primary nutritional groups are groups of
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 fu ...
s, 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. The terms ''
aerobic respiration Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be des ...
'', ''
anaerobic respiration Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain. In aerobic organisms undergoing r ...
'' and ''
fermentation 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 ...
'' (''
substrate-level phosphorylation Substrate-level phosphorylation is a metabolism reaction that results in the production of ATP or GTP by the transfer of a phosphate group from a substrate directly to ADP or GDP. Transferring from a higher energy (whether phosphate group atta ...
'') 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 Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insolu ...
(), sulfate () or fumarate in anaerobic respiration, or various metabolic intermediates in fermentation.


Primary sources of energy

''Phototrophs'' absorb light in photoreceptors and transform it into chemical energy.
''Chemotrophs'' release chemical energy. The freed energy is stored as
potential energy In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors. Common types of potential energy include the gravitational potenti ...
in ATP,
carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may o ...
s, or
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s. 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.


Primary sources of reducing equivalents

''Organotrophs'' use organic compounds as electron/hydrogen donors.
''Lithotrophs'' use inorganic compounds as electron/hydrogen donors. The
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
s or hydrogen atoms from reducing equivalents (electron donors) are needed by both phototrophs and chemotrophs in reduction-oxidation reactions that transfer energy in the anabolic processes of
ATP synthesis ATP synthase is a protein that catalyzes the formation of the energy storage molecule adenosine triphosphate (ATP) using adenosine diphosphate (ADP) and inorganic phosphate (Pi). It is classified under ligases as it changes ADP by the formation ...
(in heterotrophs) or
biosynthesis Biosynthesis is a multi-step, enzyme-catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined to form macromolecul ...
(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 anaerobic respiration and
fermentation 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 ...
are not taken into account here. For example, plants are lithotrophs because they use water as their electron donor for biosynthesis. 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.


Primary sources of carbon

''Heterotrophs'' metabolize organic compounds to obtain carbon for growth and development.
''Autotrophs'' use
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 t ...
(CO2) as their source of carbon.


Energy and carbon

A ''chemoorganoheterotrophic''
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 fu ...
is one that requires organic substrates to get its
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...
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 and carnivores are examples of organisms that obtain carbon and electrons or hydrogen from living organic matter. Chemoorganotrophs are
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 fu ...
s which use the chemical energy in
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. Th ...
s as their energy source and obtain electrons or hydrogen from the organic compounds, including sugars (i.e.
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 and proteins. Chemoheterotrophs also obtain the carbon atoms that they need for cellular function from these organic compounds. All
animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...
s are chemoheterotrophs (meaning they oxidize chemical compounds as a source of energy and carbon), as are
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 fr ...
,
protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
, and some
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
. The important differentiation amongst this group is that chemoorganotrophs oxidize only organic compounds while chemolithotrophs instead use oxidation of
inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemis ...
s as a source of energy.


Primary metabolism table

The following table gives some examples for each nutritional group: * Some authors use ''-hydro-'' when the source is water. The common final part ''-troph'' is from Ancient Greek "nutrition".


Mixotrophs

Some, usually unicellular, organisms can switch between different metabolic modes, for example between photoautotrophy, photoheterotrophy, and chemoheterotrophy in ''Chroococcales''. Rhodopseudomonas palustris - another example - can grow with or without
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
, use either light, inorganic or organic compounds for energy. Such mixotrophic organisms may dominate their
habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
, due to their capability to use more resources than either photoautotrophic or organoheterotrophic organisms.


Examples

All sorts of combinations may exist in nature, but some are more common than others. For example, most plants are ''photolithoautotrophic'', since they use light as an energy source, water as electron donor, and CO2 as a carbon source. All animals and fungi are ''chemoorganoheterotrophic'', since they use organic substances both as chemical energy sources and as electron/hydrogen donors and carbon sources. Some eukaryotic 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.
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 & Con ...
s show a great diversity of nutritional categories.Tang, K.-H., Tang, Y. J., Blankenship, R. E. (2011). "Carbon metabolic pathways in phototrophic bacteria and their broader evolutionary implications" ''Frontiers in Microbiology'' 2: Art. 165. http://dx.doi.org/10.3389/micb.2011.00165 For example,
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...
and many purple sulfur bacteria can be ''photolithoautotrophic'', using light for energy, H2O or sulfide as electron/hydrogen donors, and CO2 as carbon source, whereas green non-sulfur bacteria can be ''photoorganoheterotrophic'', using organic molecules as both electron/hydrogen donors and carbon sources. Many bacteria are ''chemoorganoheterotrophic'', using organic molecules as energy, electron/hydrogen and carbon sources. Some bacteria are limited to only one nutritional group, whereas others are
facultative {{wiktionary, facultative Facultative means "optional" or "discretionary" (antonym '' obligate''), used mainly in biology in phrases such as: * Facultative (FAC), facultative wetland (FACW), or facultative upland (FACU): wetland indicator statuses ...
and switch from one mode to the other, depending on the nutrient sources available. Sulfur-oxidizing,
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
, and
anammox Anammox, an abbreviation for anaerobic ammonium oxidation, is a globally important microbial process of the nitrogen cycle that takes place in many natural environments. The bacteria mediating this process were identified in 1999, and were a gre ...
bacteria as well as
methanogen Methanogens are microorganisms that produce methane as a metabolic byproduct in hypoxic conditions. They are prokaryotic and belong to the domain Archaea. All known methanogens are members of the archaeal phylum Euryarchaeota. Methanogens are c ...
s are ''chemolithoautotrophs'', 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 two model microorganisms from heterotrophy to chemoorganoautotrophy: * ''
Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Esc ...
'' was genetically engineered and then evolved in the laboratory to use CO2 as the sole carbon source while using the one-carbon molecule
formate Formate (IUPAC name: methanoate) is the conjugate base of formic acid. Formate is an anion () or its derivatives such as ester of formic acid. The salts and esters are generally colorless.Werner Reutemann and Heinz Kieczka "Formic Acid" in ''Ull ...
as the source of electrons. * The methylotrophic '' Pichia pastoris'' yeast was genetically engineered to use CO2 as the carbon source instead of
methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is ...
, while the latter remained the source of electrons for the cells.


See also

* Autotrophic * Chemosynthesis * Chemotrophic * Heterotrophic * Lithotrophic *
Metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run ...
*
Mixotrophic 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 com ...
* Organotrophic * Phototrophic


Notes and references

{{DEFAULTSORT:Primary Nutritional Groups Trophic ecology Physiology