Mycelium immersed, not staining in iodine.
Anamorph: conidiomata pycnidia, 150–200 μm diam., the wall brown below and greenish brown above, K– or K+ green. Conidiogenous cells 7–14 × 4.5–5 μm. Conidia (6–) 6.5–8.5 (–10.5) × (3.5–) 4–5 (–6) μm, ellipsoidal with a truncate base, hyaline, aseptate, verruculose.
Teleomorph: ascomata (110–)180–290(–400) μm diam. and (80–)110–170(–220) μm tall, superficial, blackish, often with a yellowish pruina, UV–, subglobose, with a strongly convex disc and a strongly constricted base. Hymenium hyaline to pale greenish brown, K– (becoming green in the upper part from epihymenial pigment), N–, I–, K/I–, 50–85 μm tall (including the epihymenium); epihymenium dark greenish brown, K+ green and N+ purplish brown, 13–20 μm thick. Hypothecium dark brown, K–, N–. Exciple absent. Interascal tissue of branched and anastomosed paraphyses 2.5–3 μm diam. Asci 55–65 × 12–15 μm, clavate, thick-walled, I–, K/I–, 8-spored. Ascospores (12–) 13–16.5 (–20) × 5–6 (–7) mm, brown, verruculose, 1-septate, the upper cell usually slightly shorter and broader than the lower cell.
Assessed (BLS website) as of Least Concern, but listed as Nationally Scarce.
Abrothallus microspermus is largely similar to A. parmotrematis, and collections of the latter species (on Parmotrema) have previously been identified as the former. A. microspermus occurs on Flavoparmelia rather than Parmotrema, and has slightly smaller and relatively narrower ascospores (9.5-14.5 x 3.5-5.5 versus 13-16.5 x 5-6 µm), epruinose ascomata and with more copious production of conidiomata.
On living thalli of Parmotrema species; in GB&I reported from P. crinitum and P. perlatum.
In GB&I, originally reported from Skye, but now recorded throughout the west of Britain, many records were originally submitted as A. microspermus. Widely distributed in western Europe and Macaronesia and also reported from S and SE USA.
Parasitic, but apparently causing little damage to the host thallus.