Norstictic acid is a depsidone produced as a secondary metabolites in lichens.^[1] The compound contains both an aldehyde carbonyl group and an adjacent hydroxyl group in its molecular structure, which enables it to form complexes with certain metals. This property was demonstrated in studies of lichens growing on copper-rich substrates, where norstictic acid forms complexes with copper ions in the cortex (outer layer) of the lichen thallus, resulting in a distinctive green-yellow coloration. The formation of these copper-norstictic acid complexes appears to be selective, as evidenced by comparing it to the structurally similar stictic acid, which lacks the adjacent hydroxy group and does not form copper complexes. This metal-binding capability may have implications for both lichen taxonomy and the use of lichens as biological indicators of metal mineralization, though the formation of such complexes is not the only mechanism by which lichens can tolerate high metal concentrations in their environment.^[2]