Psoroma absconditum

The lichen forms a small thallus up to about 2 cm across, made of pale-brown, shallowly incised squamules (scales) that are attached to the substrate but may lift slightly at the tips (adnate to subascending). Apothecia (disc-like sexual fruiting bodies) are infrequent; they start cup-shaped and bright brown, then expand to about 2–4 mm diameter with a thin, wrinkled thalline margin and a deep-brown, slightly concave disc. Microscopy shows a hymenium 120–160 micrometres (μm) tall and a cortex of tightly packed cells; the eight ascospores per ascus are 13–16 × 9–11 μm and have a subtly roughened surface (minutely rugulose). No lichen products were detected by chemical tests.^[2]

Small, compressed cephalodia (specialised structures housing the cyanobacterium Nostoc) occur among and on the squamules; these contribute to nitrogen fixation and are bluish-brown with finger-like projections. In the field the species can be picked out by its low, tightly packed squamules, the relatively large brown apothecia when present, and the lack of detectable secondary metabolites.^[2]

Psoroma absconditum grows among moss cushions, especially in tufts of Ditrichum cf. immersum. Mature squamules and apothecia are usually at ground level, with a few small squamules climbing the lower moss stems. It is known from sub-Antarctic Heard Island and the Kerguelen Islands, occurring from low elevations to at least 340–600 m in cold, windy habitats typical of these oceanic islands.^[2]