A review of the nature, role and control of lithobionts on stone cultural heritage: weighing-up and managing biodeterioration and bioprotection.

Lithobionts (rock-dwelling organisms) have been recognized as agents of aesthetic and physico-chemical deterioration of stonework. In consequence, their removal from cultural heritage stone surfaces (CHSS) is widely considered a necessary step in conservation interventions. On the other hand, lithobiontic communities, including microbial biofilms ('biological patinas'), can help integrate CHSS with their environmental setting and enhance biodiversity. Moreover, in some cases bioprotective effects have been reported and even interpreted as potential biotechnological solutions for conservation. This paper reviews the plethora of traditional and innovative methodologies to characterize lithobionts on CHSS in terms of biodiversity, interaction with the stone substrate and impacts on durability. In order to develop the best management and conservation strategies for CHSS, such diagnosis should be acquired on a case-by-case basis, as generalized approaches are unlikely to be suitable for all lithobionts, lithologies, environmental and cultural contexts or types of stonework. Strategies to control biodeteriogenic lithobionts on CHSS should similarly be based on experimental evaluation of their efficacy, including long-term monitoring of the effects on bioreceptivity, and of their environmental safety. This review examines what is known about the efficacy of control methods based on traditional-commercial biocides, as well as those based on innovative application of substances of plant and microbial origin, and physical techniques. A framework for providing a balanced scientific assessment of the role of lithobionts on CHSS and integrating this knowledge into management and conservation decision-making is presented.

The Essential Oil of Thymbra capitata and its Application as A Biocide on Stone and Derived Surfaces.

Many chemicals used nowadays for the preservation of cultural heritage pose a risk to both human health and the environment. Thus, it is desirable to find new and eco-friendly biocides that can replace the synthetic ones. In this regard, plant essential oils represent effective alternatives to synthetic substances for the preservation of historical monuments. Thymbra capitata (syn. Thymus capitatus) is a medicinal and aromatic plant growing in the Mediterranean area and endowed with important pharmacological properties related to its essential oil. Among them, the antimicrobial ones make the T. capitata essential oil an ideal candidate for industrial applications; for instance, as biocide for the inhibition and elimination of biological patinas of cyanobacteria and green algae on historical monuments. In the present work, we studied the chemical composition of the essential oil from T. capitata growing in Malta by gas chromatography-mass spectrometry (GC/MS). The major volatile component is the phenolic monoterpene carvacrol (73.2%), which is capable of damaging the cytoplasmic membrane and to interfere both in the growth curve and in the invasive capacity, though the contribution of minor components γ-terpinene and p-cymene cannot be disregarded. For the oil application on the stone surface, Pickering emulsions systems were prepared with an essential oil/water 1:3 mass ratio stabilized with kaolinite at 4 mass% in the presence of Laponite®; this allowed to limit the fast volatility of the oil and guaranteed a better application and an easier removal from the artefacts attacked by biodeteriogens both indoor and outdoor. This formulation caused the elimination of biodeteriogens from treated surfaces without residuals or films on artworks surface, and the effect was retained up to four months.

Culturable fungi associated with urban stone surfaces in Mexico City

Background: Urban surface stones in Mexico City are exposed to a temperate climate and a range of atmospheric conditions ranging from mildly impacted to heavily polluted areas. In this study, we focused on the characterization of the cultivable fungal component of selected biological patinas in the surrounding area of Chapultepec castle, a historic monument in Mexico City. Thirty four representative fungal isolates selected based on distinctive differential macroscopic characteristics out of a total of 300 fungi, were characterized using morphological and molecular approaches. Results: This identification strategy based on the combination of phenotypic- and molecular-based methodologies allowed us to discriminate the fungal community in some cases down to the species level. Conclusions: The characterization of this mycoflora revealed the presence of a complex fungal community mainly represented by filamentous fungi belonging to the genera Fusarium, Trichoderma, Aspergillus, Cladosporium, Alternaria, Mucor, Penicillium, Pestalotiopsis, and the dimorphic fungus Aureobasidium, along with the yeast Rhodotorula. A specific distribution of fungi could be observed based on the type of biological patina analyzed.