Heat Shock Treatments: A New Safe Approach against Lichen Growth on Outdoor Stone Surfaces

Abstract

The control of lichen growth, particularly important in the field of stone conservation of outdoor monuments, largely depends on the use of biocides, that may be dangerous for the users, the environment and the substratum. A new, alternative approach is proposed, which makes the most of a poorly known peculiarity of poikilohydrous organisms: they are thermo-tolerant (up to 65-70 °C) when dry, but thermo-sensitive when wet. The efficacy of thermal treatments (range: 20-55 °C), in parallel to the application of three biocides, was verified in the laboratory with six epi- and endolithic lichens. Chlorophyll a fluorescence emission was checked in treated and nontreated samples of all the species, whereas histochemical observations with a dead cell stain were carried out on one of them. The feasibility of the thermal treatments in the field was verified with a seventh species. The results confirm that a 6 h treatment at 55 °C is sufficient to kill the lichens if they are kept fully hydrated. At 40 °C the organisms are damaged: in this case biocides at concentrations 10× lower than in normal applications can profitably be used. The new protocol is simple, the field equipment cheap, and the negative effects associated with standard biocide treatments are absent.

The Control of Cultural Heritage Microbial Deterioration

“…Many microorganisms living on outdoor stone are thermotolerant (up to 65–70 °C) when dry but are thermosensitive when wet, and heat shock treatments on wet, metabolically active lithobionts cause a loss of membrane permeability and denaturation of proteins [ 75 ]. The method is simple, easy, and fully substratum-, operator-, and environment-friendly, and it employs plastic foils, thermal blankets, and infrared lamps [ 75 ].…”

“…The feasibility of the in situ thermal treatment was verified with several lichen species under laboratory conditions [ 75 ]. Six-hour treatment at 55 °C was able to kill the fully hydrated lichens.…”

Present and Future Perspectives for Biocides and Antifouling Products for Stone-Built Cultural Heritage: Ionic Liquids as a Challenging Alternative

“…Thermal treatments (microwave, heat irradiation, heat shock treatment) are considered an ecologically safe alternative to traditional conservation methods [16][17][18][19]. Thanks to their low-interaction with the substrate and penetration depth, they are safe for the cultural asset, for operators, and the environment.…”

Mixotrophy in the terrestrial green alga Apatococcus lobatus (Trebouxiophyceae, Chlorophyta)

“…We recommend the application of Apatococcus for any applied study focusing on either prevention strategies for terrestrial algal growth (e.g., testing the activity of photocatalytic coatings; Gladis and Schumann ) or conservation purposes (e.g., removal of biofilms from stone surfaces; Tretiach et al. ). Further, biofilm‐based algal cultivation has received increased attention as alternative to the suspension‐based systems that require expensive harvesting procedures (Gross et al.…”

“…Based on this study, we recommend the cultivation of Apatococcus on TOM, which enhances the growth rate slightly and results in a considerable increase in the overall growth capacity. We recommend the application of Apatococcus for any applied study focusing on either prevention strategies for terrestrial algal growth (e.g., testing the activity of photocatalytic coatings; Gladis and Schumann 2011) or conservation purposes (e.g., removal of biofilms from stone surfaces; Tretiach et al 2012). Further, biofilm-based algal cultivation has received increased attention as alternative to the suspensionbased systems that require expensive harvesting procedures (Gross et al 2015), and it might be worth testing the performance of mixotrophically grown Apatococcus cultures.…”