Spilonema is a genus of lichen-forming fungi in the family Coccocarpiaceae. These lichens form small, dark tufts of thread-like filaments that grow on rocks and are found in various regions from tropical to arctic climates. They partner with nitrogen-fixing cyanobacteria, which gives them their distinctive blue-green colouration and allows them to survive in nutrient-poor environments. The genus was established in 1856 by the French botanist Jean-Baptiste ÃÂdouard Bornet, who distinguished these species from similar-looking lichens based on their unique internal structure.
The genus Spilonema was circumscribed by Jean-Baptiste ÃÂdouard Bornet in 1856. He established the genus to accommodate lichens with distinctive morphological characteristics that set them apart from the closely related genus Stigonema (now Ephebe). In his original description, he characterised the genus as having a cylindrical, branched thallus with a cellular internal structure. The thallus contains large transverse and bears apothecium. The paraphyses are robust, (club-shaped), and articulate, whilst the spermogonia are closed. The spore-bearing structures (asci) are elongated and divided into segments, and the pycnidia are oblong.
The type species, S. paradoxum, was described from specimens collected on granitic rocks near Cannes, particularly in somewhat humid locations. Bornet noted that this species forms short tufts of a blackish-olive colour, closely resembling Ephebe pubescens. The thallus filaments, measuring no more than two millimetres in height, are densely packed, , and often curved in an arc shape with varying degrees of branching.
Bornet distinguished Spilonema from Stigonema primarily based on differences in cellular organisation within the thallus. Whilst Stigonema species have a homogeneous tube structure containing large cells, Spilonema species lack the interpolated cellular tissue between the photobiont cells that characterises Stigonema. The cortical portion of Stigonema is smooth and homogeneous, whereas Spilonema filaments present an (divided into small polygonal areas) surface structure.
Spilonema produces minute, dark tufts that look either filamentous or finely shrubby. Each black filament is constructed around a central strand of cyanobacterial cellsâÂÂboth Stigonema and Scytonema are âÂÂwhich are wrapped in an irregular, brickâÂÂlike sheath of fungal hyphae. These filaments anchor to the substrate by blueâÂÂgreen, rhizineâÂÂlike hyphae that turn purpleâÂÂred when treated with the standard nitrile (N) spot test.
The lichen's sexual fruiting bodies (apothecia) develop laterally on the filaments and sit directly on the surface (). They are brown to black, convex to almost spherical, and internally pigmented green or violet; both pigments react red in the N test. No rim of thallus tissue surrounds the , but a thin of radiating fungal cells is present. The hymenium itself is blueâÂÂgreen to violet and stains blue with iodine (I+). Stout, branched paraphyses thread through this layer; they are divided by crossâÂÂwalls and terminate in pointed tips. Cylindrical asci contain eight ascospores and show a blueâÂÂstaining apical dome in the combined potassiumâÂÂiodide test (K/I+). The ascospores are colourless, narrowly ellipsoidal, singleâÂÂcelled, and thinâÂÂwalled.
Asexual reproduction occurs in small, black pycnidia that sit on the filament surface. Their walls are greenâÂÂblack and give a red reaction to the N test. Short, chainâÂÂforming conidiogenous cells produce rodâÂÂshaped (), colourless conidia. Thin-layer chromatography has not detected any characteristic secondary metabolites in the genus.