Buellia badioatra var. applanata(Th.Fr.) Boistel (1903)
Rhizocarpon badioatrum is a widely distributed species of saxicolous (rock-dwelling), crustose lichen in the family Rhizocarpaceae.[2] This lichen forms a brown crust broken into small tile-like plates surrounded by a distinctive jet-black border, with scattered black disc-shaped fruiting bodies up to 1mm across. It has a very wide global distribution, being found across multiple continents including Europe, North America, Australia, Asia, Africa, and even Antarctica.
Rhizocarpon badioatrum spreads as a crust up to about 10cm across. Its surface is broken into tile-like plates ( areoles ), each no more than 2mm wide. These plates are matt , medium- to dark-brown, often tinged pinkish or red-brown, and sit within a well-developed jet-black border of fungal tissue, the prothallus . The areoles are thick and mostly flat but can have scalloped edges; where they meet one another the thallus appears as a mosaic. Internally, the white fungal layer (the medulla) shows no iodine staining reaction (I–), and a drop of potassium hydroxide solution (the K test) may leave it unchanged or briefly yellow.[4]
Scattered between the areoles are the fruit-bodies (apothecia), black discs up to 1mm in diameter that remain essentially flat. A narrow, dark wall called the true exciple surrounds each disc; in section the inner part looks red-brown and flashes purple-red with the K test. The very top layer of the disc (the epithecium ) is likewise red-brown and gives the same K-induced purple tint, sometimes releasing tiny red granules that dissolve in the reagent. Beneath this lies a clear spore-producing zone (the hymenium, about 110–120μm tall) and a medium-brown foundation layer (the hypothecium ), which shows no colour change with potassium hydroxide. Each club-shaped ascus produces eight ascospores; the mature spores measure 27–38μm by 13–19μm, are divided by a single cross-wall, and darken through blue-green to deep brown as they age. Para-phenylenediamine (the Pd test) is usually negative, though an orange flash or traces of stictic acid are occasionally detected.[4]
Distribution
Rhizocarpon badioatrum occurs in Europe, Macaronesia, southern Africa, South-East Asia, Papua New Guinea, North America, Argentina and Antarctica.[5] It is also found in Greenland.[6] In Australia, it has been recorded from south-eastern Queensland, New South Wales, Victoria, Tasmania, and the Australian Capital Territory.[5]
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