1. Species online
  2. Arthoniaceae, English version
  3. Arthonia

Arthonia atra (Pers.) A. Schneid.

Forrige Opp Neste
Svartflekklav.
Creative Commons Attribution Share-alike | Andreas Frisch

Arthonia atra collected from smooth bark of Fraxinus excelsior at Rauer vest in Fredrikstad (O-L-43264).

Arthonia atra grows on smooth bark of deciduous trees. It is known in coastal Norway from humid forests and woodlands north to Steinkjer in Trøndelag. Arthonia atra is characterized by lirellate apothecia with black margin and slit-like disc, an often mosaic forming silver-white thallus with trentepohlioid photobiont, and colorless, 3-septate spores without enlarged apical cell.

Description

Thallus

The extensive thallus is off- to silver-white with often an olivaceous hue. It is smooth and immersed in the tree bark to superficial. The margin is bordered by a thick brown prothallus line or it is not determinate. The photobiont is a species of the family Trentepohliaceae.

Fruitbodies

The apothecia are lirellate, black, and without pruina. They are highly variable in shape, randomly distributed on the thallus, with a clear parallel orientation, or in distinctly star-shaped clusters. The apothecia are raised over the thallus or occasionally immersed, 0.3–2.8 × 0.1–0.3 mm in size and 100–150 μm tall. Characteristic for the species is the clearly defined brown-black apothecial margin and slit-like disc.

The epithecium is 5–7 µm tall and brown or olive-brown.

The hymenium is unpigmented and 40–60(–90) μm tall.

The hypothecium is dark brown and up to 80 μm tall.

The paraphysoids are 1–2 μm wide. Their tips are slightly widened to 2–3 µm and covered by dark pigment caps and plaques.

The asci are clavate, without stipe, 37–55 × 12–17 µm in size, and 8-spored.

The spores are colorless, narrowly obovoid or narrowly clavate, 13–20 × 2.5–5 μm in size, and divided by 3 transverse septa. The apical cell is not enlarged.

Anamorph

The pycnidia are brownish black, immersed in the thallus or slightly raised, and ca 50 µm in size. The wall is brown. The rod-shaped conidia are 4–5 × 0.7–1 μm in size and straight or slightly bent.

Chemistry

The thallus does not react with C, K, KC, Pd, or UV (C–, K–, KC–, Pd–, UV–). Lichen secondary compounds have not been detected by TLC. 

The hymenium reacts I+ red mottled with blue and KI+ blue. A KI+ blue ring structure has been observed in the asci.

The brown pigment in the exciple, epithecium, and the wall of the pycnidia changes to green in K solution.

Svartflekklav.
Creative Commons Attribution Share-alike | Andreas Frisch

Arthonia atra collected at Fredsberg in Ålesund (O-L-83951).

Ecology

Arthonia atra is a species of humid forests and woodlands in coastal Norway including the boreal and boreo-nemoral rainforests. It grows on smooth bark of deciduous trees. Common host trees are common hazel (Corylus avellana), European ash (Fraxinus excelsior) and holly (Ilex aquifolium). Others include aspen (Populus tremula), oak (Quercus spp.), rowan (Sorbus aucuparia), and small-leaved lime (Tilia cordata). 

Distribution in Norway and the Nordic countries

Arthonia atra is widely distributed in coastal Norway from Viken in the South to Steinkjer in Trøndelag. In the Nordic countries, it is further known from Denmark, Finland and southern Sweden.

Global distribution

Outside the Nordic Countries, A. atra is widely distributed in mediterranean to southern boreal climates throughout the Northern Hemisphere. The species is further reported from South America, South Africa and Australasia.

Svartflekklav.
Creative Commons Attribution Share-alike | Andreas Frisch

Arthonia atra collected from Fraxinus excelsior near Jærnbanegården in Holmestrand (O-L-83938).

Similar species

Few Arthonia species in Norway have apothecia with a well-developed black margin and a slit-like apothecial disc. Arthonia calcarea grows on damp and shady calcareous rocks or similar anthropogenic materials like mortar, murals or roof tiles. The species further differs in the usually less elongated and less branched apothecia and longer conidia, 4–9 × 1 μm in size.

Arthonia excipienda like A. atra is a species of smooth bark of deciduous trees in humid forests and woodlands. This species lacks the well-developed, silver-white thallus of A. atra, the apothecia are much smaller, 0.3–1.5 × 0.1–0.14 μm in size und mostly unbranched, and the spores are 1-septate only.

Arthonia radiata is probably the species that is most easily mistaken for A. atra. The two species share the white and often mosaic forming thalli, highly variable, elongated, branched or star-shaped apothecia, and the narrowly obovoid or narrowly elliptical spores with 3 transverse septa. They further often grow together in the same habitats and even on the same tree trunks. Arthonia radiata lacks a well-developed black apothecial margin and slight-like apothecial disc. The apothecia are immersed in the thallus or only indistinctly raised. The two species differ furthermore in the color reaction of the hymenium in iodine, which is I+ blue in A. atra and I+ red in A. radiata.

Kalkflekklav.
Arthonia calcarea (Turner ex Sm.) Ertz & Diederich
Kantflekklav.
Arthonia excipienda (Nyl.) Leight.
Stråleflekklav.
Arthonia radiata (Pers.) Ach.

Remarks

Based on the well-defined black apothecial margin, Arthonia atra was classified in the genus Opegrapha until recently. Other characters of the apothecia including the asci and ascospores are however similar to other Arthonia species. The placement of A. atra in Arthonia is confimed by molecular phylogenetic data (Ertz et al. 2009).

Literature

Cannon P, Ertz D, Frisch A, Aptroot A, Chambers S, Coppins BJ, Sanderson N, Simkin J and Wolseley P (2020). Arthoniales: Arthoniaceae. Revisions of British and Irish Lichens 1: 1–48.

Ertz D, Miadlikowska J, Lutzoni F, Dessein S, Raspe O, Vigneron N, Hofstetter V and Diederich P (2009). Towards a new classification of the Arthoniales (Ascomycota) based on a three-gene phylogeny focusing on the genus Opegrapha. Mycological Research 113: 141–152.

Wirth V, Hauck M and Schultz M (2013). Die Flechten Deutschlands, vol. 1+2. Ulmer, Stuttgart. 1244s.