Caeruleum heppii (Nägeli ex Körb.) K. Knudsen & L. Arcadia

Caeruleum heppii is widespread in Europe and has been reported from North America. It grows usually on calcareous rock in shade to full sun, often like a pioneer species on small stones and pebbles. The lichen can occur on Ca-enriched sandstone or decaying granite or even anthropogenic substrates such as brick. It has even been reported on dried paint and old paper (Magnusson 1929). In the Czech Republic the species has sparsely been collected from lowlands to submontane areas. It is probably more common but under-collected.

It has small apothecia 0.2–0.4(–0.6) mm that are dispersed on an ecorticate white thallus of hyphae and clusters of algae which is epilithic or endolithic (especially in dry habitats). Caeruleum heppii has a high hymenium 100–150 μm and paraphyses 0.5–1.0 μm wide. Its main diagnostic character is the apical dome of asci turns dark blue in Lugol’s. The genus is currently included in the Acarosporaceae but attempts to sequence it have so far failed due to contamination.

This species was traditionally classified as a member of Acarosporaceae. Recently, Díaz-Escandón et al. (2022) placed it into the class of Lichinomycetes. In this atlas, we provisionally placed Caeruleum into Thelocarpaceae, with which it is closely related according to the mentioned study.

Literature: Magnusson A.H. (1929): A monograph of the genus Acarospora. – Kungliga Svenska Vetenskaps-Akademiens Handlingar 7: 1–400. Díaz-Escandón D. et al. (2022): Genome-level analyses resolve an ancient lineage of symbiotic ascomycetes. – Current Biology 32: 1–10.

Ascomycota → Lichinomycetes → Thelocarpales → Thelocarpaceae → Caeruleum