Evaluating the durability and performance of polyoxometalate-ionic liquid coatings on calcareous stones: Preventing biocolonisation in outdoor environments

Rock-based materials exposed to outdoor environments are naturally colonised by an array of microorganisms, which can cause dissolution and fracturing of the natural stone. Biocolonisation of monuments and architectures of important cultural heritage therefore represents an expensive and recurring problem for local authorities and private owners alike. In this area, preventive strategies to mitigate biocolonisation are generally preferred to curative approaches, such as mechanical cleaning by brush or high-pressure cleaning, to remove pre-existing patina. The aim of this work was to study the interaction between biocidal polyoxometalate-ionic liquid (POM-IL) coatings and calcareous stones and evaluate the capacity of these coatings to prevent biocolonisation through a series of accelerated ageing studies in climate chambers, carried out in parallel with a two-year period of outdoor exposure in north-eastern France. Our experiments show that POM-IL coatings did not affect water vapour transfer nor significantly alter the total porosity of the calcareous stones. Simulated weathering studies replicating harsh (hot and wet) climatic weather conditions demonstrated that the colour variation of POM-IL-coated stones did not vary significantly with respect to the natural uncoated stones. Accelerated biocolonisation studies performed on the weathered POM-IL-coated stones proved that the coatings were still capable of preventing colonisation by an algal biofilm. However, a combination of colour measurements, chlorophyll fluorescence data, and scanning electron microscopy imaging of stones aged outdoors in northern France for two years showed that coated and uncoated stone samples showed signs of colonisation by fungal mycelium and phototrophs. Altogether, our results demonstrate that POM-ILs are suitable as preventative biocidal coatings for calcareous stones, but the correct concentrations must be chosen to achieve a balance between porosity of the stone, the resulting colour variation and the desired duration of the biocidal effect over longer periods of time, particularly in outdoor environments.

Effectiveness of aerobiological dispersal and microenvironmental requirements together influence spatial colonization patterns of lichen species on the stone cultural heritage

Dispersal patterns of lichen species in monumental and archaeological sites and their relationships with spatial population structure are almost unknown, hampering predictions on colonization dynamics that are fundamental for planning conservation strategies. In this work, we tested if the local abundance and distribution pattern of some common lichen species on carbonate stones of heritage sites may be related to their patterns of propagule dispersal. We combined analyses of the spatial population structure of eight species on the calcareous balustrade of a heritage site in Torino (NW Italy) with aerobiological analyses. In situ and laboratory analyses were mainly focused on the ejection of ascospores and their air take-off and potential dispersal at short and long distance. Results indicate that the spatial distribution of lichens on the stone surfaces is influenced by both species-specific patterns of propagule dispersal and microenvironmental requirements. In particular, apotheciate species that have a higher ejection of ascospores with higher potential for long range dispersal are candidate for a much aggressive spreading on the monumental surfaces. Moreover, their occurrence on natural or artificial stone surfaces in the surroundings of the stone monumental surface may easily support recolonization dynamics after cleaning interventions, as an effective supply of propagules is expected. On the other hand, species with a lower dispersal rate have a more clustered distribution and are less effective in rapid recolonization, thus representing a minor threat for cultural heritage conservation. These results support the idea that information on the reproductive strategy and dispersal patterns of lichens should be coupled with traditional analyses on stone bioreceptivity and microclimatic conditions to plan effective restoration interventions of stone surfaces.