Xanthoria parietina (Common orange lichen)

Xanthoria parietina is a foliose, or leafy, lichen. It has wide distribution, and many common names such as common orange lichen, yellow scale, maritime sunburst lichen and shore lichen. It can be found near the shore on rocks or walls (hence the epithet parietina meaning "on walls"), and also on inland rocks, walls, or tree bark. It was chosen as a model organism for genomic sequencing (planned in 2006) by the US Department of Energy Joint Genome Institute (JGI).

Description
The vegetative body of the lichen, the thallus, is foliose, and typically less than 8 centimetres (3.1 in) wide. The lobes of the thallus are 1–4 mm in diameter, and flattened down. The upper surface is some shade of yellow, orange, or greenish yellow, while the lower surface is white, with a cortex, and with sparse pale rhizines or hapters. The vegetative reproductive structures soredia and isidia are absent in this species, however, apothecia are usually present.

The outer "skin" of the lichen, the cortex, is composed of closely packed fungal hyphae and serves to protect the thallus from water loss due to evaporation as well as harmful effects of high levels of irradiation. In Xanthoria parietina, the thickness of the thalli is known to vary depending on the habitat in which it grows. Thalli are much thinner in shady locations than in those exposed to full sunshine; this has the effect of protecting the algae that cannot tolerate high light intensities. The lichen pigment parietin gives this species a deep yellow or orange-red color.

X. parietina prefers growing on bark and wood; it is found more rarely on rock. Nutrient enrichment by bird droppings enhances the ability of X. parietina to grow on rock.

Photobiont
The photosynthetic symbionts, or photobionts, associated with X. parietina are from the green algal genus Trebouxia. Species that have been found include Trebouxia arboricola and T. irregularis. Both of these photobionts are known to occur free-living in nature, having been found on bark colonized by X. parietina as well as on bark not colonized by lichens.

In one study, the photobiont was shown to occupy 7% of the volume of the thallus. The density of pigmentation of the upper cortex also varies and seems to control the amount of light reaching the algae.

Reproduction and dispersal
A large number of lichens disperse very effectively by means of symbiotic vegetative propagules such as soredia, isidia and blastidia, and thallus fragmentation. However, X. parietina does not produce such vegetative propagules, but must establish the symbiotic state at each reproductive cycle. Two oribatid mite species, Trhypochtonius tectorum and Trichoribates trimaculatus, which are common inhabitants and consumers of X. parietina, are vectors of the photobiont cells. Faecal pellets of both species contain both viable ascospores and photobiont cells, and are suggested to be a common mode of short- and long-distance dispersal of this species.

Habitat and distribution
X. parietina occurs in hardwood forests in broad, low-elevation valleys, as well as scattered on Populus and other hardwoods in riparian areas in agricultural and populated areas. It is often associated with high level of nitrogen and favored by eutrophication, and can be often found near farmland and around livestock.

The species is widespread, and has been reported from Australia, Africa, Asia, North America and throughout much of Europe. In eastern North America and Europe, it is found more frequently near coastal locations. The increases in nitrate deposition as a result of industrial and agricultural developments in southern Ontario, Canada in the 20th century are thought to be responsible for the reappearance of this species in the local lichen flora.
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