Essential oils to contrast biodeterioration of the external marble of Florence Cathedral

Mesoporous silica and vegetal extracts combined as sustainable stone heritage protection against biodeterioration

Since biodeterioration is considered one of the main issues related to the conservation of cultural heritage stone materials, an investigation was conducted into preventive sustainable antimicrobial alternatives to protect the stone surfaces. The study focuses on using MCM-41 mesoporous silica particles and vegetal extracts: the mesoporous materials act as nanocontainers encapsulating the extracts, which instead serve as green antimicrobic compounds to inhibit microbiological proliferation. In this way, the antimicrobial features of the extracts are sustained for a more extended period, reducing the evaporation rate and diminishing the quantity required; the amount necessary to achieve the minimum inhibitory concentration was reduced due to the decrease in evaporation. Moreover, since the MCM-41 can host a higher quantity of product than is necessary to exert the antimicrobial effect, the duration of activity is further prolonged, releasing the extracts over time. Specifically, the mesoporous particles were impregnated with the vegetal extract of limonene and the essential oils of thyme and oregano. In vitro microbiological tests were conducted on two fungi (i.e., Aspergillus tubingensis and Penicillium chrysogenum), taken as model microorganisms from real-case scenarios. A combination of mesoporous silica and vegetal extracts was employed to develop a protective coating for stone surfaces, and tests were conducted on marble mock-ups. The promising synergic results show that this system could be of interest for preventing microbiological growth over stone surfaces, avoiding a visible aesthetic impact, being non-toxic for the environment or the operator, and preventing the extract from evaporating but holding it for a controlled release. KEY POINTS: • Green antimicrobial system using porous silica as nanocontainer for plant extracts • Encapsulated vegetal extracts to inhibit microbial growth on stone surfaces • Stable and efficient coating against fungal species in vitro and on marble mock-up.

The role of essential oils as eco-friendly strategy to control biofilm collected in the Colosseum (Rome, Italy)

The control of biodeteriogenic microorganisms is essential for the management of heritage sites. Many conventional biocides are no longer available because they have lost their efficacy or have been withdrawn from the market due to their danger to humans and the environment. Therefore, new effective and sustainable biocides are needed, such as plant extracts that could be a good alternative. In this study, essential oils (EOs) of Ocimum basilicum L., Cinnamomum verum Presl, Lavandula angustifolia Mill., Origanum vulgare L., Thymus vulgaris L. and Melaleuca alternifolia Maiden & Betche were tested as green biocides against microorganisms collected from biofilms in the hypogeum of the Colosseum (Rome, Italy). Biocidal screening was first carried out on phototrophic microorganisms grown on BG11 agar culture medium. The efficacy was assessed by measuring photosynthetic activity with a mini-PAM portable fluorometer, and by determining morphological changes or the absence of autofluorescence using light microscopy and confocal laser scanning microscopy. The most effective EOs against phototrophs were further tested to inhibit the growth of heterotrophic fungi and bacteria in order to identify those with a broad-spectrum action. The EOs of cinnamon, oregano and thyme at 5% concentration (v/v) were the most effective against the microorganisms isolated from the biofilms in the Colosseum. These EOs represent a green alternative to traditional chemical biocides due to their activity against a wide range of microorganisms and their complex composition which suggests the potential to reduce the risk of microbial resistance. KEY POINTS: Biofilms collected from the Colosseum hypogeum were characterized EOs tested as biocides against phototrophs and heterotrophs in Colosseum biofilms. Cinnamon, oregano, and thyme EOs show broad-spectrum action at 5% concentration.

Methodological challenges for the investigation of the dual role of biofilms on outdoor heritage

Biofilm deterioration and biofilm protection should be considered as different aspects of the complex interactions between microbes and the surfaces of outdoor heritage (e.g. stones, bricks, mortar and plaster). Thus, it is urgent to verify and quantify to what extent the biofilm can protect from different weathering processes, to eventually determine the advisability of biofilm removal from the heritage surfaces. On one hand, it is necessary to more precisely describe the decaying processes caused by the microorganisms and to quantify the extent, severity, and rate at which the microorganisms are causing the decay. On the other hand, it is necessary to define methodologies to comprehensively study the bioprotection phenomena. So far, no decision-making tool is available to guide heritage professionals in deciding whether to remove or keep biofilms on heritage surfaces, and aesthetical alteration and discoloration is often the only criterion considered. In this work the different available approaches for the study of the dual role of biofilms on outdoor heritage have been critically reviewed. The open challenges and questions are also summarised.

Recolonization dynamics of marble monuments after cleaning treatments: A nine-year follow-up study

The prevention and control of biological patinas on outdoor stone monuments represent a demanding challenge for the conservation of cultural heritage also due to some microorganisms, particularly resistant to treatments, such as black meristematic fungi, an eco-physiological group well known for its tolerance to extreme conditions. Even if several methods and eco-friendly products have been proposed as new alternatives, traditional biocides are still far from being completely replaced. Recolonization is a natural process that occurs sooner or later after cleaning. The time that elapses until its occurrence can vary considerably depending on environmental conditions and the used products; unfortunately, the papers describing the effect of treatments over time are rare. This work aims to shed light on the recolonization process of marble surfaces in the ancient monumental cemetery of Bonaria (Cagliari) after nine years from treatments, evaluating the long-term efficiency of two different cleaning methods, namely dimethyl sulfoxide-based gel (DMSO-based gel) and Biotin T (a didecyldimethylammonium chloride-based product-). In this context, the microflora present before treatments and in the following years was assessed by culture-based methods and identified by molecular techniques, with attention on black meristematic fungi, which were used as reference for the most resistant lithobiontic organisms. Different environmental parameters, such as temperature, exposition, dominant winds, and rainfall, were considered, and infrared thermography, portable light microscopy, and image analysis were used. This research evidenced the influence of water availability and lightning in recolonization processes, the transition from the pioneer fungal community versus more resistant black fungal species after Biotin T treatment, and the long-lasting efficiency of the DMSO-based gel. These findings prove that this low-impact method deserves more attention in the conservation of outdoor marble monuments, emphasizing the importance of long-term studies.

Characterization of the Community of Black Meristematic Fungi Inhabiting the External White Marble of the Florence Cathedral

Meristematic black fungi are a highly damaging group of microorganisms responsible for the deterioration of outdoor exposed monuments. Their resilience to various stresses poses significant challenges for removal efforts. This study focuses on the community of meristematic fungi inhabiting the external white marble of the Cathedral of Santa Maria del Fiore, where they contribute to its darkening. Twenty-four strains were isolated from two differently exposed sites of the Cathedral, and their characterization was conducted. Phylogenetic analysis using ITS and LSU rDNA regions revealed a wide diversity of rock-inhabiting fungal strains within the sampled areas. Eight strains, belonging to different genera, were also tested for thermal preferences, salt tolerance, and acid production to investigate their tolerance to environmental stressors and their interaction with stone. All tested strains were able to grow in the range of 5-30 °C, in the presence 5% NaCl, and seven out of eight strains were positive for the production of acid. Their sensitivities to essential oils of thyme and oregano and to the commercial biocide Biotin T were also tested. The essential oils were found to be the most effective against black fungi growth, indicating the possibility of developing a treatment with a low environmental impact.