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Arthonia calcarea (Turner ex Sm.) Ertz & Diederich

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Kalkflekklav.
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Arthonia calcarea growing in a weak rock overhang on Kvitsøy (O-L-198493).

Arthonia calcarea is species of damp calcareous or base-enriched rocks including artificial substrates. It occurs in coastal Norway from Rogaland in the South to Ørland in Trøndelag. Arthonia calcarea is characterized by lirellate, black apothecia on a thin white to pale grey thallus and trentepohlioid photobiont. The colorless spores are 3-septate and lack an enlarged apical cell.

Description

Thallus

The thin but extensive thallus is white, light olive-grey, and often shows a pinkish or ochraceous hue. It is smooth or finely cracked, and immersed or superficial. The margin is not determinate. The photobiont is a species of the family Trentepohliaceae.

Fruitbodies

The apothecia are lirellate, black, and without pruina. They are variable in shape from simple to occasionally branched or star-shaped. The apothecia are raised over the thallus or partially immersed, 0.6–2 × 0.15–0.3 mm in size, and 40–120 μm tall.

The exciple is clearly defined and brown-black in color.

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

The hymenium is unpigmented and 80–100 μm tall.

The spores are colorless, narrowly obovoid, 14–22 × 3.5–6 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 100 µm in size. The wall is brown. The rod-shaped conidia are 4–9 × 1 μm in size and straight.

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+ 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 change to green in K solution.

Kalkflekklav.
Creative Commons Attribution Share-alike | Andreas Frisch

Arthonia calcarea growing in a rock overhang at Storfosna, Lyngholmen in Ørland (O-L-157247).

Ecology

Arthonia calcarea is a species of damp and shaded calcareous or base-enriched rocks in coastal Norway. It usually grows on vertical rock faces and also occurs on artificial substrates like mortar, murals, roof tiles and even bone. 

Distribution in Norway and the Nordic countries

Arthonia calcarea is widely distributed in coastal Norway from Rogaland in the South to Ørland in Trøndelag. In the Nordic countries, it is further known from Denmark and Sweden.

Global distribution

Outside the Nordic Countries, Arthonia calcarea is widely distributed in mediterranean to southern boreal climates throughout Europe. The species is further reported from Macaronesia, North America, Africa and Asia.

Similar species

Arthonia calcarea can be mistaken for species of Opegrapha and Alyxoria growing in the same type of habitat. Opegrapha dolomitica has more robust apothecia that are often less elongated and may have a distinctly gnarled appearance. The hymenium is I+ red and the spores are larger than in Arthonia calcarea. The brown pigment in exciple, epithecium and hypothecium does not change to green in K.

Specimens of Alyxoria varia growing on rock have a distinct, greenish pruina on the margin of the apothecia that is occasionally missing, the apothecial disc is exposed, and the spores are divided by 3 to 6 transverse septa and 14–30 × 4–6.5 μm in size. Like in Opegrapha dolomitica, the hymenium is I+ red and the brown pigment in exciple, epithecium and hypothecium does not change to green in K.

Remarks

Based on the well-defined black apothecial margin, Arthonia calcarea was classified in the genus Opegrapha until recently. Other characters of the apothecia including asci and ascospores are however more similar to other Arthonia species. The placement of A. calcarea 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.