''Arthonia'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

lichen A lichen ( , ) is a hybrid colony (biology), colony of algae or cyanobacteria living symbiotically among hypha, filaments of multiple fungus species, along with yeasts and bacteria embedded in the cortex or "skin", in a mutualism (biology), m ...

s in the family

Arthoniaceae The Arthoniaceae are a family (biology), family of lichenized, Lichenicolous fungus, lichenicolous and saprobic fungi in the order Arthoniales. The Arthoniaceae is the largest family of Arthoniales, with around 800 species. Most species in Arthon ...

. It was

circumscribed In geometry, a circumscribed circle for a set of points is a circle passing through each of them. Such a circle is said to ''circumscribe'' the points or a polygon formed from them; such a polygon is said to be ''inscribed'' in the circle. * Circum ...

by Swedish botanist

Erik Acharius Erik Acharius (10 October 1757 – 14 August 1819) was a Swedish botanist who pioneered the Taxonomy (biology), taxonomy of lichens and is known as the "father of lichenology". Acharius was famously the last pupil of Carl Linnaeus. Life Ac ...

in 1806. It is a genus of thin

crustose Crustose is a Habit (biology), habit of some types of algae and lichens in which the organism grows tightly appressed to a substrate, forming a biological layer. ''Crustose'' adheres very closely to the Substrate (biology), substrates at all poin ...

of widely varying forms, commonly called comma lichens.Field Guide to California Lichens, Stephen Sharnoff, Yale University Press, 2014,

Description

''Arthonia'' species are typically

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, forming thin, often inconspicuous patches that can be either flush with the surface () or sitting on top (superficial). The

thallus Thallus (: 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. A thallus usually names the entir ...

—the main body of the lichen—may spread without clear boundaries or sometimes be neatly outlined by thin brown lines. In some species, the thallus may be absent altogether. Under chemical tests, the fungal filaments (

hypha 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 o ...

e) in the thallus can show distinctive reactions, turning red or pale blue when stained with

iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...

(I+) and then blue with a

potassium iodine Potassium iodide is a chemical compound, medication, and dietary supplement. It is a medication used for treating hyperthyroidism, in radiation emergencies, and for protecting the thyroid gland when certain types of radiopharmaceuticals are u ...

(K/I) solution. The

photosynthetic Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...

partner () is most commonly a

green alga The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ( ...

belonging to the genus '' Trentepohlia''. In some cases, however, a less well-known type of green alga is involved, and a few species are only weakly lichenised or even live partly by breaking down dead organic matter (

saprophytic Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi ...

) or parasitism, parasitising other lichens (lichenicolous fungus, lichenicolous). The reproductive structures of ''Arthonia'' are ascomata, which are often similar to apothecia (open, disc-like fruiting bodies) but can have a wide range of shapes. They may be round, elongated, linear, or even star-shaped, and on bark they often develop a subtle, thin rim of tissue that includes both lichen and host bark cells. The exposed surface of these structures (the ) can be reddish-brown to black, and sometimes appears dusted with pale or white powder (). Unlike many lichens, ''Arthonia'' usually lacks a (a protective rim made of thallus tissue) and often also lacks a (a layer of fungal tissue), though a few species develop a well-defined, boundary. Internally, the top layer () can range from colourless to reddish or dark brown. The main spore-producing region (hymenium) typically reacts to the same chemical tests as the thallus hyphae, and these reactions help distinguish ''Arthonia'' from other lichens. Beneath the hymenium, a distinct (a supporting layer) may be absent or difficult to separate from the hymenium. Thread-like fungal elements () weave through a gel-like matrix, becoming more intricately branched and pigmented near their tips, where they often form small, dark "caps". The ascus, asci, or spore-bearing cells, usually contain eight spores and have a characteristic ''Arthonia''-type structure. They are often semi- (splitting in a controlled way when releasing spores) and possess a large, domed apex with a specialised "ocular chamber" that may show subtle colour changes with chemical tests. The themselves are typically oval or elongated, divided into one to seven segments (septum, septa), and start off colourless and smooth. As they mature and age, they may darken and become slightly roughened. Initially, the spores can have a very thin, colourless outer layer (). In addition to sexual reproduction via ascospores, ''Arthonia'' often produces tiny, inconspicuous, flask-shaped structures called pycnidia. These generate small, colourless, single-celled spores (conidia) that are usually rod-shaped but occasionally ellipsoidal or thread-like. From a chemical standpoint, many ''Arthonia'' species do not contain distinct lichen products, but others produce a variety of substances, including xanthones and certain anthraquinones. These chemical differences, along with the various structural features, help distinguish different species within the genus.

Gallery

Image:Arthonia_caesia.jpg, ''Arthonia caesia'' Image:Arthonia caesia-5.jpg, Photograph of a cross section of an apothecium of ''A. caesia'' taken through a compound microscope, x 400. Image:Arthonia caesia-6.jpg, Photograph of two spores (3-septate, 4-celled) from ''Arthonia caesia'' taken through a compound microscope, x 1000. (spores measure 21 x 5 micrometres)

Selectes species

* ''Arthonia didyma'' * ''Arthonia ilicinella'' * ''Arthonia isidiata'' * ''Arthonia stereocaulina'' * ''Arthonia vinosa''

References

{{Taxonbar, from=Q4797622 Arthonia, * Arthoniomycetes genera Lichen genera Taxa named by Erik Acharius Taxa described in 1806