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

Abstract

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 invitro 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. 10-2mM) 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

Raman Characterization of the UV-Protective Pigment Gloeocapsin and Its Role in the Survival of Cyanobacteria

“…When identified in fungi, it was considered secondary, and its origin was traced back to the fungal components of lichens. In fact, parietin appears to have an antibiotic effect against parasitic fungi (Gazzano et al, 2013). According to our results, the origins of biosynthesis of these compounds point to cyanobacteria, and their further distribution may have occurred via endosymbiotic incorporation of chloroplasts to plants and via lichens to fungi.…”

Biodeterioration Patterns and Their Interpretation for Potential Applications to Stone Conservation: A Hypothesis from Allelopathic Inhibitory Effects of Lichens on the Caestia Pyramid (Rome)

“…Some secondary metabolites with acidic and chelating functions have been characterised as agents of mineral leaching [33,34], but no study has ever verified if their allelopathic activity in situ could explain the presence of some tracks of uncolonised surfaces, which are compatible with the shape and size of lichen thalli and continue to appear clean after years. This could support the hypothesis of the long-lasting allelopathic properties of these substances and their potential application as natural products to control colonisation and biodeterioration by other microorganisms, as cyanobacteria and black fungi, on stone monuments [35]. Such new products are welcomed to improve the environmental sustainability of products used in restoration, since traditional biocides are often unsafe and removed from the market because of their toxicity [36][37][38].…”

“…Such allelopathic compounds, produced by the putative past colonisers of the Pyramid, and in particular the macrolide aspicilin produced together with oxalates by Circinaria species, could be the source of new active principles to be tested for stone conservation. According to the recent proposals dealing with natural biocides [38], even very low doses of lichen metabolites seem to be valuable candidates as treatments to extend the efficacy of stone cleaning interventions [35]. Their natural long-lasting activity appears to be a further crucial element of economical sustainability in stone restoration.…”

High bacterial diversity in pioneer biofilms colonizing ceramic roof tiles

“…Cyanobacteria were described as primary colonizers of clay tiles (Gazzano et al, 2013) in studies that relied on culture-based diversity inventories (Barberousse et al, 2006;Coutinho et al, 2013;Crispim et al, 2003;Hauer et al, 2015;Macedo et al, 2009;Nowicka-Krawczyk et al, 2014). Among the most frequent cyanobacterial species identified on building materials were Aphanocapsa sp., Calothrix sp., Chroococcus sp.…”