View source for Lignin

{{short description|Structural phenolic polymer in plant cell walls}}
{{About |the wood polymer|the phytoestrogen|Lignan}}
[[Image:Lignin structure.svg|thumb|500px|Idealized structure of lignin from a softwood]]
'''Lignin''' is a class of complex [[organic polymer]]s that form key structural materials in the support tissues of most plants.<ref name="Saake-2007"/>  Lignins are particularly important in the formation of [[cell wall]]s, especially in [[wood]] and [[Bark (botany)|bark]], because they lend rigidity and do not [[decomposition|rot]] easily. Chemically, lignins are polymers made by cross-linking [[phenols|phenolic]] precursors.<ref>{{cite encyclopedia |last=Lebo |first=Stuart E. Jr. |author2=Gargulak, Jerry D. |author3=McNally, Timothy J. |encyclopedia=Kirk‑Othmer Encyclopedia of Chemical Technology |chapter-url=http://www.mrw.interscience.wiley.com/emrw/9780471238966/kirk/article/lignlin.a01/current/pdf |access-date=2007-10-14 |year=2001 |publisher=John Wiley & Sons, Inc |doi=10.1002/0471238961.12090714120914.a01.pub2 |chapter=Lignin |title=Kirk-Othmer Encyclopedia of Chemical Technology |isbn=978-0-471-23896-6 |archive-date=2020-04-10 |archive-url=https://web.archive.org/web/20200410134534/http://onlinelibrary.wiley.com/ |url-status=dead }}</ref>

== History ==
Lignin was first mentioned in 1813 by the Swiss botanist [[Augustin Pyramus de Candolle|A. P. de Candolle]], who described it as a fibrous, tasteless material, insoluble in water and alcohol but soluble in weak alkaline solutions, and which can be [[Precipitation (chemistry)|precipitated]] from solution using acid.<ref>{{cite book|last=de Candolle|first=M.A.P.|title=Theorie Elementaire de la Botanique ou Exposition des Principes de la Classification Naturelle et de l'Art de Decrire et d'Etudier les Vegetaux|date=1813|publisher=Deterville|location=Paris|url=https://archive.org/details/thorielmenc00cand}} See p. 417.</ref> He named the substance "lignine", which is derived from the Latin word ''[[wikt:lignum|lignum]]'',<ref name="Sjöström-1993">{{cite book|author= E. Sjöström|title= Wood Chemistry: Fundamentals and Applications|publisher= [[Academic Press]]|year= 1993|isbn= 978-0-12-647480-0}}</ref> meaning wood. It is one of the most abundant [[organic polymer]]s on [[Earth]], exceeded only by [[cellulose]] and [[chitin]]. Lignin constitutes 30% of terrestrial non-[[fossil]] organic [[carbon]]<ref name="Baucher-2003">{{cite journal|author1=W. Boerjan |author2=J. Ralph |author3=M. Baucher |date=June 2003| title = Lignin biosynthesis| journal = Annu. Rev. Plant Biol.| volume = 54| issue = 1| pages = 519–549| doi = 10.1146/annurev.arplant.54.031902.134938| pmid = 14503002}}</ref> on Earth, and 20 to 35% of the dry mass of wood.<ref>{{Cite web |date=2023-10-05 |title=Lignin |url=https://www.britannica.com/science/lignin |access-date=2023-10-26 |website=Encyclopedia Brittanica |language=en}}</ref>

Lignin is present in [[red algae]], which suggest that the common ancestor of plants and red algae may have been pre-adapted to synthesize lignin. This finding also suggests that the original function of lignin may have been structural as it plays this role in the red alga ''[[Calliarthron]]'', where it supports joints between [[calcification|calcified]] segments.<ref name="Ralph-2009">{{cite journal
| doi = 10.1016/j.cub.2008.12.031
| date=Jan 2009
|author1=Martone, Pt |author2=Estevez, Jm |author3=Lu, F |author4=Ruel, K |author5=Denny, Mw |author6=Somerville, C |author7=Ralph, J | title = Discovery of Lignin in Seaweed Reveals Convergent Evolution of Cell-Wall Architecture.
| volume = 19
| issue = 2
| pages = 169–75
| issn = 0960-9822
| pmid = 19167225
| journal = Current Biology | s2cid=17409200
| doi-access=free
| bibcode=2009CBio...19..169M
}}</ref>

==Composition and structure==
The composition of lignin varies from species to species. An example of composition from an [[aspen]]<ref>In the referenced article, the species of aspen is not specified, only that it was from Canada.</ref> sample is 63.4% carbon, 5.9% hydrogen, 0.7% ash (mineral components), and 30% oxygen (by difference),<ref>{{cite web|title=Modeling Tar Composition in Lignin Pyrolysis|url=https://web.anl.gov/PCS/acsfuel/preprint%20archive/Files/28_5_WASHINGTON%20DC_08-83_0319.pdf|work=Symposium on Mathematical Modeling of Biomass Pyrolysis Phenomena, Washington, D.C., 1983|author=Hsiang-Hui King|author2=Peter R. Solomon|author3=Eitan Avni|author4=Robert W. Coughlin|page=1|date=Fall 1983|access-date=2024-01-29|archive-date=2017-08-08|archive-url=https://web.archive.org/web/20170808004334/http://web.anl.gov/PCS/acsfuel/preprint%20archive/Files/28_5_WASHINGTON%20DC_08-83_0319.pdf}}</ref> corresponding approximately to the formula (C<sub>31</sub>H<sub>34</sub>O<sub>11</sub>)<sub>n</sub>.

Lignin is a collection of highly [[heterogeneous]] polymers derived from a handful of precursor lignols.  Heterogeneity arises from the diversity and degree of crosslinking between these lignols. The [[Monolignol|lignols]] that [[Cross-link|crosslink]] are of three main types, all derived from phenylpropane: [[coniferyl alcohol]] (3-methoxy-4-hydroxyphenylpropane; its radical, G, is sometimes called guaiacyl), [[sinapyl alcohol]] (3,5-dimethoxy-4-hydroxyphenylpropane; its radical, S, is sometimes called syringyl), and [[paracoumaryl alcohol]] (4-hydroxyphenylpropane; its radical, H, is sometimes called 4-hydroxyphenyl).{{citation needed|date=October 2023}}

The relative amounts of the precursor "monomers" (lignols or monolignols) vary according to the plant source.<ref name="Baucher-2003"/> Lignins are typically classified according to their syringyl/guaiacyl (S/G) ratio. Lignin from [[gymnosperm]]s is derived from the [[coniferyl alcohol]], which gives rise to G upon pyrolysis. In [[angiosperm]]s some of the coniferyl alcohol is converted to S.  Thus, lignin in angiosperms has both G and S components.<ref>{{Cite journal |last1=Letourneau |first1=Dane R. |last2=Volmer |first2=Dietrich A. |date=2021-07-22 |title=Mass spectrometry-based methods for the advanced characterization and structural analysis of lignin: A review |journal=Mass Spectrometry Reviews |volume=42 |issue=1 |language=en |pages=144–188 |doi=10.1002/mas.21716 |pmid=34293221 |s2cid=236200196 |issn=0277-7037|doi-access=free }}</ref><ref name="Li-2001">{{cite journal |doi=10.1105/tpc.010111|title=The Last Step of Syringyl Monolignol Biosynthesis in Angiosperms is Regulated by a Novel Gene Encoding Sinapyl Alcohol Dehydrogenase |year=2001 |last1=Li |first1=Laigeng |last2=Cheng |first2=Xiao Fei |last3=Leshkevich |first3=Jacqueline |last4=Umezawa |first4=Toshiaki |last5=Harding |first5=Scott A. |last6=Chiang |first6=Vincent L. |journal=The Plant Cell |volume=13 |issue=7 |pages=1567–1586 |pmid=11449052 |pmc=139549 }}</ref>

Lignin's [[molecular mass]]es exceed 10,000 [[atomic mass unit|u]]. It is [[hydrophobic]] as it is rich in [[aromatic]] subunits. The [[degree of polymerisation]] is difficult to measure, since the material is heterogeneous. Different types of lignin have been described depending on the means of isolation.<ref>{{cite web |url=http://www.lignin.org/01augdialogue.html |title=Lignin and its Properties: Glossary of Lignin Nomenclature |access-date=2007-10-14 |date=July 2001 |work=Dialogue/Newsletters Volume 9, Number 1 |publisher=Lignin Institute |archive-url=https://web.archive.org/web/20071009010219/http://www.lignin.org/01augdialogue.html |archive-date=2007-10-09 }}</ref>

[[File:Monolignols general.svg|thumb|200px|The three common monolignols: {{holist
 | [[paracoumaryl alcohol]], H
 | [[coniferyl alcohol]], G
 | [[sinapyl alcohol]], S
}}]]
Many grasses have mostly G, while some palms have mainly S.<ref>{{cite journal | journal=J Agric Food Chem | date=April 2001 | volume=49 | issue=4 | pages=1840–7 |title=Characterization of sago palm (Metroxylon sagu) lignin by analytical pyrolysis |vauthors=Kuroda K, Ozawa T, Ueno T | doi=10.1021/jf001126i| pmid=11308334 | s2cid=27962271 }}</ref> All lignins contain small amounts of incomplete or modified monolignols, and other monomers are prominent in non-woody plants.<ref>{{cite journal|author=J. Ralph|year=2001|title=Elucidation of new structures in lignins of CAD- and COMT-deficient plants by NMR|journal=Phytochemistry|volume=57|issue=6|pages=993–1003|doi=10.1016/S0031-9422(01)00109-1|pmid=11423146|bibcode=2001PChem..57..993R |display-authors=etal|url=https://naldc-legacy.nal.usda.gov/naldc/download.xhtml?id=14061&content=PDF|access-date=2018-12-29|archive-date=2021-04-28|archive-url=https://web.archive.org/web/20210428022353/https://naldc-legacy.nal.usda.gov/naldc/download.xhtml?id=14061&content=PDF|url-access=subscription}}</ref>

== Biological function ==
Lignin fills the spaces in the [[cell wall]] between [[cellulose]], [[hemicellulose]], and [[pectin]] components, especially in vascular and support tissues: [[xylem]] [[tracheid]]s, [[vessel element]]s and [[sclereid]] cells.{{citation needed|date=October 2023}}

Lignin plays a crucial part in conducting water and aqueous nutrients in [[plant]] stems. The [[polysaccharide]] components of plant [[cell wall]]s are highly [[hydrophilic]] and thus [[Semipermeable membrane|permeable]] to water, whereas lignin is more [[hydrophobic]]. The crosslinking of polysaccharides by lignin is an obstacle for water absorption to the cell wall. Thus, lignin makes it possible for the plant's vascular tissue to conduct water efficiently.<ref name="Sarkanen-1971">{{cite book
|editor1=K.V. Sarkanen
|editor2=C.H. Ludwig
|name-list-style=amp
|year=1971|title=Lignins: Occurrence, Formation, Structure, and Reactions|location=New York|publisher=Wiley Intersci.}}</ref> Lignin is present in all [[vascular plant]]s,<ref>{{Cite journal |last=Isikgor |first=Furkan H. |last2=Becer |first2=C. Remzi |date=2015 |title=Lignocellulosic biomass: a sustainable platform for the production of bio-based chemicals and polymers |url=https://xlink.rsc.org/?DOI=C5PY00263J |journal=Polymer Chemistry |language=en |volume=6 |issue=25 |pages=4497–4559 |doi=10.1039/C5PY00263J |issn=1759-9954|arxiv=1602.01684 }}</ref> but not in [[bryophyte]]s, supporting the idea that the original function of lignin was restricted to water transport.

It is [[covalent]]ly linked to [[hemicellulose]] and therefore [[cross-link]]s different plant [[polysaccharide]]s, conferring mechanical strength to the [[cell wall]] and by extension the plant as a whole.<ref>{{cite journal|last=Chabannes|first=M.|year=2001|title=''In situ'' analysis of lignins in transgenic tobacco reveals a differential impact of individual transformations on the spatial patterns of lignin deposition at the cellular and subcellular levels|journal=Plant J.|pages=271–282|volume=28|doi=10.1046/j.1365-313X.2001.01159.x|pmid=11722770|issue=3|display-authors=etal|url=http://agritrop.cirad.fr/578396/|doi-access=free}}</ref> Its most commonly noted function is the support through strengthening of wood (mainly composed of [[xylem]] cells and lignified [[sclerenchyma]] fibres) in vascular plants.<ref>{{cite book|year = 1995|title = Biology|last1= Arms|first1= Karen |last2= Camp |first2 = Pamela S. |publisher = Saunders College Pub.|isbn =978-0-03-050003-9}}</ref><ref>{{cite book|title = Anatomy of Seed Plants|last = Esau|first = Katharine|date =  1977|publisher = Wiley|isbn = 978-0-471-24520-9}}</ref><ref>{{cite journal | last1 = Wardrop | last2 = The | year = 1969 | title = ''Eryngium'' sp.; | journal = Aust. J. Bot. | volume = 17 | issue = 2| pages = 229–240 | doi=10.1071/bt9690229}}</ref>

Finally, lignin also confers disease resistance by accumulating at the site of pathogen infiltration, making the plant cell less accessible to cell wall degradation.<ref>{{Cite journal|last1=Bhuiyan|first1=Nazmul H|last2=Selvaraj|first2=Gopalan|last3=Wei|first3=Yangdou|last4=King|first4=John|date=February 2009|title=Role of lignification in plant defense|journal=Plant Signaling & Behavior|volume=4|issue=2|pages=158–159|doi=10.4161/psb.4.2.7688|issn=1559-2316|pmc=2637510|pmid=19649200|bibcode=2009PlSiB...4..158B }}</ref>

== Economic significance ==
[[Image:Zellstoffwerk Rosenthal 4.jpg|thumb|In pulp mills (like this one in [[Blankenstein]], Germany) using the [[Kraft process|kraft]]  or the [[sulfite process]], lignin is removed from lignocellulose to yield pulp for papermaking.]]
Global commercial production of lignin is a consequence of papermaking.  In 1988, more than 220 million tons of paper were produced worldwide.<ref>{{cite encyclopedia |author= Rudolf Patt ''et'' ''al''. |title= Paper and Pulp |pages= 1–92 |encyclopedia= Ullmann's Encyclopedia of Industrial Chemistry |year=2005 |publisher= Wiley-VCH |place= Weinheim |doi= 10.1002/14356007.a18_545.pub4|chapter= Pulp |isbn= 978-3-527-30673-2 }}</ref>  Much of this paper was delignified; lignin comprises about 1/3 of the mass of lignocellulose, the precursor to paper.  Lignin is an impediment to papermaking as it is colored, it yellows in air, and its presence weakens the paper. Once separated from the cellulose, it is  burned as fuel.  Only a fraction is used in a wide range of low volume applications where the form but not the quality is important.<ref>{{cite web|url = http://www.nnfcc.co.uk/publications/nnfcc-renewable-chemicals-factsheet-lignin |title =NNFCC Renewable Chemicals Factsheet: Lignin |first1= A |last1=Higson|first2= C|last2= Smith|date = 25 May 2011|archive-url = https://web.archive.org/web/20110720183626/http://www.nnfcc.co.uk/publications/nnfcc-renewable-chemicals-factsheet-lignin|archive-date = 20 July 2011}}</ref>

Mechanical, or high-yield [[wood pulp|pulp]], which is used to make [[newsprint]], still contains most of the lignin originally present in the wood. This lignin is responsible for newsprint's yellowing with age.<ref name="Sjöström-1993" /> High quality paper requires the removal of lignin from the pulp.  These delignification processes are core technologies of the papermaking industry as well as the source of significant environmental concerns.{{Citation needed|date=February 2021}}

In [[Sulfite process|sulfite pulping]], lignin is removed from wood pulp as [[lignosulfonates]], for which many applications have been proposed.<ref>{{cite web|url=http://www.lignin.org/whatis.html|title=Uses of lignin from sulfite pulping|access-date=2007-09-10|archive-url=https://web.archive.org/web/20071009004745/http://www.lignin.org/whatis.html|archive-date=2007-10-09}}</ref>  They are used as [[dispersant]]s, [[humectant]]s, [[Emulsion stabilization using polyelectrolytes|emulsion stabilizers]], and sequestrants ([[water treatment]]).<ref>{{cite encyclopedia |encyclopedia= Ullmann's Encyclopedia Of Industrial Chemistry |author= Barbara A. Tokay |year=2000 |doi= 10.1002/14356007.a04_099 |isbn = 978-3-527-30673-2|chapter = Biomass Chemicals}}</ref> Lignosulfonate was also the first family of [[water reducer]]s or [[superplasticizer]]s to be added in the 1930s as admixture to fresh [[concrete]] in order to decrease the water-to-cement (''w/c'') ratio, the main parameter controlling the concrete [[porosity]], and thus its [[mechanical strength]], its [[diffusion coefficient|diffusivity]] and its [[hydraulic conductivity]], all parameters essential for its durability. It has application in environmentally sustainable dust suppression agent for roads. Also, lignin can be used in making biodegradable plastic along with cellulose as an alternative to hydrocarbon-made plastics if lignin extraction is achieved through a more environmentally viable process than generic plastic manufacturing.<ref>{{Ullmann |doi=10.1002/14356007.a18_545|chapter=Paper and Pulp|year=2000|last1=Patt|first1=Rudolf|last2=Kordsachia|first2=Othar|last3=Süttinger|first3=Richard|last4=Ohtani|first4=Yoshito|last5=Hoesch|first5=Jochen F.|last6=Ehrler|first6=Peter|last7=Eichinger|first7=Rudolf|last8=Holik|first8=Herbert|last9=Hamm|first9=Udo|last10=Rohmann|first10=Michael E.|last11=Mummenhoff|first11=Peter|last12=Petermann|first12=Erich|last13=Miller|first13=Richard F.|last14=Frank|first14=Dieter|last15=Wilken|first15=Renke|last16=Baumgarten|first16=Heinrich L.|last17=Rentrop|first17=Gert-Heinz|isbn=3527306730}}</ref>

Lignin removed by the [[kraft process]] is usually burned for its fuel value, providing energy to power the paper mill. Two commercial processes exist to remove lignin from [[black liquor]] for higher value uses: LignoBoost (Sweden) and LignoForce (Canada). Higher quality lignin presents the potential to become a renewable source of [[aromatic]] compounds for the chemical industry, with an addressable market  of more than $130bn.<ref>{{Cite web | url=http://www.frost.com/prod/servlet/press-release.pag?docid=290584323 | title=Frost & Sullivan: Full Speed Ahead for the Lignin Market with High-Value Opportunities as early as 2017}}</ref>

Given that it is the most prevalent biopolymer after [[cellulose]], lignin has been investigated as a feedstock for biofuel production and can become a crucial plant extract in the development of a new class of biofuels.<ref name="Folkedahl-2016">{{Citation |last=Folkedahl |first=Bruce |year=2016 |access-date=2016-08-10 |title=Cellulosic ethanol: what to do with the lignin |journal=Biomass |url=http://biomassmagazine.com/articles/2928/cellulosic-ethanol-what-to-do-with-the-lignin |postscript=.}}</ref><ref name="Abengoa-2016">{{Citation |author=Abengoa |author-link=Abengoa |date=2016-04-21 |access-date=2016-08-10|title=The importance of lignin for ethanol production |url=http://www.theenergyofchange.com/importance-lignin-ethanol-production |postscript=.}}</ref>

== Biosynthesis ==
Lignin [[biosynthesis]] begins in the [[cytosol]] with the synthesis of [[Glycosylation|glycosylated]] monolignols from the [[amino acid]] [[phenylalanine]]. These first [[chemical reaction|reactions]] are shared with the [[phenylpropanoid]] pathway. The attached [[glucose]] renders them water-soluble and less [[toxic]]. Once transported through the [[cell membrane]] to the [[apoplast]], the glucose is removed, and the polymerisation commences.<ref>{{cite journal | journal=Planta |date=November 2002 | volume=216 |issue=1|pages=72–82 |  title=Cellular machinery of wood production: differentiation of secondary xylem in Pinus contorta var. latifolia |vauthors=Samuels AL, Rensing KH, Douglas CJ, Mansfield SD, Dharmawardhana DP, Ellis BE |pmid=12430016 | doi=10.1007/s00425-002-0884-4|bibcode=2002Plant.216...72A |s2cid=20529001 }}</ref> Much about its [[anabolism]] is not understood even after more than a century of study.<ref name="Baucher-2003" />

[[File:LigninPolymerisation.png|thumb|200px|Polymerisation of [[coniferyl alcohol]] to lignin. The reaction has two alternative routes [[catalysed]] by two different oxidative enzymes, [[peroxidase]]s or [[oxidase]]s.]]
The [[polymerisation]] step, that is a radical-radical coupling, is [[catalysis|catalysed]] by [[oxidative enzyme]]s. Both [[peroxidase]] and [[laccase]] enzymes are present in the [[plant]] [[cell walls]], and it is not known whether one or both of these groups participates in the polymerisation. Low molecular weight oxidants might also be involved. The oxidative enzyme catalyses the formation of monolignol [[Radical (chemistry)|radicals]]. These radicals are often said to undergo uncatalyzed coupling to form the lignin [[polymer]].<ref>{{cite journal |author=Davin, L.B. |author2=Lewis, N.G.  |year=2005 |title=Lignin primary structures and dirigent sites |journal=Current Opinion in Biotechnology |volume=16 |pages=407–415 |doi=10.1016/j.copbio.2005.06.011 |pmid=16023847 |issue=4}}</ref> An alternative theory invokes an unspecified biological control.<ref name="Saake-2007">{{Ullmann |doi=10.1002/14356007.a15_305.pub3|chapter=Lignin|year=2007|last1=Saake|first1=Bodo|last2=Lehnen|first2=Ralph|isbn=978-3527306732}}</ref>

== Biodegradation ==
In contrast to other bio-polymers (e.g. proteins, DNA, and even cellulose), lignin resists degradation.  It is immune to both acid- and base-catalyzed hydrolysis.  The degradability varies with species and plant tissue type. For example, syringyl (S) lignin is more susceptible to degradation by fungal decay as it has fewer aryl-aryl bonds and a lower redox potential than guaiacyl units.<ref name="Vane-2003" /><ref name="Vane-2006" /> Because it is cross-linked with the other cell wall components, lignin minimizes the accessibility of cellulose and hemicellulose to microbial enzymes (e.g. [[Steric hindrance]]), leading to a reduced digestibility of biomass.<ref name="Sarkanen-1971" />

Some ligninolytic enzymes include [[Haem peroxidase|heme peroxidases]] such as [[lignin peroxidase]]s, [[manganese peroxidase]]s, [[versatile peroxidase]]s, and [[Dye decolorizing peroxidase|dye-decolourizing peroxidases]] as well as copper-based [[laccase]]s. Lignin peroxidases oxidize non-phenolic lignin, whereas manganese peroxidases only oxidize the phenolic structures. Dye-decolorizing peroxidases, or DyPs, exhibit catalytic activity on a wide range of lignin model compounds, but their ''in vivo'' substrate is unknown. In general, laccases oxidize phenolic substrates but some fungal laccases have been shown to oxidize non-phenolic substrates in the presence of synthetic redox mediators.<ref name="Gadd-2013">{{Cite book |title=Advances in applied microbiology. Vol. 82 |date=2013 |publisher=Academic |author1-link=Geoffrey Michael Gadd |last1=Gadd |first1=Geoffrey M|last2=Sariaslani |first2=Sima |isbn=978-0-12-407679-2 |location=Oxford |pages=1–28 |oclc=841913543}}</ref><ref name="de Gonzalo-2016">{{Cite journal|last1=de Gonzalo|first1=Gonzalo|last2=Colpa|first2=Dana I.|last3=Habib|first3=Mohamed H.M.|last4=Fraaije|first4=Marco W.|title=Bacterial enzymes involved in lignin degradation|journal=Journal of Biotechnology|volume=236|pages=110–119|doi=10.1016/j.jbiotec.2016.08.011|pmid=27544286|year=2016|doi-access=free}}</ref>

=== Lignin degradation by fungi ===
Well-studied ligninolytic enzymes are found in ''[[Phanerochaete chrysosporium]]''<ref>{{cite journal|last1=Tien|first1=M|date=1983|title=Lignin-Degrading Enzyme from the Hymenomycete Phanerochaete chrysosporium Burds.|journal=Science|volume=221|issue=4611|pages=661–3|doi=10.1126/science.221.4611.661|pmid=17787736|bibcode=1983Sci...221..661T|s2cid=8767248}}</ref> and other [[White rot|white rot fungi]]. Some white rot fungi, such as ''Ceriporiopsis subvermispora'', can degrade the lignin in [[Lignocellulosic biomass|lignocellulose]], but others lack this ability.  Most fungal lignin degradation involves secreted [[peroxidase]]s. Many fungal [[laccase]]s are also secreted, which facilitate degradation of phenolic lignin-derived compounds, although several intracellular fungal laccases have also been described. An important aspect of fungal lignin degradation is the activity of accessory enzymes to produce the H<sub>2</sub>O<sub>2</sub> required for the function of [[lignin peroxidase]] and other [[heme peroxidase]]s.<ref name="Gadd-2013" />

=== Lignin degradation by bacteria ===
Bacteria lack most of the enzymes employed by fungi to degrade lignin, and lignin derivatives (aliphatic acids, furans, and solubilized phenolics) inhibit the growth of bacteria.<ref>{{Cite journal|last1=Cerisy|first1=Tristan|date=May 2017|title=Evolution of a Biomass-Fermenting Bacterium To Resist Lignin Phenolics|journal=Applied and Environmental Microbiology|volume=83|issue=11|doi=10.1128/AEM.00289-17|pmid= 28363966|pmc=5440714|bibcode=2017ApEnM..83E.289C |doi-access=free}}</ref> Yet, bacterial degradation can be quite extensive,<ref>{{Cite journal|last1=Pellerin|first1=Brian A.|last2=Hernes|first2=Peter J.|last3=Saraceno|first3=JohnFranco|last4=Spencer|first4=Robert G. M.|last5=Bergamaschi|first5=Brian A.|date=May 2010|title=Microbial degradation of plant leachate alters lignin phenols and trihalomethane precursors|journal=Journal of Environmental Quality|volume=39|issue=3|pages=946–954|doi=10.2134/jeq2009.0487|issn=0047-2425|pmid=20400590|bibcode=2010JEnvQ..39..946P }}</ref> especially in aquatic systems such as lakes, rivers, and streams, where inputs of terrestrial material (e.g. [[leaf litter]]) can enter waterways.  The ligninolytic activity of bacteria has not been studied extensively even though it was first described in 1930. Many bacterial DyPs have been characterized. Bacteria do not express any of the plant-type peroxidases (lignin peroxidase, Mn peroxidase, or versatile peroxidases), but three of the four classes of DyP are only found in bacteria. In contrast to fungi, most bacterial enzymes involved in lignin degradation are intracellular, including two classes of DyP and most bacterial laccases.<ref name="de Gonzalo-2016" />

In the environment, lignin can be degraded either biotically via bacteria or abiotically via photochemical alteration, and oftentimes the latter assists in the former.<ref>{{Cite journal|url=https://www.researchgate.net/publication/228990186|title=Photochemical and microbial degradation of dissolved lignin phenols: Implications for the fate of terrigenous dissolved organic matter in marine environments|journal=Journal of Geophysical Research|volume=108|language=en|access-date=2018-11-27|doi=10.1029/2002JC001421|year=2003|last1=Hernes|first1=Peter J.|issue=C9|page=3291|bibcode=2003JGRC..108.3291H|doi-access=free}}</ref>  In addition to the presence or absence of light, several of environmental factors affect the [[biodegradability]] of lignin, including bacterial community composition, mineral associations, and redox state.<ref>{{Cite web|url=https://www.researchgate.net/publication/51699032|title=Persistence of Soil Organic Matter as an Ecosystem Property|website=ResearchGate|language=en|access-date=2018-11-27}}</ref><ref>{{Cite book|date=2015-01-01|chapter=Reasons Behind the Long-Term Stability of Dissolved Organic Matter|title=Biogeochemistry of Marine Dissolved Organic Matter|language=en|pages=369–388|doi=10.1016/B978-0-12-405940-5.00007-8|last1=Dittmar|first1=Thorsten|isbn=978-0-12-405940-5}}</ref>

In [[shipworm]]s, the lignin it ingests is digested by "[[Alteromonas|Alteromonas-like sub-group]]" bacteria [[Symbiosis|symbionts]] in the [[typhlosole]] sub-organ of its [[cecum]].<ref name="Goodell-2024">{{cite journal |last1=Goodell |first1=Barry |last2=Chambers |first2=James |last3=Ward |first3=Doyle V. |last4=Murphy |first4=Cecelia |last5=Black |first5=Eileen |last6=Mancilio |first6=Lucca Bonjy Kikuti |last7=Perez- Gonzalez |first7=Gabriel |last8=Shipway |first8=J. Reuben |date=2024 |title=First report of microbial symbionts in the digestive system of shipworms; wood boring mollusks |journal=International Biodeterioration & Biodegradation |volume=192 |article-number=105816 |doi=10.1016/j.ibiod.2024.105816|doi-access=free |bibcode=2024IBiBi.19205816G }}</ref>

== Pyrolysis ==
[[Pyrolysis]] of lignin during the [[combustion]] of wood or [[charcoal]] production yields a range of products, of which the most characteristic ones are [[methoxy]]-substituted [[phenols]]. Of those, the most important are [[guaiacol]] and [[syringol]] and their derivatives.  Their presence can be used to trace a [[smoke]] source to a wood fire. In [[cooking]], lignin in the form of [[hardwood]] is an important source of these two compounds, which impart the characteristic aroma and taste to [[smoking (food)|smoked foods]] such as [[barbecue]]. The main flavor compounds of [[smoked ham]] are [[guaiacol]], and its 4-, 5-, and 6-methyl derivatives as well as 2,6-dimethylphenol. These compounds are produced by thermal breakdown of lignin in the  wood used in the smokehouse.<ref>{{cite book|last1=Wittkowski|first1= Reiner|last2= Ruther|first2= Joachim|last3= Drinda|first3= Heike|last4= Rafiei-Taghanaki|first4= Foroozan |title = Formation of smoke flavor compounds by thermal lignin degradation |series = ACS Symposium Series (Flavor Precursors)|date= 1992| volume= 490|pages= 232–243|isbn = 978-0-8412-1346-3}}</ref>

== Chemical analysis ==
{{Main|Lignin characterization}}
The conventional method for lignin quantitation in the pulp industry is the Klason lignin and acid-soluble lignin test, which is standardized procedures. The cellulose is digested thermally in the presence of acid. The residue is termed Klason lignin. Acid-soluble lignin (ASL) is quantified by the intensity of its [[Ultraviolet spectroscopy]]. The carbohydrate composition may be also analyzed from the Klason liquors, although there may be sugar breakdown products (furfural and [[5-hydroxymethylfurfural]]).<ref>{{Cite web|url=https://www.tappi.org/content/SARG/T222.pdf|title=TAPPI. T 222 om-02 – Acid-insoluble lignin in wood and pulp|access-date=2018-03-19|archive-date=2018-12-22|archive-url=https://web.archive.org/web/20181222192223/http://www.tappi.org/content/SARG/T222.pdf|url-status=dead}}</ref>

A solution of hydrochloric acid and [[phloroglucinol]] is used for the detection of lignin (Wiesner test). A brilliant red color develops, owing to the presence of [[coniferaldehyde]] groups in the lignin.<ref>{{cite web|title = Lignin production and detection in wood|first = John M.|last = Harkin|publisher = U.S. Forest Service Research|id = Note FPL-0148|date = November 1966|url = http://www.fpl.fs.fed.us/documnts/fplrn/fplrn0148.pdf|access-date = 2012-12-30|archive-date = 2020-03-05|archive-url = https://web.archive.org/web/20200305091429/http://www.fpl.fs.fed.us/documnts/fplrn/fplrn0148.pdf}}</ref>

[[Thioglycolysis]] is an analytical technique for lignin [[quantitation]].<ref>{{cite journal | last1 = Lange | first1 = B. M. | last2 = Lapierre | first2 = C. | last3 = Sandermann | first3 = Jr | year = 1995 | title = Elicitor-Induced Spruce Stress Lignin (Structural Similarity to Early Developmental Lignins) | journal = Plant Physiology | volume = 108 | issue = 3| pages = 1277–1287 | doi = 10.1104/pp.108.3.1277 | pmid=12228544 | pmc=157483}}</ref> Lignin structure can also be studied by computational simulation.<ref>{{cite journal |last1=Glasser |first1=Wolfgang G. |last2=Glasser |first2=Heidemarie R. |title=Simulation of Reactions with Lignin by Computer (Simrel). II. A Model for Softwood Lignin |journal=Holzforschung |volume=28 |issue=1 |pages=5–11, 1974 |year=1974 |doi=10.1515/hfsg.1974.28.1.5|s2cid=95157574 }}</ref>

Thermochemolysis (chemical break down of a substance under vacuum and at high temperature) with [[tetramethylammonium hydroxide]] (TMAH) or [[Copper(II) oxide|cupric oxide]]<ref>{{Cite journal|last1=Hedges|first1=John I.|last2=Ertel|first2=John R.|date=February 1982|title=Characterization of lignin by gas capillary chromatography of cupric oxide oxidation products|journal=Analytical Chemistry|language=EN|volume=54|issue=2|pages=174–178|doi=10.1021/ac00239a007|issn=0003-2700}}</ref> has also been used to characterize lignins.  The ratio of syringyl lignol (S) to vanillyl lignol (V) and cinnamyl lignol (C) to vanillyl lignol (V) is variable based on plant type and can therefore be used to trace plant sources in aquatic systems (woody vs. non-woody and angiosperm vs. gymnosperm).<ref>{{Cite journal|date=1979-11-01|title=The characterization of plant tissues by their lignin oxidation products|journal=Geochimica et Cosmochimica Acta|language=en|volume=43|issue=11|pages=1803–1807|doi=10.1016/0016-7037(79)90028-0|issn=0016-7037|last1=Hedges|first1=John I.|last2=Mann|first2=Dale C.|bibcode=1979GeCoA..43.1803H}}</ref> Ratios of carboxylic acid (Ad) to aldehyde (Al) forms of the lignols (Ad/Al) reveal diagenetic information, with higher ratios indicating a more highly degraded material.<ref name="Vane-2003">{{cite journal | last1 = Vane | first1 = C. H. | display-authors = etal   | year = 2003 | title = Biodegradation of Oak (Quercus alba) Wood during Growth of the Shiitake Mushroom (Lentinula edodes): A Molecular Approach | journal = Journal of Agricultural and Food Chemistry | volume = 51 | issue = 4| pages = 947–956 | doi=10.1021/jf020932h | pmid=12568554}}</ref><ref name="Vane-2006">{{cite journal | last1 = Vane | first1 = C. H. | display-authors = etal   | year = 2006 | title = Bark decay by the white-rot fungus Lentinula edodes: Polysaccharide loss, lignin resistance and the unmasking of suberin | journal = International Biodeterioration & Biodegradation | volume = 57 | issue = 1| pages = 14–23 | doi=10.1016/j.ibiod.2005.10.004| bibcode = 2006IBiBi..57...14V }}</ref> Increases in the (Ad/Al) value indicate an oxidative cleavage reaction has occurred on the alkyl lignin side chain which has been shown to be a step in the decay of wood by many [[Wood-decay fungus|white-rot]] and some  [[Wood-decay fungus|soft rot fungi]].<ref name="Vane-2003" /><ref name="Vane-2006" /><ref name="Vane-2001b">{{cite journal | last1 = Vane | first1 = C. H. | display-authors = etal   | year = 2001 | title = The effect of fungal decay (Agaricus bisporus) on wheat straw lignin using pyrolysis–GC–MS in the presence of tetramethylammonium hydroxide (TMAH) | journal = Journal of Analytical and Applied Pyrolysis | volume = 60 | issue = 1| pages = 69–78 | doi=10.1016/s0165-2370(00)00156-x| bibcode = 2001JAAP...60...69V }}</ref><ref name="Vane-2001a">{{cite journal |last1=Vane |first1=C. H. |display-authors=etal |year=2001 |title=Degradation of Lignin in Wheat Straw during Growth of the Oyster Mushroom (''Pleurotus ostreatus'') Using Off-line Thermochemolysis with Tetramethylammonium Hydroxide and Solid-State <sup>13</sup>C NMR |journal=Journal of Agricultural and Food Chemistry |doi=10.1021/jf001409a |pmid=11409955 |volume=49 |issue=6 |pages=2709–2716}}</ref><ref name="Vane-2005">{{cite journal |last1=Vane |first1=C. H. |display-authors=etal |year=2005 |title=Decay of cultivated apricot wood (''Prunus armeniaca'') by the ascomycete ''Hypocrea sulphurea'', using solid state <sup>13</sup>C NMR and off-line TMAH thermochemolysis with GC–MS. |journal=International Biodeterioration & Biodegradation |doi=10.1016/j.ibiod.2004.11.004 |volume=55 |issue=3 |pages=175–185}}</ref>

Lignin and its models have been well examined by <sup>1</sup>H and <sup>13</sup>C NMR spectroscopy.  Owing to the structural complexity of lignins, the spectra are poorly resolved and quantitation is challenging.<ref>{{cite book |last1=Ralph |first1=John |last2=Landucci |first2=Larry L. |year=2010
|title=Lignin and Lignans: Advances in Chemistry |chapter=NMR of Lignin and Lignans |publisher=Taylor & Francis|location=Boca Raton, FL |isbn=978-1-57444-486-5 |pages=137–244}}</ref>

== See also ==

* [[Carboniferous|Carboniferous period]]
* [[Coal|Coal formation]]

== References ==
{{Reflist|30em}}

== Further reading ==
* {{cite book |editor1-last=Freudenberg |editor1-first=K. |editor2-last=Nash |editor2-first=A. C. |name-list-style=amp |year=1968 |title=Constitution and Biosynthesis of Lignin |publisher=Springer-Verlag |location=Berlin}}
* Lignins: Occurrence, formation, structure and reactions; edited by [[Kyosti Vilho Sarkanen|K. V. Sarkanen]] and C. H. Ludwig, John Wiley & Sons, Inc., New York, 1971

== External links ==
* [https://web.archive.org/web/20140725132911/http://www.tecnaro.de/english/willkommen.htm?section=we Tecnaro website]

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