Lithobiontic recolonization following cleaning and preservative treatments on the rock engravings of Valle Camonica, Italy: A 54-months monitoring

: Both the indirect control of microclimate conditions and the direct application of preservative products to contrast stone bioreceptivity may contribute to limit lithobiontic recolonization of cultural heritage surfaces after cleaning interventions. However, the priority deserved by these different preventive approaches has still been poorly evaluated, particularly in outdoor environments. This work dealt with the engraved sandstone surfaces of the National Park of Rock Engravings of Naquane (Italy, UNESCO WHS), widely colonized by lichens, mosses and a dark cyanobacterial biofilm, and thus requiring frequent cleaning interventions to preserve their legibility for visitors and scholars. In particular, post-cleaning recolonization by the different lithobionts was seasonally monitored along 54 months in different zones of an engraved outcrop, primarily differing in levels of shading, on parcels exposed to nine different conservative treatments. These included (or not) a pre-cleaning devitalization of lithobionts and the post-cleaning application of biocidal (benzalkonium chloride, plant essential oils, usnic acid) and other restoration products (nanocrystalline anatase, polysiloxane-based water repellent, ethyl-silicate-based consolidant). The combination of surface image analyses, fluorimetric and colorimetric measurements showed that mosses and the cyanobacterial biofilm rapidly recolonized all the parcels in the more shaded zone, irrespective of conservative treatments. In the other areas, recolonization significantly differed depending on the treatment. The post-cleaning application of biocides determined the best results through two vegetative seasons, but only nanocrystalline anatase and the polysiloxane-based water repellent maintained the surfaces lighter than uncleaned controls along the whole monitoring period. Recolonization primarily proceeded by the uncleaned surfaces surrounding the parcels and, at least in the examined case of lichens, did not show substantial shifts in community composition, although some nitrophytic species increased their frequency. In conclusion, the effectiveness of preservative treatments to prevent a rapid recolonization of heritage stone surfaces appeared subordinate to the presence of microenvironmental conditions less favourable to lithobionts

Litobionti e arte rupestre: strategie per la conservazione

Prova a guardare da vicino la superficie di una roccia esposta in ambiente esterno, che sia in un bosco o al bordo di una strada, in alta montagna, in prossimità del mare o anche in città: vedrai che è colonizzata da organismi viventi! Sono tanti e diversi fra loro per dimensioni e strategie di vita, nel loro complesso sono chiamati litobionti. I più grandi, ad esempio i muschi e i licheni, hanno per lo più dimensioni centimetriche e si vedono bene ad occhio nudo o con l’aiuto di una lente d’ingrandimento, mentre i più piccoli possono essere distinti solo con l'aiuto di un microscopio: sono cianobatteri, alghe verdi unicellulari e funghi neri a sviluppo microcoloniale. Questi microrganismi spesso si organizzano in biofilm ossia patine di vari colori spesso visibili sulle rocce; avvolti da una matrice gelatinosa che li protegge dagli agenti esterni, convivono tanti organismi diversi sia autotrofi che eterotrofi. Continua a leggere..

Litobionti e arte rupestre: strategie per la conservazione

Prova a guardare da vicino la superficie di una roccia esposta in ambiente esterno, che sia in un bosco o al bordo di una strada, in alta montagna, in prossimità del mare o anche in città: vedrai che è colonizzata da organismi viventi! Sono tanti e diversi fra loro per dimensioni e strategie di vita, nel loro complesso sono chiamati litobionti. I più grandi, ad esempio i muschi e i licheni, hanno per lo più dimensioni centimetriche e si vedono bene ad occhio nudo o con l’aiuto di una lente d’ingrandimento, mentre i più piccoli possono essere distinti solo con l'aiuto di un microscopio: sono cianobatteri, alghe verdi unicellulari e funghi neri a sviluppo microcoloniale. Questi microrganismi spesso si organizzano in biofilm ossia patine di vari colori spesso visibili sulle rocce; avvolti da una matrice gelatinosa che li protegge dagli agenti esterni, convivono tanti organismi diversi sia autotrofi che eterotrofi. Continua a leggere..

The influence of structural organization of epilithic and endolithic lichens on limestone weathering

Hyphal penetration, mineral dissolution and neoformation at the lichen–rock interface have been widely characterized by microscopic and spectroscopic studies, and considered as proxies of lichen deterioration of stone substrates. However, these phenomena have not been clearly related to experimental data on physical properties related to stone durability, and the physical consequences of lichen removal from stone surfaces have also been overlooked. In this study, we combine microscopic and spectroscopic characterization of the structural organization of epi- and endolithic lichens (Caloplaca marina (Wedd.) Du Rietz, Caloplaca ochracea (Schaer.) Flagey, Bagliettoa baldensis (A.Massal.) Vězda, Porina linearis (Leight.) Zahlbr., Verrucaria nigrescens Pers.) at the interface with limestones of interest for Cultural Heritage (Portland Limestone, Botticino Limestone), with analysis of rock properties (water absorption, surface hardness) relevant for durability, before and after the removal or scraping of lichen thalli. Observations using reflected-light and electron microscopy, and Raman analyses, showed lichen–limestone stratified interfaces, differing in the presence/absence and depth of lichen anatomical layers (lithocortex, photobiont layer, pervasive and sparse hyphal penetration component) depending on species and lithology. Specific structural organizations of lichen–rock interface were found to be associated with differential patterns of water absorption increase, evaluated by Karsten tube, in comparison with surfaces with microbial biofilms only, even more pronounced after the removal or scraping of the upper structural layers. Equotip measurements on surfaces bearing intact thalli showed lower hardness in comparison with control surfaces. By contrast, after the removal or scraping procedures, Equotip values were similar to or higher than those of controls, suggesting that the increasing open porosity may be related to a biogenic hardening process. Such counterposed patterns of porosity increase and hardening need to be considered when models relating lichen occurrence on limestones and biogeomorphological surface evolution are proposed, and to evaluate the consequences of lichen removal from stone-built cultural heritage