About Porpidia flavicunda (Ach.) Gowan
Porpidia flavicunda grows as a bright yellow-orange crust spread across siliceous rock. Each colony is clearly bounded by a narrow, black, felt-like line of fungal threads called the prothallus, which separates it from neighboring thalli. Its overall surface is typically even, but a hand lens reveals a mosaic of tiny areoles that merge to form a continuous crust. Black fruiting discs known as apothecia are commonly scattered across the thallus, growing either singly or in small clusters of up to ten. These apothecia start out embedded within the thallus, then soon emerge to sit on top as shallow bowls, later become nearly level, and eventually become domed. If apothecia grow close together, their outlines shift from round to irregular. A distinct, raised rim made entirely of fungal tissue, called the true exciple, surrounds each disc and remains visible even on older specimens. The apothecial disc itself is matte to slightly glossy, and is often covered in a fine, whitish bloom; in very mature apothecia, the center may split into wrinkled, sterile patches.
Microscopically, the clear hymenium measures 85–150 micrometres (μm) tall, and sits beneath a thin, olive-tinged brown cap called the epithecium. Supporting layers change color from pale yellow-brown just below the hymenium to dark reddish-brown at the base, the hypothecium. Asci produce smooth, single-celled ascospores that measure 15–19 × 8.5–10 μm, which are released to form new colonies. Spot test chemical analysis shows confluentic acid is the dominant compound in European samples of Porpidia flavicunda; North American populations sometimes contain stictic acid, norstictic acid, or both instead. No other lichen products have been consistently reported for this species.
Porpidia flavicunda has a circumpolar arctic and boreal distribution. Evidence indicates that populations of this species frequently disperse their propagules to other populations, even across different continents. This results in low levels of genetic drift in isolated populations, and higher genetic uniformity between continents than expected. Porpidia flavicunda is one of the most common and widely encountered lichens in Iceland, where it grows on rocks.
