Arthonia toensbergii

Arthonia toensbergii is a species of lichenicolous (lichen-dwelling) fungus in the family Arthoniaceae. It occurs in old-growth boreal rainforests in Norway, where it parasitises the lichen Mycoblastus affinis growing on trunks and branches of Norway spruce.

Taxonomy
The fungus was formally described as a new species in 2018 by the lichenologists Håkon Holien and Andreas Frisch. The type specimen was collected by Holien along the rover Svordka in Meldal (Sør-Trøndelag), where it was found growing on Picea abies twigs in an old spruce forest. The species epithet honours the Norwegian lichenologist Tor Tønsberg, on the occasion of his 70th birthday.

Molecular phylogenetics analysis shows that Arthonia toensbergii has a sister taxon relationship with the rare Scandinavian species Arthonia protoparmeliae, and that these two species form a clade that is itself sister to a clade containing Arthonia peltigera and Bryostigma muscigenum. All of these species are members of the so-called Bryostigma clade, which includes many lichenicolous Arthoniaceae.

Description
Arthonia toensbergii may cause pale brownish discolouration on the thallus of its host. The vegetative hyphae are thick-walled, measuring around 2–3 μm wide and 0.5–1.0 μm thick, and have specific chemical reactions (I+ vinose, KI+ pale blue). These hyphae are present in the host thallus near the ascomata. The ascomata form in small, non-aggregated colonies, with an irregularly rounded to short elliptical shape. They are adnate, moderately to strongly convex, and do not break through the host thallus. The ascomata are brownish-black to black with a finish and have a minutely roughened surface, measuring 0.07–0.15 mm in diameter and 0.07–0.1 mm in height.

The, , hymenium, and all feature a medium olivish-brown colouration. The is composed of compacted paraphysoidal hyphae, while the hymenium is  and 35–40 μm tall, with distantly spaced asci. are loosely branched, netted, and embedded in a dense gelatinous matrix, with tips that widen and have dark brownish pigment or plaques. The asci are broadly (club-shaped), contain eight spores in irregular rows, and have a broadly triangular ocular chamber. The ascospores are hyaline (translucent), contain a single septum, and slipper-shaped, measuring around 12.6 by 4.7 μm on average. They are constricted at the septa, with thin walls and a narrow, smooth. No pycnidia were observed to occur in this species.