3.7: Lichens

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Biology

Lichens are not individual organisms, but a single body formed from multiple symbiotic organisms. Lichens contain a fungal partner (the mycobiont) that forms the majority of the lichen body (called a thallus) and one or more photosynthetic partners (the photobiont) that are typically found in a thin layer or in isolated pockets. The mycobiont is typically an ascoymycete, but can very rarely be a basidiomycete (basidiolichen). The photobiont is usually green algae, but can also be cyanobacteria (cyanolichen) or both green algae and cyanobacteria (tripartite lichen). The mycobiont is responsible for maintenance of the lichen thallus, while the photobiont is responsible for producing food for both partners through photosynthesis. A cyanobacterial partner might also be fixing nitrogen for the lichen. As the study of lichens progresses, we are uncovering more partners with as yet unknown roles, such as basidiomycete yeasts and bacteria.

The Lichen Mutualism

A cross section through a lichen thallus. The layers, a-e, are shown in order from the upper surface to the lower surface of the thallus. — Figure \(\PageIndex{1}\): Cross-section through a lichen thallus, showing the general organization: a) upper cortex b) layer containing the photobiont, shown as green algal cells wrapped in fungal hyphae c) medulla d) lower cortex e) rhizines (root-like structures that protrude from the lower cortex and anchor the lichen to its substrate). Diagram by Nefronus, CC BY-SA 4.0, via Wikimedia Commons.

Lichens and orange free-living algae on a log — Figure \(\PageIndex{2}\): A log covered in foliose lichens and free-living green algae. The lichens shown in this picture are the result of a mutualistic relationship between fungi and green algae. The "green" alga in this image appears orange, due to the production of carotenoids. This algae, *Trentepohlia*, is likely the same algal species that is present within the lichens shown. Photo by Maria Morrow, CC-BY.

A lichen growing in a crack with two apothecia labelled — Figure \(\PageIndex{3}\): A lichen growing in a crack with a moss. This lichen is producing many round, orange apothecia. You may recognize these as belonging to the Ascomycota. Most lichens are formed with an ascomycete as the mycobiont. When apothecia are formed, the spores are reproducing only the mycobiont, not the photobiont. Photo by Maria Morrow, CC-BY.

A light micrograph of a cross section through the apothecium of a lichen (labeled) — Figure \(\PageIndex{4}\): A cross section through an apothecium of the lichen *Physcia*. The apothecium is circled by a dotted line, A) hymenium composed of asci and paraphyses, B) upper cortex, C) photobiont layer (stained pink), D) medulla. Micrograph by Curtis Clark, CC BY-SA 3.0, via Wikimedia Commons with labels added by Maria Morrow.

Basidiolichens

A basidiolichen growing on a log, producing a mushroom — Figure \(\PageIndex{5}\): In the image on the left, there is a green slime covering the log. That green slime is primarily composed of the algal partner (the photobiont) of the basidiolichen. There is a small brown mushroom protruding from the slime. That mushroom is the fruiting body of the basidiomycete that is the mycobiont in this lichen. The image on the right shows a (dried out) mushroom produced by the basidiolichen *Lichenomphalia*. Photo by Maria Morrow, CC-BY.

A close up of a basidiolichen mushroom — Figure \(\PageIndex{5}\): In the image on the left, there is a green slime covering the log. That green slime is primarily composed of the algal partner (the photobiont) of the basidiolichen. There is a small brown mushroom protruding from the slime. That mushroom is the fruiting body of the basidiomycete that is the mycobiont in this lichen. The image on the right shows a (dried out) mushroom produced by the basidiolichen *Lichenomphalia*. Photo by Maria Morrow, CC-BY.

Cyanolichens

A foliose cyanolichen growing amongst moss and spruce needles — Figure \(\PageIndex{6}\): Peltigera membranacea is a cyanolichen. Cyanolichens often appear darker in color than lichens with an algal partner. This lichen is a dark, steely blue color when wet. This lichen is called the dog tooth lichen because the orange-brown apothecia are produced in rolls on the margins, looking like horns or teeth. Photo by Maria Morrow, CC-BY.

Another foliose cyanolichen growing amongst moss and another, lighter-colored lichen — Figure \(\PageIndex{7}\): A dark brown, foliose lichen is growing intermixed with other lichens and moss. This *Nephroma* sp. is a cyanolichen. Here, the apothecia are also produced on the edges, but the fertile side is facing downward. Photo by Maria Morrow, CC-BY.

Brown, amorphous, gelatinous looking blobs on a field of green. — Figure \(\PageIndex{8}\): Cyanobacteria in the genus *Nostoc* are housed in structures called cephalodia (cephalodium sing.). This image shows a close up (scale bar = 1 mm) of cephalodia on the lichen *Nephroma arcticum,* which appear dark brown. This contrasts the surrounding green pigment produced by the algal photobiont in the medulla. Photo by Jason Hollinger, CC BY 2.0, via Wikimedia Commons.

Additional Features

Rhizines on the underside of a foliose lichen — Figure \(\PageIndex{8}\): The first image (left) shows a foliose lichen that is a minty green on the upper surface and black on the lower surface. If you look closely at the undersurface, you can see many short hair-like protrusions. These rhizines help anchor the lichen to its substrate. In the next photo (right) a dark blue-grey foliose lichen (*Peltigera*) has many long white rhizines protruding from the lower cortex. First photo by Maria Morrow (CC-BY). Second photo by Kyle Copas (CC-BY).

Blue-grey foliose lichen with long white projections from the underside. — Figure \(\PageIndex{8}\): The first image (left) shows a foliose lichen that is a minty green on the upper surface and black on the lower surface. If you look closely at the undersurface, you can see many short hair-like protrusions. These rhizines help anchor the lichen to its substrate. In the next photo (right) a dark blue-grey foliose lichen (*Peltigera*) has many long white rhizines protruding from the lower cortex. First photo by Maria Morrow (CC-BY). Second photo by Kyle Copas (CC-BY).

A lichen with marginal soralia — Figure \(\PageIndex{9}\): A foliose lichen with marginal **soralia**. The powdery regions on the edges of this lichen are called soralia. In these soralia, the lichen is producing many soredia--loose bundles of fungal hyphae and algal cells that allow the lichen to reproduce asexually. Three of these soralia have been indicated by white arrows. Photo by Maria Morrow, CC-BY.

A highly branched yellow fruticose lichen with shiny projections emerging from the cortex — Figure \(\PageIndex{9}\): Lichens with **isidia**. Much like soralia, these are asexual reproductive structures containing both fungal and algal cells. Unlike soralia, these structures appear shiny (not powdery) because they are covered in a thin layer of cortex. First photo of *Flavoparmelia baltimorensis* by Jason Hollinger, CC BY 2.0, via Wikimedia Commons. Second photo of *Letharia vulpina* by Walter Siegmund, CC BY-SA 3.0, via Wikimedia Commons, has been cropped and labeled by Maria Morrow.

Forms

Lichens can generally be classified into three general forms: crustose, foliose, and fruticose. However, there are some lichens that don't really fit into these categories as nicely, such as the "dustose" lichens that form powdery crusts.

Crustose

Figure \(\PageIndex{10}\): This image shows several different crustose lichens (and one foliose lichen) on the bark of a young pear tree. The crustose lichen closest to the camera is in the genus Graphis. It looks as though it has been painted onto the tree bark, but has small raised black lines running through it, giving it the common name "script lichen". These lines are the elongated apothecia. Photo by Maria Morrow, CC-BY.

A crustose lichen growing on wax myrtle bark — Figure \(\PageIndex{12}\): This image shows a single crustose lichen (*Lepra*) growing out radially from a central point. There are not other crustose lichens growing adjacent to it, so it does not have the distinct boundaries that we saw with the crustose lichens on the pear bark above. The dots forming concentric circles on the surface of the lichen are apothecia. Photo by Maria Morrow, CC-BY.

Foliose

Foliose lichens have a distinct upper and lower surface, much like a leaf. Sometimes these can grow appressed to a substrate, but unlike crustose lichens, they can usually be separated from it (with some effort).

Lower surface of a foliose lichen (white) — Figure \(\PageIndex{13}\): A foliose lichen (*Lobaria pulmonaria*). The first image shows the upper cortex. The second image shows the lower cortex. These two surfaces make a distinct 'upper' and 'lower', unlike the fruticose lichens. Photos by Maria Morrow, CC-BY.