Biocidal effect of lichen secondary metabolites against rock-dwelling microcolonial fungi, cyanobacteria and green algae

The use of commercial biocides in outdoor environments is increasingly discouraged because of their ecotoxicity, new methods being thus invoked to control patinas of biological origin on the stone cultural heritage. The effects of secondary metabolites (usnic acid, norstictic acid, parietin) produced by saxicolous lichens, natural competitors of rock dwelling microorganisms, were investigated in vitro against microcolonial fungi (MCF: Coniosporium apollinis, Coniosporium perforans, Coniosporium uncinatum, Phaeococcomyces-like sp.), coccoid cyanobacteria (Chroococcus minutus) and green algae (Scenedesmus ecornis) which commonly occur on stonework. An acetone/water 10/90 vol/vol mixture was screened as suitable to solubilise the lichen metabolites and to not affect the bioassay results. Benzalkonium chloride 1% was used as positive control. All the three metabolites (approx. 10−2 mM) inhibited the growth of the assayed MCF species, displaying the same effect of benzalkonium chloride. Chroococcus and Scenedesmus exhibited sensibility to the lichen metabolites when exposed to high incubation temperatures (35 °C), chemicals and temperature synergically yielding percentage decreases of intact cells with red chlorophyll epifluorescence. These findings suggest lichen secondary metabolites as allelopathic agents against rock dwelling microorganisms and as potential natural sources for their control on stone materials in restoration and conservation programmes. In this perspective, the detection of a negligible chromatic alteration (ΔE<0.5) caused by LSM to the white Carrara marble is reported as the first step of the necessary extensive evaluation of the LSM-stone material interactions.