Lichen Anatomy And Physiology

Lichen anatomy and physiology is very different from the anatomy and physiology of the fungus and/or algae and/or cyanobacteria that make up the lichen when growing apart from the lichen, either naturally, or in culture. The fungal partner is called the mycobiont. The photosynthetic partner, algae or cyanobacteria, is called the photobiont. The thallus is different from those of either the fungus or alga growing separately. The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations. In many species the fungus penetrates the algal cell wall, forming penetration pegs or haustoria similar to those produced by pathogenic fungi.F.S. Dobson (2000) Lichens, an illustrated guide to the British and Irish species. Richmond Publishing Co. Ltd., Slough, UKR. Honegger (1988) Mycobionts. Chapter 3 in T.H. Nash (ed.) (1996) Lichen Biology. Cambridge University Press. Lichens are capable of surviving extremely low levels of water content (poikilohydric). However, the re-configuration of membranes following a period of dehydration requires several minutes at least.

The algal or cyanobacterial cells are photosynthetic, and as in plants they reduce atmospheric carbon dioxide into organic carbon sugars to feed both symbionts. 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. If a cyanobacterium is present, as a primary partner or another symbiont in addition to green alga as in certain tripartite lichens, they can fix atmospheric nitrogen, complementing the activities of the green alga.

Although strains of cyanobacteria found in various cyanolichens are often closely related to one another, they differ from the most closely related free-living strains.Sciencemag.org
/ref> The lichen association is a close symbiosis. It extends the ecological range of both partners but is not always obligatory for their growth and reproduction in natural environments, since many of the algal symbionts can live independently. A prominent example is the alga '' Trentepohlia'' which forms orange-coloured populations on tree trunks and suitable rock faces. Lichen propagules ( diaspores) typically contain cells from both partners, although the fungal components of so-called "fringe species" rely instead on algal cells dispersed by the “core species.”

Lichen associations may be examples of mutualism, commensalism or even parasitism, depending on the species. Cyanobacteria in laboratory settings can grow faster when they are alone rather than when they are part of a lichen.

In tests, lichen survived and showed remarkable results on the adaptation capacity of photosynthetic activity within the simulation time of 34 days under Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR).

Symbionts

::''"Lichens are fungi that have discovered agriculture"'' - Trevor GowardLICHEN BIOLOGY AND THE ENVIRONMENT, LICHENS OF NORTH AMERICA, Sylvia and Stephen Sharnoff

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Living as a Symbiosis, symbiont in a lichen appears to be a successful way for a fungus to derive essential nutrients, as about 20% of all fungal species have acquired this mode of life. The fungal partner may be an Ascomycete or Basidiomycete.Lichens: Systematics, University of California Museum of Paleontology
/ref> Common algal partners are ''Trebouxia'', '' Pseudotrebouxia'', or ''Myrmecia''. Common cyanobacterium partners include are ''Nostoc'' or ''Scytonema''.

The largest number of lichenized fungi occur in the Ascomycota, with about 40% of species forming such an association.Kirk ''et al''., pp. 378–81. Some of these lichenized fungi occur in orders with nonlichenized fungi that live as saprotrophs or plant parasites (for example, the Leotiales, Dothideales, and Pezizales). Other lichen fungi occur in only five orders in which all members are engaged in this habit (Orders Graphidales, Gyalectales, Peltigerales, Pertusariales, and Teloschistales). Lichenized and nonlichenized fungi can even be found in the same genus or species. Overall, about 98% of lichens have an ascomycetous mycobiont. Next to the Ascomycota, the largest number of lichenized fungi occur in the unassigned fungi imperfecti. Comparatively few basidiomycetes are lichenized, but these include agarics, such as species of ''Lichenomphalia'', clavarioid fungi, such as species of ''Multiclavula'', and corticioid fungi, such as species of '' Dictyonema''.

The autotrophic symbionts occurring in lichens are a wide variety of simple, photosynthetic organisms commonly and traditionally known as algae. These symbionts include both prokaryotic and eukaryotic organisms. Approximately 100 species of photosynthetic partners from 40 genera and five distinct classes (prokaryotic: Cyanophyceae; eukaryotic: Trebouxiophyceae, Phaeophyceae, Chlorophyceae) have been found to associate with the lichen-forming fungi. The prokaryotes belong to the Cyanobacteria, whose representatives are often called bluegreen algae. The bluegreen algae occur as symbionts in about 8% of the known lichens. The most commonly occurring genus is ''Nostoc''. The majority of the lichens contain eukaryotic autotrophs belonging to the Chlorophyta (green algae) or to the Xanthophyta ( yellow-green algae). About 90% of all known lichens have a green alga as a symbiont, and among these, ''Trebouxia'' is the most common genus, occurring in about 40% of all lichens. The second most commonly represented green alga genus is '' Trentepohlia''. Overall, about 100 species are known to occur as autotrophs in lichens. All the algae are probably able to exist independently in nature as well as in the lichen.

A particular fungus species and algal species are not necessarily always associated together in a lichen. One fungus, for example, can form lichens with a variety of different algae. The thalli produced by a given fungal symbiont with its differing partners will be similar, and the secondary metabolites identical, indicating that the fungus has the dominant role in determining the morphology of the lichen. Further, the same algal species can occur in association with different fungal partners. Lichens are known in which there is one fungus associated with two or even three algal species. Rarely, the reverse can occur, and two or more fungal species can interact to form the same lichen.

Both the lichen and the fungus partner bear the same scientific name, and the lichens are being integrated into the classification schemes for fungi. The alga bears its own scientific name, which bears no relationship to that of the lichen or fungi.

Fungus component

Depending on context, the entire lichen, or just the fungus that is part of the lichen. Both the lichen and the fungus that is a part of the lichen are currently (2014) given the same species name, which creates an ambiguity. An example of when "lichenized fungus" refers to just the fungus is when the fungus is grown in culture without a phycobiont. An example where "lichenized fungus" refers to the entire lichen is in a list of classified lichens.

Some fungi can only be found living on lichens ( obligate parasites), but are not considered part of the lichen. These are referred to as lichenolous fungi.

Photosynthetic component

The photosynthetic component of a lichen is called the photobiont

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