In
biology
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
, syntrophy, synthrophy, or cross-feeding (from
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
''syn'' meaning together, ''trophe'' meaning nourishment) is the phenomenon of one
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
feeding on the metabolic products of another species to cope up with the energy limitations by
electron transfer
Electron transfer (ET) occurs when an electron relocates from an atom or molecule to another such chemical entity. ET is a mechanistic description of certain kinds of redox reactions involving transfer of electrons.
Electrochemical processes ar ...
.
In this type of
biological interaction
In ecology, a biological interaction is the effect that a pair of organisms living together in a community have on each other. They can be either of the same species (intraspecific interactions), or of different species (interspecific interaction ...
, metabolite transfer happens between two or more metabolically diverse
microbial
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
species that lives in close proximity to each other.
The growth of one partner depends on the
nutrient
A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
s,
growth factor
A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Usually it is a secreted protein or a steroid hormone. Growth factors are important for regul ...
s, or
substrates provided by the other partner. Thus, syntrophism
can be considered as an obligatory interdependency and a mutualistic metabolism between two different bacterial species.
Microbial syntrophy
Syntrophy is often used synonymously for mutualistic
symbiosis
Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
especially between at least two different bacterial species. Syntrophy differs from
symbiosis
Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
in a way that syntrophic relationship is primarily based on closely linked metabolic interactions to maintain thermodynamically favorable lifestyle in a given environment.
Syntrophy plays an important role in a large number of microbial processes especially in oxygen limited environments, methanogenic environments and anaerobic systems.
In anoxic or methanogenic environments such as wetlands, swamps, paddy fields, landfills, digestive tract of
ruminant
Ruminants (suborder Ruminantia) are ungulate, hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by Enteric fermentation, fermenting it in a specialized stomach prior to digestion, principally ...
s, and anerobic digesters syntrophy is employed to overcome the energy constraints as the reactions in these environments proceed close to
thermodynamic equilibrium
Thermodynamic equilibrium is an axiomatic concept of thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable walls. In the ...
.
Mechanism of microbial syntrophy
The main mechanism of syntrophy is removing the metabolic end products of one species so as to create an energetically favorable environment for another species.
This obligate metabolic cooperation is required to facilitate the degradation of complex organic substrates under anaerobic conditions. Complex organic compounds such as ethanol,
propionate
Propionic acid (, from the Greek words πρῶτος : ''prōtos'', meaning "first", and πίων : ''píōn'', meaning "fat"; also known as propanoic acid) is a naturally occurring carboxylic acid with chemical formula CH3CH2CO2H. It is a liqu ...
,
butyrate
The conjugate acids are in :Carboxylic acids.
{{Commons category, Carboxylate ions, Carboxylate anions
Carbon compounds
Oxyanions ...
, and
lactate cannot be directly used as substrates for
methanogenesis by methanogens.
On the other hand,
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 ...
of these organic compounds cannot occur in fermenting microorganisms unless the hydrogen concentration is reduced to a low level by the methanogens. The key mechanism that ensures the success of syntrophy is interspecies electron transfer.
The interspecies electron transfer can be carried out via three ways:
interspecies hydrogen transfer, interspecies formate transfer and interspecies direct electron transfer.
Reverse electron transport is prominent in syntrophic metabolism.
The metabolic reactions and the energy involved for syntrophic degradation with H
2 consumption:
A classical syntrophic relationship can be illustrated by the activity of ‘''Methanobacillus omelianskii''’. It was isolated several times from anaerobic sediments and sewage sludge and was regarded as a pure culture of an anaerobe converting ethanol to acetate and methane. In fact, however, the culture turned out to consist of a methanogenic archaeon "organism M.o.H" and a Gram-negative Bacterium "Organism S" which involves the oxidization of
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...
into acetate and
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
mediated by
interspecies hydrogen transfer. Individuals of organism S are observed as obligate
anaerobic bacteria
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 oxygenat ...
that use ethanol as an
electron donor
In chemistry, an electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process.
Typical reducing agents undergo permanent chemi ...
, whereas M.o.H are
methanogens
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 co ...
that oxidize hydrogen gas to produce methane.
Organism S: 2 Ethanol + 2 H
2O → 2 Acetate
− + 2 H
+ + 4 H
2 (ΔG°' = +9.6 kJ per reaction)
Strain M.o.H.: 4 H
2 + CO
2 → Methane + 2 H
2O (ΔG°' = -131 kJ per reaction)
Co-culture:2 Ethanol + CO
2 → 2 Acetate
− + 2 H
+ + Methane (ΔG°' = -113 kJ per reaction)
The oxidization of ethanol by organism S is made possible thanks to the methanogen M.o.H, which consumes the hydrogen produced by organism S, by turning the positive
Gibbs free energy
In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and pr ...
into negative Gibbs free energy. This situation favors growth of organism S and also provides energy for methanogens by consuming hydrogen. Down the line, acetate accumulation is also prevented by similar syntrophic relationship.
Syntrophic degradation of substrates like butyrate and benzoate can also happen without hydrogen consumption.
An example of propionate and butyrate degradation with interspecies formate transfer carried out by the mutual system of ''
Syntrophomonas wolfei'' and ''Methanobacterium formicicum'':
Propionate+2H
2O+2CO
2 → Acetate
- +3Formate
- +3H
+ (ΔG°'=+65.3 kJ/mol)
Butyrate+2H2O+2CO
2 → 2Acetate- +3Formate- +3H
+ ΔG°'=+38.5 kJ/mol)
Direct interspecies electron transfer (DIET) which involves electron transfer without any electron carrier such as H
2 or formate was reported in the co-culture system of ''Geobacter mettalireducens'' and
''Methanosaeto'' or ''
Methanosarcina
''Methanosarcina'' is a genus of euryarchaeote archaea
Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as ...
''
Examples
In ruminants
The defining feature of
ruminant
Ruminants (suborder Ruminantia) are ungulate, hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by Enteric fermentation, fermenting it in a specialized stomach prior to digestion, principally ...
s, such as cows and goats, is a stomach called a
rumen
The rumen, also known as a paunch, is the largest stomach compartment in ruminants and the larger part of the reticulorumen, which is the first chamber in the alimentary canal of ruminant animals. The rumen's microbial favoring environment al ...
. The rumen contains billions of microbes, many of which are syntrophic.
Some anaerobic fermenting microbes in the rumen (and other gastrointestinal tracts) are capable of degrading organic matter to
short chain fatty acids Short-chain fatty acids (SCFAs) are fatty acids with fewer than six carbon atoms. Derived from intestinal microbial fermentation of indigestible foods, SCFAs are the main energy source of colonocytes, making them crucial to gastrointestinal health. ...
, and hydrogen.
The accumulating
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, an ...
inhibits the microbe's ability to continue degrading organic matter, but the presence of syntrophic hydrogen-consuming microbes allows continued growth by metabolizing the waste products.
In addition, fermentative bacteria gain maximum energy yield when
protons
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mas ...
are used as electron acceptor with concurrent
H2 production. Hydrogen-consuming organisms include
methanogens
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 co ...
, sulfate-reducers,
acetogens, and others.
Some fermentation products, such as
fatty acids
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, f ...
longer than two carbon atoms, alcohols longer than one carbon atom, and branched chain and aromatic fatty acids, cannot directly be used in
methanogenesis. In
acetogenesis Acetogenesis is a process through which acetate is produced either by the reduction of CO2 or by the reduction of organic acids, rather than by the oxidative breakdown of carbohydrates or ethanol, as with acetic acid bacteria.
The different bac ...
processes, these products are oxidized to
acetate and H
2 by obligated proton reducing bacteria in syntrophic relationship with methanogenic
archaea
Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...
as low H
2 partial pressure is essential for acetogenic reactions to be thermodynamically favorable (ΔG < 0).
Biodegradation of pollutants
Syntrophic microbial
food webs
A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Another name for food web is consumer-resource system. Ecologists can broadly lump all life forms into one ...
play an integral role in bioremediation especially in environments contaminated with crude oil and petrol. Environmental contamination with
oil
An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
is of high ecological importance and can be effectively mediated through syntrophic degradation by complete mineralization of
alkane
In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which ...
,
aliphatic
In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated, like hexane, ...
and
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
chains.
The hydrocarbons of the oil are broken down after activation by
fumarate
Fumaric acid is an organic compound with the formula HO2CCH=CHCO2H. A white solid, fumaric acid occurs widely in nature. It has a fruit-like taste and has been used as a food additive. Its E number is E297.
The salts and esters are known as f ...
, a chemical compound that is regenerated by other microorganisms.
Without regeneration, the microbes degrading the oil would eventually run out of fumarate and the process would cease. This breakdown is crucial in the processes of
bioremediation
Bioremediation broadly refers to any process wherein a biological system (typically bacteria, microalgae, fungi, and plants), living or dead, is employed for removing environmental pollutants from air, water, soil, flue gasses, industrial effluent ...
and global carbon cycling.
Syntrophic microbial communities are key players in the breakdown of
aromatic compounds
Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member of aromatic compounds is benzene. The word "aromatic" originates from the past groupin ...
, which are common pollutants.
The degradation of aromatic
benzoate
Benzoic acid is a white (or colorless) solid organic compound with the formula , whose structure consists of a benzene ring () with a carboxyl () substituent. It is the simplest aromatic carboxylic acid. The name is derived from gum benzoin, wh ...
to
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
produces intermediate compounds such as
formate,
acetate, and H
2.
The buildup of these products makes benzoate degradation thermodynamically unfavorable. These intermediates can be metabolized syntrophically by
methanogens
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 co ...
and makes the degradation process thermodynamically favorable
Degradation of amino acids
Studies have shown that bacterial degradation of
amino acids
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
can be significantly enhanced through the process of syntrophy.
Microbes growing poorly on amino acid substrates
alanine
Alanine (symbol Ala or A), or α-alanine, is an α-amino acid that is used in the biosynthesis of proteins. It contains an amine group and a carboxylic acid group, both attached to the central carbon atom which also carries a methyl group side ...
,
aspartate,
serine,
leucine
Leucine (symbol Leu or L) is an essential amino acid that is used in the biosynthesis of proteins. Leucine is an α-amino acid, meaning it contains an α- amino group (which is in the protonated −NH3+ form under biological conditions), an α- ...
,
valine
Valine (symbol Val or V) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α- amino group (which is in the protonated −NH3+ form under biological conditions), an α- carboxylic acid group (which is in the deprotona ...
, and
glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid (carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinogeni ...
can have their rate of growth dramatically increased by syntrophic H
2 scavengers. These scavengers, like ''
Methanospirillum
In taxonomy, ''Methanospirillum'' is a genus of microbes within the family Methanospirillaceae. All its species are methanogenic archaea. The cells are bar-shaped and form filaments. Most produce energy via the reduction of carbon dioxide with ...
''and''
Acetobacterium
''Acetobacterium'' is a genus of anaerobic, Gram-positive bacteria that belong to the Eubacteriaceae family.
The type species of this genus is ''Acetobacterium woodii''. The name, ''Acetobacterium'', has originated because they are acetogens, ...
,'' metabolize the H
2 waste produced during amino acid breakdown, preventing a toxic build-up.
Another way to improve amino acid breakdown is through interspecies
electron transfer
Electron transfer (ET) occurs when an electron relocates from an atom or molecule to another such chemical entity. ET is a mechanistic description of certain kinds of redox reactions involving transfer of electrons.
Electrochemical processes ar ...
mediated by formate. Species like ''
Desulfovibrio
''Desulfovibrio'' is a genus of Gram-negative sulfate-reducing bacteria. ''Desulfovibrio'' species are commonly found in aquatic environments with high levels of organic material, as well as in water-logged soils, and form major community member ...
'' employ this method.
Amino acid fermenting anaerobes such as ''
Clostridium'' species, ''Peptostreptococcus asacchaarolyticus'', ''Acidaminococcus fermentans'' were known to breakdown amino acids like
glutamate
Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can syn ...
with the help of hydrogen scavenging methanogenic partners without going through the usual
Stickland fermentation pathway
Anaerobic digestion
Effective syntrophic cooperation between propionate oxidizing bacteria, acetate oxidizing bacteria and H
2/acetate consuming methanogens is necessary to successfully carryout anaerobic digestion to produce biomethane
Examples of syntrophic organisms
* ''
Syntrophomonas wolfei''
* ''
Syntrophobacter funaroxidans''
* ''Pelotomaculum thermopropinicium''
* ''
Syntrophus aciditrophicus''
* ''
Syntrophus buswellii''
* ''Syntrophus gentianae''
References
{{Reflist
Biological interactions