Mycobilimbia olivacea is a species of corticolous and lignicolous (bark- and wood-dwelling) crustose lichen in the family Ramalinaceae. Found in Spain, it was described as a new species in 2003.
Mycobilimbia olivacea was formally described in 2003 by Francisco José Sarrión, Gregorio Aragón, and Josef Hafellner. The specific epithet olivacea refers to the olive-green colouration of its thallus. While M. olivacea shows some characteristics comparable to the genus BiatoraâÂÂsuch as its fruiting body development, and thin-walled spores with smooth âÂÂits well-developed thallus and distinctive asci place it within Mycobilimbia.
Mycobilimbia olivacea has a thallus that is crustose (crust-like), continuous, thick, and minutely , composed of small scales. The thallus is dark green to dark olive-brown in colour. The individual measure 0.2âÂÂ0.7 mm in diameter, occasionally up to 1 mm, are up to 0.1 mm high, and are typically arranged in a continuous, slightly overlapping pattern. These squamules have flat to convex surfaces with (wavy) to finely margins.
The upper is thin, up to 10 üm, consisting of one or two rows of rounded cells with cell interiors 4.5âÂÂ9 üm wide, pigmented brown to brown-orange at the surface. Within the lichen body is the containing green, algal cells, 5âÂÂ9 üm in diameter, arranged in a continuous layer 45âÂÂ60 üm thick.
The apothecia (fruiting bodies) are numerous, to in form (with -like structures), measuring 0.2âÂÂ0.8 mm in diameter, occasionally up to 1 mm. They are simple, more or less circular to compressed, commonly sessile with a somewhat constricted base. The disc is flat to convex and dark reddish-brown. The , which is the outer layer of the apothecia, is well-developed, 48âÂÂ55 üm wide, with reddish-brown pigmented cell walls at the surface. Above the spore-producing layer is the , which is ochraceous-brown and 5âÂÂ12 üm high.
The hymenium, or spore-producing layer, is 50âÂÂ70 üm high, ranging from 45âÂÂ80 üm, and is colourless. Beneath it lies the , which is also 50âÂÂ70 üm high, occasionally up to 75 üm, colourless, and contains cells in the inner part. The sterile filaments known as paraphyses are 1.5âÂÂ2.5 üm wide, sparsely branched and , with tips 3âÂÂ4 üm, occasionally up to 5 üm, that become club-shaped with an orange-brown cap.
The asci are club-shaped, containing eight spores, with a thick apical apparatus that shows a dark blue reaction with K/I (potassium iodide), containing a more densely amyloid tubular pore. The are arranged in two rows, colourless, single-celled, with smooth , measuring 11âÂÂ15 üm by 5âÂÂ6.5 üm, ranging from 9âÂÂ17 üm by 5âÂÂ7.5 üm, with a length to width ratio averaging 2.36, elongate to cylindrical or ovoid in shape. No lichen products were detected in this species by thin-layer chromatography. The cortex does not react with any standard chemical spot tests.
Mycobilimbia olivacea is found in Spain at middle to high elevations (900âÂÂ1750 m) in humid to subhumid meso- and supramediterranean bioclimatic belts. It inhabits two primary ecological niches: the fissures of tree bases in dense Pinus nigra (black pine) formations and the exposed wood (lignum) of Quercus pyrenaica (Pyrenean oak) in southern Iberia. The black pine habitats occur on calcareous mountains in eastern Spain, particularly in the well-preserved forests of Sierra de Segura and Sierra de Alcaraz, while the Pyrenean oak habitats are found on siliceous substrata in Sierra Madrona, in age-structured stands showing little human disturbance. In both environments, M. olivacea associates with various lichen species, suggesting it prefers mature forest ecosystems with stable conditions and minimal recent human influence. A similar, unnamed species (referred to as Mycobilimbia aff. olivacea) was reported from Italy; it differs from the Spanish material in its paler thallus and lack of biseriate asci.