Placynthium nigrum

Placynthium nigrum forms distinctive black, radiating rosettes of crinkled lobes that spread across damp rock and stonework surfaces. As a cyanolichen, its dark coloration comes from the cyanobacterial photobiont within its tissues, giving it a striking appearance against pale calcareous backgrounds.

Yes, P. nigrum is a cyanolichen, meaning it partners with cyanobacteria rather than green algae for photosynthesis. This cyanobacterial association contributes to its black coloration and enables nitrogen fixation, supporting its growth on nutrient-poor stonework.

P. nigrum is commonly found on damp, calcareous rocks and stonework, including limestone surfaces and lime-mortared walls. It favors base-rich substrates that retain moisture, distinguishing it from lichens that prefer acidic or siliceous rock.

Look for black, radiating rosettes with crinkled, wrinkled lobes spreading across damp limestone or calcareous stone surfaces. The combination of its dark coloration, rosette growth form, and preference for lime-rich, moist substrates makes it one of the more recognizable cyanolichens on stonework.