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Cetraria
	Cetraria aculeata
Scientific classification
Domain:	Eukaryota
Kingdom:	Fungi
Division:	Ascomycota
Class:	Lecanoromycetes
Order:	Lecanorales
Family:	Parmeliaceae
Genus:	Cetraria; Ach. (1803)
Type species
	Cetraria islandica; (L.) Ach. (1803)
Synonyms
	Cetraria sect. Platysma Körb. (1859); Cetrariomyces E.A.Thomas (1939); Coelocaulon Link (1833); Geissodea Vent. (1799); Platisma Hoffm. (1796); Platyphyllum Vent. (1799); Pseudocornicularia Gyeln. (1933); Squamaria Hoffm. (1789);

Cetraria is a genus of fruticose lichens that associate with green algae as photobionts. Most species are found at high latitudes, occurring on sand or heath. Species have a characteristic "strap-like" form, with spiny lobe edges.

Cetraria islandica, Iceland moss, is one of the few culinary lichens, ground and eaten by Northern Europeans in times of famine.

Another species is used today as a "bulking" agent in garam masala of traditional Indian cuisine.

Description

Genus Cetraria includes lichens that typically form dorsiventral thalli, which are distinctly differentiated between an upper and lower surface. The thallus can be leaf-like (foliose) and loosely attached to the substrate, often with ascending lobe margins that may form rosette-like patterns, or it can be shrubby and erect (fruticose) with lobes that are channelled, sometimes tubular, and occasionally flat. In some species, the thallus forms conspicuous spine-like projections, growing in tufted clusters. The colour of the thallus varies, ranging from dark brown or olive green to grey-green or even yellowish in certain conditions.^[2]

The upper cortex of Cetraria lichens may have one or two layers, with thick-walled hyphal cells forming the external layer. Beneath this lies a more delicate layer of hyphae arranged parallel to the surface (periclinal arrangement). The internal medulla, which forms the core of the thallus, is typically white or bright yellow. Pseudocyphellae (small pores that facilitate gas exchange) are often present on the lower surface or at the margins of the lobes. The lower surface generally lacks rhizines (root-like structures for attachment) or has them only sparsely. Soralia, which are specialised structures for vegetative reproduction, are rarely found. The primary photobiont (photosynthetic partner) in Cetraria is a trebouxioid or chlorococcoid green alga.^[2]

The sexual reproductive structures of Cetraria are apothecia (fruiting bodies), typically positioned at the margins of the thallus and often attached at an angle. The margin of the apothecium can sometimes appear notched or crenulate, and the thalline margin (a rim formed by the thallus tissue) is often curved inward. The epithecium, or uppermost layer of the apothecium, is usually red-brown to dark brown. The hymenium (fertile layer) stains blue when exposed to iodine (I+ blue), while the hypothecium (tissue beneath the hymenium) is colourless. The hamathecium, composed of paraphyses (sterile filamentous structures), is typically straight, sparsely branched, with swollen tips.^[2]

The asci (spore-producing cells) are eight-spored, narrowly club-shaped (clavate), and feature a moderately large tholus (internal structure) with an apical ring and a conical ocular chamber that may have a narrow to broad beak. This configuration is characteristic of the Lecanora-type ascus. The ascospores are colourless, single-celled (aseptate), and vary in shape from ellipsoidal to nearly spherical (subglobose).^[2]

Asexual reproductive structures, the pycnidia, are also present in Cetraria. These are flask-shaped structures that release conidia (asexual spores) through a blackened opening (ostiole). The pycnidia may be located on the surface of the thallus or at the ends of marginal projections. The walls of the pycnidia are two-layered, with the outer layer being thin (around 5 μm thick) and sometimes darkened. The conidia themselves are colourless and may be cylindrical, bottle-shaped, crescent-shaped, or lemon-shaped.^[2]

The chemical composition of Cetraria species includes a range of secondary metabolites, such as β-orcinol depsidones (e.g., fumarprotocetraric acid, norstictic acid, gyrophoric acid, and hiascinic acid), fatty acids (lichesterinic and protolichesterinic acids), as well as usnic acid and derivatives of pulvinic acid in some species. These compounds contribute to the lichen's defence mechanisms and other ecological roles.^[2]

Species

Cetraria aculeata (Schreb.) Fr. (1826)
Cetraria arenaria Kärnefelt (1977)
Cetraria australiensis W.A.Weber ex Kärnefelt (1977)
Cetraria corrugata (R.F.Wang, X.L.Wei & J.C.Wei) Divakar, A.Crespo & Lumbsch (2017)
Cetraria endochrysea (Lynge) Divakar, A.Crespo & Lumbsch (2017)
Cetraria ericetorum Opiz (1852)
Cetraria flavonigrescens (A.Thell & Randlane) Divakar, A.Crespo & Lumbsch (2017)
Cetraria isidiigera (Kurok. & M.J.Lai) Divakar, A.Crespo & Lumbsch (2017)
Cetraria islandica (L.) Ach. (1803)
Cetraria kamczatica Savicz (1914)
Cetraria laevigata Rass. (1943)
Cetraria minuscula (Elenkin & Savicz) McCune (2018)
Cetraria muricata (Ach.) Eckfeldt (1895)
Cetraria nigricans Nyl (1859)
Cetraria odontella (Ach.) Ach. 1814)
Cetraria racemosa (Lynge) Øvstedal (2009)
Cetraria sepincola (Hoffm.) Ach. (1803)
Cetraria sinensis (X.Q.Gao) Divakar, A.Crespo & Lumbsch (2017)
Cetraria wangii Divakar, A.Crespo & Lumbsch (2017)
Cetraria weii Divakar, A.Crespo & Lumbsch (2017)