Xanthoria aureola

Xanthoria aureola was first described as Parmelia aureola in 1809; it was found on seaside rocks in Boshuslän, Sweden and named by Acharius.[2]  In 1930, Erichsen transferred P. aureola to the genus Xanthoria at the species rank, resulting in the accepted binomial X. aureola.[2]  However, from 1965 to 1984, the classification X. aureola was mistakenly applied to X. calcicola, a closely related species first described in 1937.[2]  Within the genus Xanthoria, DNA sequencing has confirmed that X. aureola is most closely related to X. calcicola and more distantly related to X. parietina.[5]  

X. aureola grows on exposed maritime rocks in sunny areas.[3][4]  It generally grows on nutrient-rich, siliceous rocks, as well as limestone and lignum.[2][3]  It is found on European coasts 0–150 meters above sea level.[2]  Some countries in which X. aureola is commonly found include Spain, Portugal, France, Ireland, Denmark, Sweden, Norway, Italy, and the UK.[1]  It usually grows next to X. parietina, but in greater abundance and on exposed rock.[2] 

X. aureola apothecia on a rock in Tórshavn, Faroe Islands.

The thallus of X. aureola is bright yellow, orange, or orange-red with a foliose morphology.[2]  It is characterized by overlapping strap-shaped lobes that exhibit dichotomous branching.[2][3]  When treated with potassium hydroxide, the thallus turns deep red (K+ red).[2]  Average lobe width is 0.46-1.6 mm and average lobe thickness is 135 μm.[3]  X. aureola has a lower cortex, although no true rhizines.[2]  There are scattered hapters on the cream-colored underside of thick lobes.[3]  The upper cortex is rough with a layer of crystals, dotted with few apothecia.[2]  Chemicals such as parietin, fallacina, emodin, teloistin, and parietinic acid are present, as well as the dominant carotenoid mutatoxanthin.[2][6]  Mutatoxanthin, a carotenoid important in the protection of the photosynthetic component against harsh sunlight, represents 94.4% of the total carotenoid content in X. aureola.[6]  Of all Xanthoria species, X. aureola contains the most mutatoxanthin.[6]  

X. aureola is often confused with closely related species X. parietina and X. calcicola.[5]  In comparison, X. aureola has a brighter thallus color as well as a considerably thicker medulla (187 mm compared to 114–120 mm).[5]  Additionally, the rough upper surface of X. aureola contains few apothecia and does not contain soredia or isidia; laminar structures are lobules.[2][5]  Last, substrate is important: X. aureola is restricted to seashore rocks, while X. calcicola and X. parietinia can be found on almost any rock or wall.[5]    

The algal symbiont in X. aureola is Trebouxia.[7]  Trebouxia fixes ¹⁴C mainly into ribitol during photosynthesis; approximately 80% is ribitol, 5% is sucrose, 4% is organic acids, and 9% is baseline CH.[7][8]  Therefore, ribitol is the main way in which carbohydrates are transferred among symbionts in the thallus.[7]  The flow of carbon from Trebouxia to the fungus is efficient, with a steady rate of 15 minutes.[4]  Little carbon (~2%) is stored as insoluble compounds in the thallus.[4][8]  The mean chlorophyll content per algal cell is 3.0-4.8 x 10⁻⁶ mg.[8]  

Environmental disturbance plays an important role in efficiency and productivity.  Lichen species are often used to monitor pollution since they are sensitive to SO₂, heavy metals, salt, and high levels of UV.[9]  Environmental stress (i.e., increased UV light) enhances the creation of reactive oxygen species (ROS), including superoxide and hydrogen peroxide.[9]  The formation of ROS was higher in all treatments with greater UV, although Xanthoria species showed the greatest resilience under harsh light conditions.[9]  Additionally, pre-treatment with salicylic acid coupled with high-energy radiation resulted in fewer amino acids, notably Glu, Tyr, and Pro.[9]  Amino acids are essential in the formation of proteins and basic biochemical functions.  Another experiment underscored the sensitivity of X. aureola to high concentrations of heavy metals and salt.[10]  Membrane integrity was measured via conductivity and potential photosystem II (PSII) efficiency (Fv/Fm), with the former being a more accurate measure.[10]  High UV, salt, and heavy metal concentrations increased membrane dissolution and electrolyte leakage.[10]  X. aureola was more resistant to salt than other lichenized species Lobaria pulmonaria and Parmelia sulcata.[10]  Copper and zinc had no effect on Fv/Fm of X. aureola.[10]  It is likely that zinc and iodine in seawater protect Trebouxia and increase resistance to high salt and UV.[10]  Increasing environmental stress may exacerbate ROS formation and electrolyte leakage.