''Astrothelium mediocrassum'' is a species of corticolous (bark-dwelling),

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

in the family

Trypetheliaceae The Trypetheliaceae are a family of mainly lichen-forming fungi in the order Trypetheliales. Genera *'' Alloarthopyrenia'' – 1 sp. *'' Aptrootia'' – 3 spp. *'' Architrypethelium'' – 8 spp. *''Astrothelium'' – ca. 275 spp. *'' Bathel ...

. Found in Guyana, it was formally described as a new species in 2016 by Dutch lichenologist

André Aptroot André Aptroot ( Heemskerk, 1961) is a Dutch mycologist and lichenologist. In 1993 he did his PhD at the University of Utrecht under the supervision of Robbert Gradstein (nl). His dissertation was titled "Systematic studies on pyrenocarpous l ...

. The

type specimen In biology, a type is a particular specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally attached. In other words, a type is an example that serves to anchor or centralizes the ...

was collected by

Harrie Sipman Henricus (Harrie) Johannes Maria Sipman (born 1945) is a Dutch lichenologist. He specialises in tropical and subtropical lichens, and has authored or co-authored more than 250 scientific publications. He was the curator of the lichen herbarium ...

south of the Kuyuwini Landing (about south of

Aishalton Aishalton is an Amerindian village that is situated in the Rupununi savannah of southern Guyana, in the Upper Takutu-Upper Essequibo Region (Region 9) of the country. It is the administrative centre for the southern sub-district of Region 9. I ...

, Upper Takutu District) at an altitude of . The lichen has a somewhat shiny, smooth, pale green

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 ...

that covers an area of up to in diameter. Its

ascospore 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 o ...

s are

hyaline A hyaline substance is one with a glassy appearance. The word is derived from el, ὑάλινος, translit=hyálinos, lit=transparent, and el, ὕαλος, translit=hýalos, lit=crystal, glass, label=none. Histopathology Hyaline cartilage is ...

, spindle-shaped (''fusiform''), and muriform (i.e., divided into internal chambers by transverse and longitudinal

septa The Southeastern Pennsylvania Transportation Authority (SEPTA) is a regional public transportation authority that operates transit bus, bus, rapid transit, commuter rail, light rail, and electric trolleybus services for nearly 4 million people ...

), with dimensions of 70–80 by 22–25

μm The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...

. This is the smallest ascospore size of '' Astrothelia'' species. The middle, or median, septum of the spore is thickened in comparison with the others. No

lichen product Lichen products, also known as lichen substances, are organic compounds produced by a lichen. Specifically, they are secondary metabolites. Lichen products are represented in several different chemical classes, including terpenoids, orcinol de ...

s were detected using

thin-layer chromatography Thin-layer chromatography (TLC) is a chromatography technique used to separate non-volatile mixtures. Thin-layer chromatography is performed on a sheet of an inert substrate such as glass, plastic, or aluminium foil, which is coated with a t ...

. It is somewhat similar in appearance to ''

Astrothelium octosporum ''Astrothelium'' is a genus of corticolous (bark-dwelling) lichens in the family Trypetheliaceae. The genus was circumscribed in 1824 by German botanist Franz Gerhard Eschweiler, with ''Astrothelium conicum'' assigned as the type species. Spe ...

'', but that species has

lichexanthone Lichexanthone is an organic compound in the structural class of chemicals known as xanthones. Lichexanthone was first isolated and identified by Japanese chemists from a species of leafy lichen in the 1940s. The compound is known to occur in ma ...

in its thallus and pseudostromata.

References

mediocrassum Lichen species Lichens described in 2016 Lichens of Guyana Taxa named by André Aptroot {{Dothideomycetes-stub