# Athallia saxifragarum

Athallia saxifragarum is a species of terricolous (ground-dwelling) crustose lichen in the family Teloschistaceae. First described by Josef Poelt in 1955 as a member of Caloplaca, it was reclassified into the genus Athallia in 2013 following molecular phylogenetics studies. This lichen is characterized by its thin white thallus and small yellowish-orange apothecia. It primarily grows on bryophytes and plant detritus, showing a particular affinity for Saxifraga species, and is typically found in calciferous or alkaline environments in Arctic and alpine regions, including Greenland, Svalbard, and the European Alps, with recent documentation in Fennoscandia. It also occurs in North America.

## Taxonomy

It was formally described as a new species in 1955 by Josef Poelt, who classified it as a member of the genus Caloplaca. Ulf Arup, Patrik Frödén, and Ulrik Søchting transferred it to Athallia following a molecular phylogenetics-informed reorganization of the Teloschistaceae.

## Description

Athallia saxifragarum is a lichen species characterized by its thin, irregularly shaped thallus (the main body of the lichen) that grows either within the substrate (endosubstratic) or on its surface (episubstratic), appearing white. Its apothecia (fruiting bodies) are moderately abundant and somewhat clustered. These small, disc-shaped structures measure 0.2–0.7 mm in diameter and sit directly on the surface (sessile), slightly constricted at the base and pressed closely against the substrate. The apothecia are lecanorine in form, meaning they have a rim containing algal cells. The central disc is flat and yellowish-orange in colour, occasionally developing a brownish tinge as it matures, with a surface that ranges from smooth to finely granular. The margin around the disc is not prominent or wavy (not flexuose) and appears darker than the rest of the thallus, usually yellowish-orange like the disc or slightly paler, with a smooth texture.
The internal structure includes an excipulum (the outer layer of the apothecium) measuring 20–50 μm wide, with a poorly defined cortex containing few algal cells. The proper exciple is broad, composed of parallel hyphae (prosoplectenchymatous), strongly glutinized (gel-like), and colorless (hyaline). The hypothecium, which is the layer below the spore-producing layer, is 100 μm high, made up of unoriented to vertically aligned hyphae with mostly wide openings, and is colourless. The subhymenium (the layer just below the hymenium) is also colourless. The hymenium, where spores are produced, is 70–80 μm high and colorless. The paraphyses (sterile filaments among the spore-producing cells) have simple terminal cells that are swollen up to 6 μm wide and colorless, with subterminal cells up to 3 μm wide. The epihymenium (the uppermost layer of the hymenium) is distinct and yellow to yellowish-orange.
The asci (spore-containing sacs) measure approximately 50 by 17 μm. Each ascus typically contains 8 broadly ellipsoid ascospores that measure 10–12 by 6-7 μm, with a septum (dividing wall) of about 5.5 μm.

## Habitat and distribution

Athallia saxifragarum primarily grows on bryophytes (mosses and liverworts) and plant detritus, showing a particular affinity for various Saxifraga species. In Arctic regions, it is frequently found on Saxifraga oppositifolia (purple saxifrage) and Dryas integrifolia (mountain avens). The species appears to be restricted to calciferous or alkaline environments in Arctic habitats, growing in locations that are either fully exposed to sunlight or partially shaded, typically in dry conditions.
This lichen can be distinguished from the similar Athallia pyracea by its substrate preference and spore characteristics. While A. saxifragarum grows primarily on plant detritus, especially from Saxifraga species, A. pyracea is typically found on woody substrates. Additionally, A. saxifragarum has a slightly larger spore septum than A. pyracea.
Geographically, Athallia saxifragarum has been documented in several northern and alpine regions, including Greenland, Svalbard (a Norwegian archipelago in the Arctic Ocean), and throughout the European Alps. It was reported as new to Finland and Norway in 2014, and to Sweden in 2023. It is also found in North America.