Local granite has been used throughout history in Galicia (NW Spain), forming the basis of much of the region’s architecture. Like any other rock, granite provides an ecological niche for a multitude of organisms that form biofilms that can affect the physical integrity of the stone. In this study, for the first time, characterization of the microbial consortium forming biofilms that developed on historical rural granite buildings is carried out using a combination of culture-dependent and next generation sequencing (NGS) techniques. Results pointed to differences in biofilm composition on the studied rural granite buildings and that of previously analyzed urban granite buildings, especially in terms of abundance of cyanobacteria and lichenized fungi. Exposure was corroborated as an important factor, controlling both the diversity and abundance of microorganisms on walls, with environmental factors associated with a northern orientation favoring a higher diversity of fungi and green algae, and environmental factors associated with the west orientation determining the abundance of lichenized fungi. The orientation also affected the distribution of green algae, with one of the two most abundant species, Trentepohlia cf. umbrina, colonizing north-facing walls, while the other, Desmococcus olivaceus, predominated on west-facing walls.

The society has become increasingly interested in using natural products over chemicals for cleaning activities. In this study, the cleaning potential of formulations embedded in a hydrogel matrix and composed respectively of essential oils (EOs) of Origanum vulgare, Thymus vulgaris, and Calamintha nepeta, and their respective main active components (EO-ACs), viz., Carvacrol, Thymol, and Pulegone, on a phototropic biofilm growing on granite was investigated. In addition, and for comparative purposes, analysis with the combination of the three EOs, the combination of the three EO-ACs, and Preventol RI-80^® (one of the most effective commercial cleaning agents based on quaternary ammonium salts) in all three cases embedded in a hydrogel matrix, as well as only the hydrogel matrix, distilled water, and Preventol RI-80^®, in both latter cases applied with brush, were also studied. The cleaning effect of the treatments was assessed immediately after the treatment and after one and two weeks by color spectrophotometry, a reliable tool to evaluate the presence and vitality of the phototrophs and the cleaning effectiveness in granite. C. nepeta and its active component Pulegone proved to be the most effective and yielded similar results, comparable to those of uncolonized granite, and better than those obtained with Preventol RI-80^® applied with brush (most common way), especially at the end of the experiment. These promising first results support the suitable use of the phytochemical compounds used on phototrophs field where there are still few published studies and encourage further investigation toward the evaluation of their exhibited biocidal activity.

Accurate measuring and monitoring methods available since the 1980s have shown that the amount of Ultraviolet B (UV-B) radiation reaching the Earth's surface has increased as a result of degradation of the ozone layer. Since the adoption of the Montreal Protocol in 1987, ozone levels have been recovering successfully. However, in the context of the current climate change, other factors such as changes in cloud patterns and an increased incidence of natural disasters (e.g. fires) may be disrupting this recovery. The present study aimed to investigate the effects of different UV-B radiation levels on biofilms colonising heritage monuments. For this purpose, the effects of current UV-B levels on a biofilm composed of Synechocystis sp. (a cyanobacterium), Bracteacoccus minor (a green alga) and Fusarium sp. (a fungus) were compared at three points along a South-North transect: Portugal, Galicia (NW Spain) and Ireland (from highest to lowest UV-B radiation, respectively). Increased levels of UV-B radiation caused changes in the growth, physiological state and composition of subaerial biofilms, with cyanobacteria being more resistant than green algae to high levels of UV-B. A reduction of fungal growth and extracellular polymer substances (EPS) production was also observed, related to the reduction of biofilm aggregation at high UV-B levels.

Natural derivatives, such as essential oils, are presented as an alternative to classical biocides to the treatment of biocolonization. Thus, in this work, the cleaning and biocidal potential of some natural derivatives towards two natural biofilms’ growth on the same granite wall, with different microbial composition, was evaluated. For this purpose, three essential oils (EOs) (from Origanum vulgare, Thymus vulgaris and Calamintha nepeta) and their main active principles (APs) (carvacrol, thymol and R-(+)-pulegone, respectively) were embedded in a hydrogel matrix, with different combinations of EOs and APs, in order to evaluate the synergistic action of different actives. For comparative purposes, pure hydrogel and a mechanical method (brushing) were also used. Colorimetric measurements and chlorophyll a fluorescence analyses were performed to evaluate the cleaning action of the treatments on the biofilms. Overall, the EOs and APs present in the hydrogel proved to be reliable treatments to limit natural biocolonization, with O. vulgare being one of the most effective treatments in combination with other compounds, due to the majority presence of carvacrol. Moreover, the effect of the different treatments strictly depended on the biofilm in question, as well as its ability to adhere to the substrate.