Editing Lichen (section)

=== 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/>