# Flavoplaca citrina

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 pulverulent (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 areoles (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 prothallus (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 (thalline margins) are persistent, granular, and sorediate, sometimes appearing frosty (pruinose). The disc is yellow-orange with a green-yellow margin.
Paraphyses (sterile filamentous structures in the apothecia) often have narrow tips or are club-shaped (clavate) 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 bacilliform (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.