''Physcia caesia'', known colloquially as blue-gray rosette lichen and powder-back lichen, is a species of foliose

lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.Georg Franz Hoffmann in 1784, it is common across much of Europe, North America and New Zealand, and more patchily distributed in South America, Asia, Australia and Antarctica. There are 2

subspecies In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (morphology), but that can successfully interbreed. Not all species ...

: ''P. c. caesia'' and ''P. c. ventosa'', as well as a number of distinct forms and varieties. Molecular studies suggest that the species as currently defined may be polyphyletic. It is typically pale gray shading to darker gray in the center (though some forms are considerably darker), and grows in a small rosette, usually some across at maturity. It only rarely has apothecia, instead reproducing most often vegetatively via soredia, which are piled in round blue-gray mounds across the

thallus Thallus (plural: thalli), from Latinized Greek (), meaning "a green shoot" or "twig", is the vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts, lichens, and the Myxogastria. Many of these organisms wer ...

's upper surface. It grows most often on rock—principally calcareous, but also

basaltic Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...

and siliceous—and also occurs on bone, bark and soil. It is

nitrophilic Nitrophily is a botanical term that indicates a preference of certain plant species for a habitat rich in nitrate. This term was first introduced by George Fuller during the 1930s. The word is a contraction of the Greek words νἰτρον (nitro ...

and is particularly common on substrates where birds perch. Capable of growing at a multitude of angles on a variety of surfaces, ''Physcia caesia'' also tolerates a wide range of environmental extremes from the high temperatures of desert locations to the low temperatures of the Antarctic. It grows on both dry stone and that moistened by seepage, and can survive being irregularly submerged for extended periods underwater. Like many lichens that grow on rock, ''Physcia caesia'' is able to extract nutrients from the substrate upon which it grows, as well as from rainwater and atmospheric dust. It is threatened by habitat loss through development, as well as trampling or overgrowth of its location. A number of lichenicolous species are known

parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

Taxonomy

''Physcia caesia'' was first described by German botanist Georg Franz Hoffmann in 1784 as ''Lichen caesius''. Other lichenologists assigned it to various other

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

both before and after it was moved into its current genus by Georg Ernst Ludwig Hampe in 1839. It has two

, ''P. c. caesia'' and ''P. c. ventosa'', as well as a number of distinct forms and varieties, some of which were thought to be distinct species in the past. Although this species and '' Physcia aipolia'' have been considered a species pair, some molecular studies suggest that they should instead be considered conspecific. Other molecular studies suggest that both ''Physcia caesia'' and ''Physcia aipolia'' are polyphyletic taxa, with various forms more closely related to other species than to each other. The nomenclatural database MycoBank considers the taxon ''Physcia wainioi'', one of its many synonyms, to be a distinct species. The genus name ''

Physcia ''Physcia'' is a genus of lichenized fungi in the family Physciaceae. The genus name means "inflated" or "sausage-like", referring to the form of the type species. According to a 2008 estimate, the widespread genus contains 73 species. Species ...

'' means "inflated" or "sausage-like", referring to the form of the

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...

. The

specific epithet In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...

''caesia'' is a Latin word meaning "blue-gray". ''Physcia caesia'' is known colloquially as ''blue-gray rosette lichen'' and ''powder-back lichen''.

Description

''Physcia caesia'' is a

foliose lichen Foliose lichen is one of the morphological classes of lichens, which are complex organisms that arise from the symbiotic relationship between fungi and a photosynthetic partner, typically algae. This partnership allows lichen to live in diverse ...

that forms small rosettes, typically measuring across at maturity, though it can reach diameters of up to . The

is pale gray shading to darker gray in the center, and adheres closely to the substrate on which it grows. Its convex lobes are linear, typically measuring in width, though occasionally as wide as . Forms in some areas, including Greenland, can be considerably darker, with a thallus ranging from gray to dark gray; they can also have broader or narrower lobes. The lichen's surface is white-spotted, though this is not always obvious. It has blue-gray soredia, which are piled in round mounds (such aggregations are known as soralia) across the thallus's upper surface. The lower surface is white to brown with short, dark rhizines. ''Physcia caesia'' only rarely has apothecia. Where present, these are black discs measuring up to across, with prominent thalline margins (which means the thallus extends up around the edges of the apothecia) and a grayish

pruinescence Pruinescence , or pruinosity, is a "frosted" or dusty-looking coating on top of a surface. It may also be called a pruina (plural: ''pruinae''), from the Latin word for hoarfrost. The adjectival form is pruinose . Entomology In insects, a "blo ...

. Each

ascus An ascus (; ) is the sexual spore-bearing cell produced in ascomycete fungi. Each ascus usually contains eight ascospores (or octad), produced by meiosis followed, in most species, by a mitotic cell division. However, asci in some genera or s ...

contains 8 spores, which are thick-walled and brown, measuring x . Its

photobiont A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.green alga The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...

'' Trebouxia impressa'', a species associated with many ''Physcia'' lichens. The lichen's cortex and medulla react positively with

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which exp ...

(K), turning yellow. Cortex and medulla react positively with para-phenylenediamene (Pd) as well, also turning yellow. They produce no reaction with calcium or sodium hypochlorite (

bleach Bleach is the generic name for any chemical product that is used industrially or domestically to remove color (whitening) from a fabric or fiber or to clean or to remove stains in a process called bleaching. It often refers specifically, to ...

– C), nor with KC (potassium hydroxide quickly followed by bleach). Among the substances produced by the lichen are atranorin and

zeorin Zeorin is a triterpene with the molecular formula C30H52O2 which occurs in many lichens A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among hypha, filaments of multiple fungus, fungi species in a mutu ...

Similar species

''Physcia caesia'' was long considered to be a species pair with the very similar ''Physcia aipolia''; the latter is regularly covered with black apothecia, and typically grows

epiphytically An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphytes grow are called phoroph ...

. ''Physcia caesia'' may also be confused with ''Physcia poncinsii'', but the latter has "crateriform" (hollowed, like a bowl or saucer) rather than rounded soralia, and obviously convex lobes. It may also be confused with ''Physcia dubia'' which, like ''Physcia caesia'', can be quite variable; however, ''P. dubia'' has lip-shaped soralia (which tend to be primarily apical), and its thallus does not react with potassium hydroxide.

Distribution and habitat

''Physcia caesia'' is widespread across much of the world, found in

Arctic The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenla ...

boreal Boreal may refer to: Climatology and geography *Boreal (age), the first climatic phase of the Blytt-Sernander sequence of northern Europe, during the Holocene epoch *Boreal climate, a climate characterized by long winters and short, cool to mild ...

and

temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...

vegetation zones. Widely distributed across Europe, it is one of the most common species of ''Physcia'' in Greenland, and is abundant throughout Britain. Though found across much of North America, it is absent from central and southeastern United States, parts of the

Great Basin The Great Basin is the largest area of contiguous endorheic basin, endorheic watersheds, those with no outlets, in North America. It spans nearly all of Nevada, much of Utah, and portions of California, Idaho, Oregon, Wyoming, and Baja California ...

, and northern Alaska. In South America, it is found in Argentina, Chile, and Peru; common and widespread in temperate areas, it occurs only rarely and only at elevations above in tropical regions. It occurs patchily across Asia—in India, Bhutan, Nepal and Japan—as well as in East Africa. It is described as "cosmopolitan" in New Zealand, has been reported from a number of sites in southeastern Australia and Tasmania, and also occurs in Antarctica. It is common on calcareous substrates, growing on

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

(including

tufa Tufa is a variety of limestone formed when carbonate minerals precipitate out of water in unheated rivers or lakes. Geothermally heated hot springs sometimes produce similar (but less porous) carbonate deposits, which are known as travertine. ...

) and

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wi ...

. It also grows on

and siliceous rock, as well as on bone. It is particularly common on rocks where birds perch and defecate. It is uncommon on bark; however, in the Colorado

Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico in ...

, it is one of the predominant lichens on

juniper Junipers are coniferous trees and shrubs in the genus ''Juniperus'' () of the cypress family Cupressaceae. Depending on the taxonomy, between 50 and 67 species of junipers are widely distributed throughout the Northern Hemisphere, from the Arcti ...

trees, growing primarily within of the ground on the northern and eastern side of trunks. In

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...

, it grows on mosses, soil and gravel, and may be among the species contributing to the formation of a soil crust on the continent. In California, it is found in montane and subalpine forests ranging from in elevation.

Ecology

''Physcia caesia'' tolerates a variety of locations, occurring on vertical, angled and horizontal surfaces, as well as rocky overhangs. It is found on both dry stone and stone moistened by seepage, and can tolerate being irregularly submerged underwater. It survives in hot desert conditions, primarily on the north side of slopes. It also thrives in cold conditions, and is able to

photosynthesize Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in c ...

at temperatures as low as . Like many lichens that grow on rocky substrates, ''Physcia caesia'' is able to accumulate a number of necessary nutrients—including nitrogen, phosphorus, potassium and iron—from the substrate upon which it grows, as well as from rainwater and atmospheric dust. Its

hyphae A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...

can extend into substrate as much as . It is a nitrophyte and can tolerate higher levels of atmospheric ammonia than can many other species of lichen, because it can survive on substrates with a higher pH. It has been recorded growing on a lead surface (probably a weathered oxide). ''Physcia caesia'' is known to accumulate high concentrations of heavy metals, including

chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hardne ...

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

and

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

. It is slow-growing, with an increase in size of only per year for a colony observed in one study. In Greenland, it often grows in association with other lichen species tolerant of enriched environments, including ''

Umbilicaria arctica Rock tripe is the common name for various lichens of the genus ''Umbilicaria'' that grow on rocks. They are widely distributed, including on bare rock in Antarctica, and throughout northern parts of North America such as New England and the Rocky ...

'', '' Xanthoria candelaria'', ''

Xanthoria elegans ''Xanthoria elegans'', commonly known as the elegant sunburst lichen, is a lichenized species of fungus in the genus ''Xanthoria'', family Teloschistaceae. Recognized by its bright orange or red pigmentation, this species grows on rocks, often ne ...

'', ''

Physcia dubia ''Physcia'' is a genus of lichenized fungi in the family Physciaceae. The genus name means "inflated" or "sausage-like", referring to the form of the type species. According to a 2008 estimate, the widespread genus contains 73 species. Species ...

'', and '' Rhizoplaca melanophthalma''. In Poland, it is part of bryophyte communities found on

asbestos Asbestos () is a naturally occurring fibrous silicate mineral. There are six types, all of which are composed of long and thin fibrous crystals, each fibre being composed of many microscopic "fibrils" that can be released into the atmosphere b ...

tile roofs, occurring with '' Tortula ruralis'', ''

Orthotrichum anomalum ''Orthotrichum'' is a genus of moss in the family Orthotrichaceae. It is distributed throughout the world. There are about 125 species in the genus. Species include:Grimmia pulvinata ''Grimmia pulvinata'', otherwise known as grey-cushioned grimmia or pulvinate dry rock moss, is a bryophyte moss common in temperate climates worldwide. Characteristics ''Grimmia pulvinata'' grows in small cushion-like shaped, around 1-2 cent ...

'', '' Schistidium apocarpum'' and others. Foliose lichens such as ''Physcia caesia'' are intermediate in their response to air pollution — less sensitive than fruticose lichens, but more sensitive than

crustose lichen Crustose lichens are lichens that form a crust which strongly adheres to the Substrate (biology), substrate (soil, rock, tree bark, etc.), making separation from the substrate impossible without destruction. The basic structure of crustose lichen ...

s. ''Physcia caesia'' is found in cities, though at lower frequencies than more pollution-tolerant species. As with most lichens, ''Physcia caesia'' is impacted by habitat loss through development, as well as by the trampling or overgrowth of its location. However, the retention of old stone walls, buildings and bridges can help to support its populations, even in cities. ''Physcia caesia'' is attacked by lichenicolous fungi, including '' Polycoccum galligenum'', ''

Polycoccum pulvinatum ''Polycoccum'' is a genus of lichenicolous fungi in the family Polycoccaceae. It has about 60 species. Species *'' Polycoccum acarosporicola'' *'' Polycoccum aksoyi'' *''Polycoccum alboatrum'' *'' Polycoccum amygdalariae'' *''Polycoccum ana ...

'', '' Zwackhiomyces physciicola'' and '' Arthonia epiphyscia''.

References

Cited books

* * * * * * * * * * * * * * *

External links

*
''Physcia caesia'' photos on Stephen Sharnoff's lichen website
(photographer for ''Lichens of North America'') {{Taxonbar, from=Q1572772 Caliciales Lichen species Lichens described in 1784 Taxa named by Georg Franz Hoffmann