View source for Decomposer

{{short description|Organism that breaks down dead or decaying organisms}}
{{For|The Matches album of the same name|Decomposer (album)}}'''Decomposers''' are [[Organism|organisms]] that break down dead organisms and release the nutrients from the dead matter into the environment around them'''.''' [[Decomposition]] relies on chemical processes similar to [[digestion]] in animals; in fact, many sources use the words digestion and decomposition interchangeably.<ref>{{Cite web |last1=Clark |first1=Mary Ann |last2=Douglas |first2=Matthew |last3=Choi |first3=Jung |date=2018-03-28 |title=6.1 Energy and Metabolism - Biology 2e {{!}} OpenStax |url=https://openstax.org/books/biology-2e/pages/6-1-energy-and-metabolism |access-date=2024-10-30 |website=openstax.org |language=English}}</ref> In both processes, complex molecules are chemically broken down by [[Enzyme|enzymes]] into simpler, smaller ones. The term "digestion," however, is commonly used to refer to food breakdown that occurs within animal bodies, and results in the absorption of nutrients from the gut into the animal's bloodstream.<ref>{{Citation |last1=Patricia |first1=Justin J. |title=Physiology, Digestion |date=2024 |work=StatPearls |url=https://www.ncbi.nlm.nih.gov/books/NBK544242/ |access-date=2024-10-30 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=31334962 |last2=Dhamoon |first2=Amit S.}}</ref> This is contrasted with external digestion, meaning that, rather than swallowing food and then digesting it using enzymes located within a GI tract, an organism instead releases enzymes directly onto the food source, which is what decomposers do as compared to animals.<ref>{{Cite web |date=2021-12-05 |title=31.2: Fungal Forms, Nutrition, and Reproduction |url=https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Map:_Raven_Biology_12th_Edition/31:_Fungi/31.02:_Fungal_Forms_Nutrition_and_Reproduction |access-date=2024-10-30 |website=Biology LibreTexts |language=en}}</ref> After allowing the enzymes time to digest the material, the decomposer then absorbs the nutrients from the environment into its cells.<ref name=":1">[http://www.eoearth.org/article/Trophic_level?topic=58074 ''Trophic level''. Eds. M. McGinley & C. J. Cleveland. Encyclopedia of Earth. National Council for Science and the Environment. Washington DC]</ref> Decomposition is often erroneously conflated with this process of external digestion, probably because of the strong association between fungi, which are external digesters, and decomposition.

The term "decomposer" refers to a role in an ecosystem, not to a particular class or type of organism, or even to a specific capacity of those organisms.<ref>{{Cite web |last1=Clark |first1=Mary Ann |last2=Douglas |first2=Matthew |last3=Choi |first3=Jung |date=2018-03-28 |title=46.1 Ecology of Ecosystems - Biology 2e {{!}} OpenStax |url=https://openstax.org/books/biology-2e/pages/46-1-ecology-of-ecosystems |access-date=2024-11-11 |website=openstax.org |language=English}}</ref> The definition of "decomposer" therefore centers on the ''outcome'' of the decomposition process, rather than the types of organisms performing it. At the center of this definition are the organisms that benefit most directly from the increase in nutrient availability that results from decomposition; plants and other non-mobile (sessile) [[Autotroph|autotrophs]] cannot travel to seek out nutrients, and most cannot digest other organisms themselves. They must therefore rely on decomposers to free up nutrients from dead matter that they can then absorb.<ref name=":0">{{Cite journal |last1=Griffiths |first1=Hannah M. |last2=Ashton |first2=Louise A. |last3=Parr |first3=Catherine L. |last4=Eggleton |first4=Paul |date=September 2021 |title=The impact of invertebrate decomposers on plants and soil |url=https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.17553 |journal=New Phytologist |language=en |volume=231 |issue=6 |pages=2142–2149 |doi=10.1111/nph.17553 |pmid=34128548 |bibcode=2021NewPh.231.2142G |issn=0028-646X |hdl-access=free |hdl=10072/406155}}</ref>

Note that this definition does not focus on where digestion takes place (i.e. inside or outside of an organism's body), but rather on where the products of that digestion end up. "Decomposer" as a category, therefore, would include not just fungi and bacteria, which perform external digestion, but also [[Invertebrate|invertebrates]] such as [[Earthworm|earthworms]], [[woodlice]], and [[sea cucumbers]] that digest dead matter internally and release nutrients locally via their feces.<ref name=":0" /> In some definitions of decomposition that center on the means and location of digestion, these invertebrates, which digest their food internally, are set apart from decomposers and placed in a separate category called [[Detritivore|detritivores.]]<ref name=":1" /> These categories are not, in fact, mutually exclusive. "Detritivore" describes behavior and physiology, while "decomposer" describes an ecosystem role. Therefore, an organism can be both a detritivore and a decomposer.

While there are also purely physical processes, like [[weathering]] and [[ultraviolet light]], that contribute to decomposition, "decomposer" refers only to living organisms that contribute to the process, whether by physical or chemical breakdown of dead matter.

== Terrestrial decomposers ==
In terrestrial environments, decomposition happens mainly in or on soil, and decomposers' activities lead to increased [[soil fertility]].<ref>{{Cite web |title=Soil Carbon Storage {{!}} Learn Science at Scitable |url=https://www.nature.com/scitable/knowledge/library/soil-carbon-storage-84223790/ |access-date=2024-11-30 |website=www.nature.com |language=en}}</ref> The main nutrients plants have to derive from soils are nitrogen, phosphorus, and potassium, and all three have to be available in forms that are accessible to and absorbable by the plants. Decomposition is the process of breaking large molecules in dead matter down into smaller molecules that nearby plants are able to take up through their roots. Some steps of the process occur via mechanical grinding and churning by things like earthworms and plant roots in a process called [[bioturbation]].<ref>{{Cite journal |last1=Ruiz |first1=Siul |last2=Or |first2=Dani |last3=Schymanski |first3=Stanislaus J. |date=2015-06-18 |title=Soil Penetration by Earthworms and Plant Roots—Mechanical Energetics of Bioturbation of Compacted Soils |journal=PLOS ONE |language=en |volume=10 |issue=6 |pages=e0128914 |doi=10.1371/journal.pone.0128914 |doi-access=free |issn=1932-6203 |pmc=4472233 |pmid=26087130|bibcode=2015PLoSO..1028914R }}</ref> Further breakdown, beyond those physical means, requires the presence of enzymes. The paired processes are akin to what occurs in mammal [[Gastrointestinal tract|digestive tracts]]: food is mechanically ground up by teeth, and then chemically broken down by enzymes.

A given organism's ability to contribute to decomposition is largely dependent on what enzymes that organism possesses. Enzymes for the digestion of molecules like fats, proteins, and starch are widespread and many organisms, from microbes to mammals, have them.<ref>{{Cite journal |last1=Griffiths |first1=Hannah M. |last2=Ashton |first2=Louise A. |last3=Parr |first3=Catherine L. |last4=Eggleton |first4=Paul |date=2021 |title=The impact of invertebrate decomposers on plants and soil |url=https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.17553 |journal=New Phytologist |language=en |volume=231 |issue=6 |pages=2142–2149 |doi=10.1111/nph.17553 |pmid=34128548 |bibcode=2021NewPh.231.2142G |hdl=10072/406155 |issn=1469-8137|hdl-access=free }}</ref> As a result, those molecules are the first to decompose in the environment. [[Cellulose]] in dead plants is broken down by cellulase enzymes, which are present in far fewer organisms, and the enzymes needed to digest [[lignin]], a chemically complex molecule in woody trees and shrubs, in fewer still.<ref>{{Cite journal |last1=Nakazawa |first1=Takehito |last2=Yamaguchi |first2=Iori |last3=Zhang |first3=Yufan |last4=Saka |first4=Chinami |last5=Wu |first5=Hongli |last6=Kayama |first6=Keita |last7=Kawauchi |first7=Moriyuki |last8=Sakamoto |first8=Masahiro |last9=Honda |first9=Yoichi |date=October 2023 |title=Experimental evidence that lignin-modifying enzymes are essential for degrading plant cell wall lignin by Pleurotus ostreatus using CRISPR/Cas9 |url=https://pubmed.ncbi.nlm.nih.gov/37218079/ |journal=Environmental Microbiology |volume=25 |issue=10 |pages=1909–1924 |doi=10.1111/1462-2920.16427 |issn=1462-2920 |pmid=37218079|bibcode=2023EnvMi..25.1909N |doi-access=free }}</ref>
[[File:Fungi_on_fallen_Birch_Branch_-_geograph.org.uk_-_239255.jpg|thumb|Fungi acting as decomposers of a fallen tree branch]]

=== Fungi ===
The primary decomposer of litter in many [[ecosystem]]s is [[fungi]].<ref>{{Cite journal|last1=Godbold|first1=Douglas L.|last2=Hoosbeek|first2=Marcel R.|last3=Lukac|first3=Martin|last4=Cotrufo|first4=M. Francesca|author-link4=M. Francesca Cotrufo|last5=Janssens|first5=Ivan A.|last6=Ceulemans|first6=Reinhart|last7=Polle|first7=Andrea|last8=Velthorst|first8=Eef J.|last9=Scarascia-Mugnozza|first9=Giuseppe|last10=De Angelis|first10=Paolo|last11=Miglietta|first11=Franco|date=2006-03-01|title=Mycorrhizal Hyphal Turnover as a Dominant Process for Carbon Input into Soil Organic Matter|url=https://doi.org/10.1007/s11104-005-3701-6|journal=Plant and Soil|language=en|volume=281|issue=1|pages=15–24|doi=10.1007/s11104-005-3701-6|bibcode=2006PlSoi.281...15G |s2cid=24926892 |issn=1573-5036|url-access=subscription}}</ref><ref>{{Cite journal|last1=Talbot|first1=J. M.|last2=Allison|first2=S. D.|last3=Treseder|first3=K. K.|date=2008|title=Decomposers in disguise: mycorrhizal fungi as regulators of soil C dynamics in ecosystems under global change|journal=Functional Ecology|language=en|volume=22|issue=6|pages=955–963|doi=10.1111/j.1365-2435.2008.01402.x|issn=1365-2435|doi-access=free|bibcode=2008FuEco..22..955T }}</ref> Unlike [[bacteria]], which are unicellular organisms and are decomposers as well, most [[saprotroph]]ic fungi grow as a branching network of [[hypha]]e. Bacteria are restricted to growing and feeding on the exposed surfaces of organic matter, but fungi can use their hyphae to penetrate larger pieces of organic matter below the surface. Additionally, only [[wood-decay fungus|wood-decay fungi]] have evolved [[Lignin-modifying enzyme|lignin-modifying enzymes]] necessary to decompose [[lignin]].<ref>{{cite journal|last1=Blanchette|first1=Robert|title=Delignification by Wood-Decay Fungi|journal=Annual Review of Phytopathology|date=September 1991|volume=29|pages=281–403|doi=10.1146/annurev.py.29.090191.002121}}</ref> These two factors make fungi the primary decomposers in [[forest]]s, where litter has high concentrations of lignin and often occurs in [[coarse woody debris|large pieces]] like fallen trees and branches. Fungi decompose organic matter by releasing enzymes to break down the decaying material, after which they absorb the nutrients in the decaying material.<ref name="berkeley">{{cite web |url=http://www.ucmp.berkeley.edu/fungi/fungilh.html |title=Fungi: Life History and Ecology |first1=Ben |last1=Waggoner |first2=Brian |last2=Speer |work=Introduction to the Funge=24 January 2014}}</ref> Hyphae are used to break down matter and absorb nutrients and are also used in reproduction. 

== See also ==
* [[Chemotroph]]
* [[Micro-animals]]
* [[Microorganism]]

== References ==
{{reflist}}
{{Refbegin}}

== Further reading ==
* {{cite journal | last1 = Bear | first1 = MH | last2 = Hendrix | first2 = PF | last3 = Cheng | first3 = W | s2cid = 86031411 | year = 1992 | title = Microbial and faunal interactions and effects on litter nitrogen and decomposition in agroecosystems | journal = Ecological Monographs | volume = 62 | issue = 4| pages = 569–591 | doi=10.2307/2937317| jstor = 2937317 | bibcode = 1992EcoM...62..569B }}
* Hunt HW, Coleman DC, Ingham ER, Ingham RE, Elliot ET, Moore JC, Rose SL, Reid CPP, Morley CR (1987) "The detrital food web in a shortgrass prairie". ''Biology and Fertility of Soils 3'': 57-68
* Smith TM, Smith RL (2006) ''Elements of Ecology''. Sixth edition. Benjamin Cummings, San Francisco, CA.

{{Refend}}

{{Feeding}}
{{Modelling ecosystems}}

[[Category:Microbiology]]