Species of lichen
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Flavoplaca citrina, the mealy firedot lichen, is a species of saxicolous (rock-dwelling), 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 (crust-like) lichen with a yellow-green thallus (the main body of the lichen). The thallus is ;dfn id="" ;pulverulent ;/dfn ; (powdery) and entirely covered with granular soredia (tiny, powdery reproductive propagules). It can vary from thin to moderately thick and is usually soft. Thicker thalli often break into irregular, coarse ;dfn id="" ;areoles ;/dfn ; (small, discrete areas), typically appearing green-yellow or green in shaded conditions. Very thick thalli, which lack soredia, have convex (rounded) 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 ;dfn id="" ;prothallus ;/dfn ; (the initial growth stage of the lichen) or, if present, it is white.
Apothecia (the fruiting bodies) are frequent, ranging from 0.3 to 1.5 mm in diameter. They start immersed in the thallus but become sessile (sitting on the surface) as they mature. These apothecia are flat and can swell slightly. The edges of the apothecia ( ;dfn id="" ;thalline margin ;/dfn ;s) are persistent, granular, and sorediate, sometimes appearing frosty ( ;dfn id="" ;pruinose ;/dfn ;). The ;dfn id="" ;disc ;/dfn ; is yellow-orange with a green-yellow margin.
Paraphyses (sterile filamentous structures in the apothecia) often have narrow tips or are club-shaped ( ;dfn id="" ;clavate ;/dfn ;) up to 3 µm wide. The asci (spore-producing cells) 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 (dividing partition) 3–5 μm wide, making up about one-third of the spore's length. Conidia (asexual spores) are mostly ;dfn id="" ;bacilliform ;/dfn ; (rod-shaped).
All parts of the lichen react with potassium hydroxide solution (K) solution to turn purple.
Habitat and distribution.
"Flavoplaca citrina" is commonly found on calcareous (chalky) 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 (corticolous). 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 (adapted to nitrogen-rich environments) 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.
References.
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