Flavoplaca citrina
Flavoplaca citrina, the mealy firedot lichen, is a species of saxicolous, crustose lichen in the family Teloschistaceae. It is a common species with a cosmopolitan distribution.
Taxonomy
Flavoplaca citrina first scientifically described and named in 1796 by the German lichenologist Georg Franz Hoffmann, who initially placed it in the genus Verrucaria. Ulf Arup and colleagues transferred the taxon to the genus Flavoplaca in 2013, following a molecular phylogenetics-based restructuring of the family Teloschistaceae. Throughout its taxonomic history, the species has accumulated numerous synonyms due to multiple redescriptions and reclassifications, including various designations as varieties or forms of other species.Description
Flavoplaca citrina is a crustose lichen with a yellow-green thallus. The thallus is and entirely covered with granular soredia. It can vary from thin to moderately thick and is usually soft. Thicker thalli often break into irregular, coarse, typically appearing green-yellow or green in shaded conditions. Very thick thalli, which lack soredia, have convex areoles.The soredia, measuring between 0.02 and 0.07 mm in diameter, form at the margins of these areoles and are the same colour as the thallus. There is no or, if present, it is white.
Apothecia are frequent, ranging from 0.3 to 1.5 mm in diameter. They start immersed in the thallus but become sessile as they mature. These apothecia are flat and can swell slightly. The edges of the apothecia are persistent, granular, and sorediate, sometimes appearing frosty. The is yellow-orange with a green-yellow margin.
Paraphyses often have narrow tips or are club-shaped up to 3 μm wide. The asci are typically long and narrow, containing broadly ellipsoid ascospores that measure 10.5–14.8 μm in length and 5–7.5 μm in width, with a central septum 3–5 μm wide, making up about one-third of the spore's length. Conidia are mostly .
All parts of the lichen react with potassium hydroxide solution solution to turn purple.
Habitat and distribution
Flavoplaca citrina is commonly found on calcareous or nutrient-rich substrates, such as limestone, concrete, mortar, bone, and asbestos cement. It especially thrives on walls in sunny locations, but is rarely found on wood or bark. It is less common on naturally calcareous and occasionally nutrient-enriched siliceous rocks and metalwork. This lichen is cosmopolitan in distribution.Flavoplaca citrina is nitrophilic and is commonly found on cliffs below birds' nests, where it benefits from nitrogen enrichment from bird excrement. Research has identified F. citrina as an indicator species for such bird-influenced habitats, particularly on cliff faces where peregrine falcons and ravens nest. This adaptation allows the lichen to thrive in these localised nutrient-rich zones within otherwise nutrient-poor cliff ecosystems.