Thallus effuse, thin, pale creamy-brown, usually covering relatively large areas (to 15-20 cm diam.). Photobiont chlorococcoid, cells (9)12-15(-18)m diam.
Apothecia scattered, dark brown, arthonioid, sessile, (0.2)0.3-0.4(0.6))mm diam., round, becoming irregular when old, flat to slightly convex, without visible exciple. Hymenium hyaline, I+ blue, 125-150m tall; epihymenium brown (K+ olivaceous), granular, 12-20m wide; paraphysoids very fine, 1.0-1.5m, branched and anastomosing, strongly conglutinate in K. Asci clavate, 70-80 x 35-40m; ascospores hyaline, 8/ascus, 27-35 x 12-13.5m, muriform, 5-7 longitudinal septa, 1-2 transverse septa, outer wall I+ blue. Hypothecium pale brown, 70-80 m high, clearly separated from the thin dark-brown excipulum that extends below the hypothecium. Excipulum cupular, poorly developed, thin, 15-20m wide hyaline with a dark brown (K+ olivaceous) outer zone c.7.5m wide.
Conidiomata pycnidia, usually present, brown, immersed, 0.1mm wide, walls brown (K+ olivaceous) above, hyaline below, Roccella-type. Conidia simple, bacilliform c. 7-8.5 x 0.8m, arising from, simple, barely differentiated conidiogenous cells, Type I of Vobis (1980).
Chemistry: Thallus and apothecia C-, K-, KC-, Pd-,

Southern South America (including Falkland Islands). Arthothelium evanescens vst. (2001) from the Prince Edward Islands is also a probable synonym.
From the details of the collections in MSC, A. diffluens appears to be a maritime species. Associated species are few and not easily identified beyond genus but include Buellia sp., Caloplaca sp., Verrucaria sp.

Notes: Arthothelium and Arthonia have traditionally been separated on ascospore septation ascospores of Arthonia having only transverse septa, those of Arthothelium being muriform. Grube & Giralt (1996) showed that ascospore septation is not a good character for the separation of these two genera but as generic concepts are still unclear in the group (Tehler 1993a, Grube 1998) the species is provisionally referred to Arthothelium.
Arthothelium diffluens is separated from all other saxicolous species of Arthothelium s. lat. by its sessile apothecia and large ascospores.

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Thallus and apothecia

Apothecial section