Editing Lichen (section)

==Physiology==

=== Symbiotic relation ===
{{Main|Symbiosis in lichens}}

{{blockquote|Lichens are fungi that have discovered agriculture|Trevor Goward<ref name=LNAILBE>Sharnoff, Sylvia and Sharnoff, Stephen. [http://www.sharnoffphotos.com/lichen_info/biology.html "Lichen Biology and the Environment"] {{Webarchive|url=https://web.archive.org/web/20151017093641/http://sharnoffphotos.com/lichen_info/biology.html |date=17 October 2015 }}. sharnoffphotos.com</ref>}}
[[File:Abb4.1 Lichenes lichens interaction fungus alga Mykobiont Photobiont 2021 (M. Piepenbring).png|thumb|Lichens interaction]]
A lichen is a composite organism that emerges from [[algae]] or [[cyanobacteria]] living among the filaments ([[hyphae]]) of the [[Fungus|fungi]] in a mutually beneficial [[symbiotic]] relationship.<ref>{{cite web |last=Walker |first=Mark |date=2025 |title=What are lichens? |url=https://www.researchgate.net/publication/383947523_What_are_Lichens}}</ref> The fungi benefit from the carbohydrates produced by the algae or cyanobacteria via [[photosynthesis]]. The algae or cyanobacteria benefit by being protected from the environment by the filaments of the fungi, which also gather moisture and nutrients from the environment, and (usually) provide an anchor to it. Although some photosynthetic partners in a lichen can survive outside the lichen, the lichen symbiotic association extends the ecological range of both partners, whereby most descriptions of lichen associations describe them as symbiotic. Both partners gain water and mineral nutrients mainly from the atmosphere, through rain and dust. The fungal partner protects the alga by retaining water, serving as a larger capture area for mineral nutrients and, in some cases, provides minerals obtained from the [[Substrate (biology)|substrate]]. If a [[Cyanobacteria|cyanobacterium]] is present, as a primary partner or another symbiont in addition to a green alga as in certain [[Tripartite symbiosis|tripartite]] lichens, they can [[nitrogen fixation|fix atmospheric nitrogen]], complementing the activities of the green alga.

In three different lineages the fungal partner has independently lost the mitochondrial gene atp9, which has key functions in mitochondrial energy production. The loss makes the fungi completely dependent on their symbionts.<ref>{{Cite journal |title=Reductions in complexity of mitochondrial genomes in lichen-forming fungi shed light on genome architecture of obligate symbioses – Wiley Online Library |journal=Molecular Ecology |volume=27 |issue=5 |pages=1155–1169 |doi=10.1111/mec.14519 |pmid=29417658 |year=2018 |last1=Pogoda |first1=C. S. |last2=Keepers |first2=K. G. |last3=Lendemer |first3=J. C. |last4=Kane |first4=N. C. |last5=Tripp |first5=E. A. |s2cid=4238109 }}</ref>

The algal or cyanobacterial cells are [[photosynthesis|photosynthetic]] and, as in plants, they [[redox|reduce]] atmospheric [[carbon dioxide]] into organic carbon sugars to feed both symbionts. Phycobionts (algae) produce [[sugar alcohols]] ([[ribitol]], [[sorbitol]], and [[erythritol]]), which are absorbed by the mycobiont (fungus).<ref name=UNOLP/> Cyanobionts produce [[glucose]].<ref name=UNOLP/> Lichenized fungal cells can make the photobiont "leak" out the products of photosynthesis, where they can then be absorbed by the fungus.<ref name=SSFGCL/>{{rp|5}}

It appears many, probably the majority, of lichen also live in a symbiotic relationship with an order of [[basidiomycete]] yeasts called [[Cystobasidiomycetes|Cyphobasidiales]]. The absence of this third partner could explain why growing lichen in the laboratory is difficult. The yeast cells are responsible for the formation of the characteristic cortex of the lichen thallus, and could also be important for its shape. An example of this lichen-yeast symbiosis is the North American beard-like lichens.<ref>{{Cite journal|title=Basidiomycete yeasts in the cortex of ascomycete macrolichens|first1=Toby|last1=Spribille|first2=Veera|last2=Tuovinen|first3=Philipp|last3=Resl|first4=Dan|last4=Vanderpool|first5=Heimo|last5=Wolinski|first6=M. Catherine|last6=Aime|first7=Kevin|last7=Schneider|first8=Edith|last8=Stabentheiner|first9=Merje|last9=Toome-Heller|first10=Göran|last10=Thor|first11=Helmut|last11=Mayrhofer|first12=Hanna|last12=Johannesson|first13=John P.|last13=McCutcheon|date=29 July 2016|journal=Science|volume=353|issue=6298|pages=488–492|doi=10.1126/science.aaf8287|pmid=27445309|pmc=5793994|bibcode=2016Sci...353..488S }}</ref>

The lichen combination of alga or cyanobacterium with a fungus has a very different form (morphology), physiology, and biochemistry than the component fungus, alga, or cyanobacterium growing by itself, naturally or in culture. The body ([[thallus]]) of most lichens is different from those of either the fungus or alga growing separately. When grown in the laboratory in the absence of its photobiont, a lichen fungus develops as a structureless, undifferentiated mass of fungal filaments ([[hyphae]]). If combined with its photobiont under appropriate conditions, its characteristic form associated with the photobiont emerges, in the process called [[morphogenesis]].<ref name=LNA/> In a few remarkable cases, a single lichen fungus can develop into two very different lichen forms when associating with either a green algal or a cyanobacterial symbiont. Quite naturally, these alternative forms were at first considered to be different species, until they were found growing in a conjoined manner.{{Citation needed|date=August 2022}}

Evidence that lichens are examples of successful [[symbiosis]] is the fact that lichens can be found in almost every habitat and geographic area on the planet.<ref name=TT/> Two species in two genera of green algae are found in over 35% of all lichens, but can only rarely be found living on their own outside of a lichen.<ref>{{cite journal|pmid=19853051 |year=2010 |last1=Skaloud |first1=P |title=Evolutionary inferences based on ITS rDNA and actin sequences reveal extensive diversity of the common lichen alga ''Asterochloris'' (Trebouxiophyceae, Chlorophyta) |journal=Molecular Phylogenetics and Evolution |volume=54 |issue=1 |pages=36–46 |last2=Peksa |first2=O |doi=10.1016/j.ympev.2009.09.035 }}</ref>

In a case where one fungal partner simultaneously had two green algae partners that outperform each other in different climates, this might indicate having more than one photosynthetic partner at the same time might enable the lichen to exist in a wider range of habitats and geographic locations.<ref name=TT/>

Phycobionts can have a net output of sugars with only water vapor.<ref name=UNOLP/> The thallus must be saturated with liquid water for cyanobionts to photosynthesize.<ref name=UNOLP/>

Algae produce sugars that are absorbed by the fungus by diffusion into special fungal hyphae called [[appressoria]] or [[haustoria]] in contact with the wall of the algal cells.<ref name=ELWWL>{{cite web |author=Ramel, Gordon |title=What is a Lichen? |publisher=Earthlife Web |url=http://www.earthlife.net/lichens/lichen.html |access-date=20 January 2015 |archive-url=https://web.archive.org/web/20150119021440/http://www.earthlife.net/lichens/lichen.html |archive-date=19 January 2015 |url-status=live }}</ref> The appressoria or haustoria may produce a substance that increases permeability of the algal cell walls, and may penetrate the walls.<ref name=ELWWL/> The algae may contribute up to 80% of their sugar production to the fungus.<ref name=ELWWL/>

===Ecology===
Lichen associations may be examples of [[Mutualism (biology)|mutualism]] or [[commensalism]], but the lichen relationship can be considered [[parasitism|parasitic]]<ref name=Ahmadjian>{{cite book|author=Ahmadjian V.|date= 1993 |title=''The Lichen Symbiosis'' |publisher=New York: John Wiley & Sons |isbn=978-0-471-57885-7}}</ref> under circumstances where the photosynthetic partner can exist in nature independently of the fungal partner, but not vice versa. Photobiont cells are routinely destroyed in the course of [[nutrient]] exchange. The association continues because reproduction of the photobiont cells matches the rate at which they are destroyed.<ref name=Ahmadjian/> The [[fungus]] surrounds the algal cells,<ref name=LLHE/> often enclosing them within complex fungal tissues unique to lichen associations. In many species the fungus penetrates the algal cell wall,<ref name=LLHE/> forming penetration pegs ([[haustoria]]) similar to those produced by [[pathogenic fungus|pathogenic fungi]] that feed on a host.<ref name=Dobson/><ref name=Honegger1988/> [[Cyanobacteria]] in laboratory settings can grow faster when they are alone rather than when they are part of a lichen.

====Miniature ecosystem and holobiont theory====
Symbiosis in lichens is so well-balanced that lichens have been considered to be relatively self-contained miniature ecosystems in and of themselves.<ref name=TT>{{cite journal|pmid=21134099|year=2011|last1=Casano|first1=L. M.|title=Two ''Trebouxia'' algae with different physiological performances are ever-present in lichen thalli of ''Ramalina farinacea''. Coexistence versus competition?|journal=Environmental Microbiology|volume=13|issue=3|pages=806–818|last2=Del Campo|first2=E. M.|last3=García-Breijo|first3=F. J.|last4=Reig-Armiñana|first4=J|last5=Gasulla|first5=F|last6=Del Hoyo|first6=A|last7=Guéra|first7=A|last8=Barreno|first8=E|doi=10.1111/j.1462-2920.2010.02386.x|bibcode=2011EnvMi..13..806C |hdl=10251/60269|type=Submitted manuscript|hdl-access=free}}
</ref><ref name="FESM">[[Rosmarie Honegger|Honegger, R.]] (1991) ''Fungal evolution: symbiosis and morphogenesis, Symbiosis as a Source of Evolutionary Innovation'', Margulis, L., and Fester, R. (eds). Cambridge, MA, US: The MIT Press, pp. 319–340.</ref> It is thought that lichens may be even more complex symbiotic systems that include non-photosynthetic bacterial communities performing other functions as partners in a [[holobiont]].<ref name=SSSFD>{{cite journal |pmid=19554038|year=2009|last1=Grube|first1=M|title=Species-specific structural and functional diversity of bacterial communities in lichen symbioses|journal=The ISME Journal |volume=3 |issue=9 |pages=1105–1115 |last2=Cardinale|first2=M|last3=De Castro|first3=J. V. Jr.|last4=Müller|first4=H|last5=Berg|first5=G|doi=10.1038/ismej.2009.63|bibcode=2009ISMEJ...3.1105G |doi-access=free}}</ref><ref name=NPB>Barreno, E., Herrera-Campos, M., García-Breijo, F., Gasulla, F., and Reig-Armiñana, J. (2008) [https://www.researchgate.net/publication/258264055 "Non photosynthetic bacteria associated to cortical structures on Ramalina and ''Usnea'' thalli from Mexico"]. Asilomar, Pacific Grove, CA, USA: Abstracts IAL 6- ABLS Joint Meeting.</ref>

Many lichens are very sensitive to environmental disturbances and can be used to cheaply<ref name=LLHE>{{cite web |last1=Speer |first1=Brian R |author2=Ben Waggoner |title=Lichens: Life History & Ecology |url=http://www.ucmp.berkeley.edu/fungi/lichens/lichenlh.html |publisher=University of California Museum of Paleontology |date=May 1997 |access-date=28 April 2015 |archive-url=https://web.archive.org/web/20150502160300/http://www.ucmp.berkeley.edu/fungi/lichens/lichenlh.html |archive-date=2 May 2015 |url-status=live }}</ref> assess [[air pollution]],<ref name=ferry/><ref name=rosehawksworth/><ref name=Hawksworthrose1976/> [[ozone]] depletion, and metal contamination. Lichens have been used in making [[dye]]s, perfumes ([[oakmoss]]),<ref name=OMAO>{{cite web|url=http://www.victorie-inc.us/oakmoss.html|title=Oak Moss Absolute Oil, Evernia prunastri, Perfume Fixative|access-date=19 September 2014|archive-url=https://web.archive.org/web/20141225173039/http://www.victorie-inc.us/oakmoss.html|archive-date=25 December 2014|url-status=live}}</ref> and in [[traditional medicine]]s. A few lichen species are eaten by insects<ref name=LLHE/> or larger animals, such as reindeer.<ref>{{cite journal|doi=10.1111/j.1600-0587.1984.tb01138.x |title=Wild reindeer foraging-niche organization |journal=Ecography |volume=7 |issue=4 |page=345 |year=1984 |last1=Skogland |first1=Terje |bibcode=1984Ecogr...7..345S }}</ref> Lichens are widely used as environmental indicators or bio-indicators. When air is very badly polluted with sulphur dioxide, there may be no lichens present; only some green algae can tolerate those conditions. If the air is clean, then shrubby, hairy and leafy lichens become abundant. A few lichen species can tolerate fairly high levels of pollution, and are commonly found in urban areas, on pavements, walls and tree bark. The most sensitive lichens are shrubby and leafy, while the most tolerant lichens are all crusty in appearance. Since industrialisation, many of the shrubby and leafy lichens such as ''[[Ramalina]]'', ''[[Usnea]]'' and ''[[Lobaria]]'' species have very limited ranges, often being confined to the areas which have the cleanest air.

====Lichenicolous fungi====
Some fungi can only be found living ''on'' lichens as [[obligate parasite]]s. These are referred to as [[lichenicolous fungi]], and are a different species from the fungus living inside the lichen; thus they are not considered to be part of the lichen.<ref name=LFIEB>{{cite journal|doi=10.1639/0007-2745(2003)106[0080:LFIEAB]2.0.CO;2|year=2003|volume=106|page=80|title=Lichenicolous Fungi: Interactions, Evolution, and Biodiversity|journal=The Bryologist|last1=Lawrey|first1=James D.|last2=Diederich|first2=Paul|s2cid=85790408 |url=http://www.lichenology.info/pdf/LawreyDiederich.pdf|access-date=2 May 2011|archive-url=https://web.archive.org/web/20110103065336/http://www.lichenology.info/pdf/LawreyDiederich.pdf|archive-date=3 January 2011|url-status=live}}</ref>

===Reaction to water===
Moisture makes the cortex become more transparent.<ref name=SSFGCL/>{{rp|4}} This way, the algae can conduct photosynthesis when moisture is available, and is protected at other times. When the cortex is more transparent, the algae show more clearly and the lichen looks greener.

===Metabolites, metabolite structures and bioactivity===
Lichens can show intense antioxidant activity.<ref>{{cite journal | pmid = 25808912 | doi=10.1111/1462-2920.12850 | title=Comparative analysis of the antioxidant properties of Icelandic and Hawaiian lichens |date=March 2015 | journal=Environmental Microbiology  |vauthors=Hagiwara K, Wright PR, etal | volume=18 | issue=8 | pages=2319–2325| s2cid=13768322 }}</ref><ref>{{cite journal | pmid = 15752633 | doi=10.1016/j.fitote.2004.05.012 | volume=76 | issue=2 | title=Antioxidant activity, reducing power and total phenolic content of some lichen species |date=March 2005 | journal=Fitoterapia | pages=216–219  |vauthors=Odabasoglu F, Aslan A, Cakir A, etal }}</ref> [[Secondary metabolites]] are often deposited as crystals in the [[apoplast]].<ref name=HSLNA>{{cite journal|doi=10.1016/j.envexpbot.2010.01.003|title=Norstictic acid: Correlations between its physico-chemical characteristics and ecological preferences of lichens producing this depsidone|journal=Environmental and Experimental Botany|volume=68|issue=3|page=309|year=2010|last1=Hauck|first1=Markus|last2=Jürgens|first2=Sascha-René|last3=Leuschner|first3=Christoph|bibcode=2010EnvEB..68..309H }}</ref> Secondary metabolites are thought to play a role in preference for some substrates over others.<ref name=HSLNA/>

=== Growth rate ===
Lichens often have a regular but very slow growth rate of less than a millimeter per year.

In crustose lichens, the area along the margin is where the most active growth is taking place.<ref name=VMBMLF/>{{rp|159}} Most crustose lichens grow only 1–2&nbsp;mm in diameter per year.

=== Life span ===
Lichens may be [[longevity|long-lived]], with some considered to be among the oldest living organisms.<ref name=LNA/><ref name="Morris2007"/> Lifespan is difficult to measure because what defines the "same" individual lichen is not precise.<ref name=ELNGDL>{{cite web|url=http://www.earthlife.net/lichens/growth.html|title=The Earth Life Web, Growth and Development in Lichens|publisher=earthlife.net|access-date=12 October 2014|archive-url=https://web.archive.org/web/20150528035845/http://www.earthlife.net/lichens/growth.html|archive-date=28 May 2015|url-status=dead}}</ref> Lichens grow by vegetatively breaking off a piece, which may or may not be defined as the "same" lichen, and two lichens can merge, then becoming the "same" lichen.<ref name=ELNGDL/> One specimen of ''[[Rhizocarpon geographicum]]'' on East [[Baffin Island]] has an estimated age of 9500 years.<ref>{{Cite journal |last1=Rosenwinkel |first1=Swenja |last2=Korup |first2=Oliver |last3=Landgraf |first3=Angela |last4=Dzhumabaeva |first4=Atyrgul |date=2015 |title=Limits to lichenometry |url=https://www.sciencedirect.com/science/article/abs/pii/S0277379115301529 |journal=Quaternary Science Reviews |volume=129 |pages=229–238 |doi=10.1016/j.quascirev.2015.10.031|bibcode=2015QSRv..129..229R }}</ref><ref>{{Cite journal |last1=Miller |first1=G. H. |last2=Andrews |first2=J. T. |date=April 1972 |title=Quaternary History of Northern Cumberland Peninsula, East Baffin Island, N.W.T., Canada Part VI: Preliminary Lichen Growth Curve for Rhizocarpon geographicum |url=https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/83/4/1133/7545/Quaternary-History-of-Northern-Cumberland |journal=GSA Bulletin |volume=83 |issue=4 |pages=1133–1138 |doi=10.1130/0016-7606(1972)83[1133:QHONCP]2.0.CO;2}}</ref> [[Thallus|Thalli]] of ''Rhizocarpon geographicum'' and ''[[Rhizocarpon]] eupetraeoides''/''inarense'' in the central [[Brooks Range]] of northern Alaska have been given a maximum possible age of 10,000–11,500 years.<ref>{{Cite journal |last1=Haworth |first1=Leah A. |last2=Calkin |first2=Parker E. |last3=Ellis |first3=James M. |date=1986 |title=Direct Measurement of Lichen Growth in the Central Brooks Range, Alaska, U.S.A., and Its Application to Lichenometric Dating, Arctic and Alpine Research |url=https://www.tandfonline.com/doi/abs/10.1080/00040851.1986.12004090 |journal=Arctic and Alpine Research |volume=18 |issue=3 |pages=289–296 |doi=10.2307/1550886|jstor=1550886 }}</ref><ref>{{Cite journal |last=Benedict |first=James B. |date=January 2009 |title=A Review of Lichenometric Dating and Its Applications to Archaeology |url=https://www.cambridge.org/core/journals/american-antiquity/article/abs/review-of-lichenometric-dating-and-its-applications-to-archaeology/21C29EAAF0361CCE2B98793DC3CB9F5F |journal=American Antiquity |volume=74 |issue=1 |pages=143–172 |doi=10.1017/S0002731600047545|s2cid=83108496 }}</ref>

===Response to environmental stress===
Unlike simple dehydration in plants and animals, lichens may experience a ''complete'' loss of body water in dry periods.<ref name=LLHE/> Lichens are capable of surviving extremely low levels of [[water]] content ([[poikilohydric]]).<ref name="Nash2008-intro"/>{{rp|5–6}} They quickly absorb water when it becomes available again, becoming soft and fleshy.<ref name=LLHE/>

In tests, lichen survived and showed remarkable results on the [[adaptive capacity|adaptation capacity]] of [[photosynthesis|photosynthetic activity]] within the [[simulation|simulation time]] of 34 days under [[Life on Mars (planet)#Life on Earth under Martian conditions|Martian conditions]] in the Mars Simulation Laboratory (MSL) maintained by the [[German Aerospace Center]] (DLR).<ref name="Skymania-20120426">{{cite web |last=Baldwin |first=Emily |title=Lichen survives harsh Mars environment |url=http://www.skymania.com/wp/2012/04/lichen-survives-harsh-martian-setting.html |date=26 April 2012 |publisher=Skymania News |access-date=27 April 2012 |archive-url=https://web.archive.org/web/20120528145425/http://www.skymania.com/wp/2012/04/lichen-survives-harsh-martian-setting.html/ |archive-date=28 May 2012 |url-status=dead }}</ref><ref>{{Cite book|last=Sheldrake|first=Merlin|title=Entangled Life: How Fungi Make Our Worlds, Change Our Minds and Shape Our Futures|publisher=Bodley Head|year=2020|isbn=978-1847925206|pages=94}}</ref>

The [[European Space Agency]] has discovered that lichens can survive unprotected in space. In an experiment led by Leopoldo Sancho from the Complutense University of Madrid, two species of lichen—''[[Rhizocarpon geographicum]]'' and ''[[Rusavskia elegans]]''—were sealed in a capsule and launched on a Russian Soyuz rocket 31 May 2005. Once in orbit, the capsules were opened and the lichens were directly exposed to the vacuum of space with its widely fluctuating temperatures and cosmic radiation. After 15 days, the lichens were brought back to earth and were found to be unchanged in their ability to photosynthesize.<ref name="urlESA - Human Spaceflight and Exploration - Lichen survives in space"/><ref name=Sancho2007/>