Chrysothrix chlorina, the sulphur dust lichen, is a species of leprose (powdery) crustose lichen in the family Chrysotrichaceae. Originally described scientifically by the Swedish lichenologist Erik Acharius over 200 years ago, it has been shuffled to many different genera in its taxonomic history before finally being transferred to Chrysothrix in 1981. The lichen has a circumboreal distribution. It is typically saxicolous (rock-dwelling), particularly on the underside of rock overhangs, but has in rare instances been recorded growing on bark and various other substrates.
The lichen thallus is a yellow to yellow-green layer of fungi and green algae that are bundled together in powdery clumps called soredia. It lacks apothecia and pycnidia, which are reproductive structures found in many other lichens. Chrysothrix chlorina resembles several other lichen species but can be differentiated based on chemical composition and morphological characteristics. Historical confusions and misidentifications have occurred, especially with species such as Chrysothrix candelaris and Psilolechia lucida. In India, Chrysothrix chlorina is used for dyeing, and 19th-century studies in Sweden revealed it could be used to produce 14 distinct colours.
Taxonomic history
Originally described by Swedish lichenologist Erik Acharius in 1799, Chrysothrix chlorina was first named Lepraria chlorina. Acharius's Latin () characterized the lichen as crusty, powdery, slightly woolly, cushioned, clustered, soft, and very yellow. The species epithet chlorina is derived from the Latin word , meaning yellow-green.
In Lichenographia Britannica (1839), Dawson Turner and William Borrer praised the lichen, highlighting its elegant appearance. They likened it to a "beautifully" branched Conferva (a genus of algae) dusted with a fine yellow powder. They noted that when scraped from rocks, the yellow propagules remain adhered in cushion-like masses, which float on water and resist submersion. The texture of the lichen, they said, was soft and easily marred by touch.
The species underwent numerous taxonomic shifts, having been proposed for or transferred to different genera throughout its history. It was finally transferred to the genus Chrysothrix by the British lichenologist Jack Laundon in his 1981 monograph. Laundon drew parallels between C. chlorina and C. candelaris based on their similar coarse, yellow thallus appearance. However, C. chlorina is distinct in its sterile state, lacking sexual reproductive structures like apothecia, which complicates classification efforts.
In 1959, Elke Mackenzie highlighted Rolf Santesson's belief had mentioned that the taxon Farinaria sparsa, introduced by James Sowerby in 1803, likely referred to C. candelaris. By 2008, several taxa were identified as synonymous with Chrysothrix chlorina. In 2013, lichenologists Kerry Knudsen and Frank Bungartz expressed concerns regarding the consistent classification of C. candelaris, suggesting it remains a heterogenous species and is often misidentified, particularly in Europe and North America.
Vernacular names for Chrysothrix chlorina in North America include "sulphur dust lichen", "greenish gold dust", and "lime dust". In the United Kingdom, it has been called the "brimstone-coloured lepraria".
Description
Chrysothrix chlorina is characterised by its crustaceous and (powdery) thallus. It typically forms a continuous, ranging from non- to strongly areolate, that is yellow to yellow-green in colour. This crust comprises (powdery) with diameters between 0.1 and 0.2 mm, and can be up to about 1 mm thick. These granules, known as soredia (a type of reproductive propagule), consist of algal cells encased in fungal hyphae. The species does not have a thallus margin or a , and is devoid of apothecia (sexual reproductive structures) and pycnidia (asexual reproductive structures).
The symbiotic in Chrysothrix chlorina belongs to the genus Chlorella, which comprises single-celled green algae. These algal cells are up to 15 μm in diameter, while the fungal hyphae have a diameter of 2–4 μm.
Chemistry
Chrysothrix chlorina contains a suite of lichen products, including calycin, leprapinic acid, vulpinic acid, zeorin, and trace amounts of pulvinic acid and unidentified terpenoids. These chemical signatures help differentiate it from other similar sterile lichens that lack distinct features. Historically, Laundon identified these pigments using s, but contemporary analyses prefer more sensitive methods like thin-layer chromatography, are typically used instead. Standard chemical spot tests on the thallus typically yield results of C−, K± (faint orange), KC± (red), Pd−, and UV+ (orange).