Insight on bacteria communities in outdoor bronze and marble artefacts in a changing environment

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.

Taxonomic and metabolic characterisation of biofilms colonising Roman stuccoes at Baia's thermal baths and restoration strategies

Stuccoes are very delicate decorative elements of Roman age. Very few of them survived almost intact to present days and, for this reason, they are of great interest to restorers and conservators. In this study, we combined metabarcoding and untargeted metabolomics to characterise the taxonomic and metabolic profiles of the microorganisms forming biofilms on the stuccoes located on the ceiling of the laconicum, a small thermal environment in the archaeological park of Baia (southern Italy). We found that some samples were dominated by bacteria while others by eukaryotes. Additionally, we observed high heterogeneity in the type and abundance of bacterial taxa, while the eukaryotic communities, except in one sample (at prevalence of fungi), were dominated by green algae. The metabolic profiles were comparable across samples, with lipids, lipid-like molecules and carbohydrates accounting for roughly the 50% of metabolites. In vitro and in vivo tests to remove biofilms on stuccoes using essential oils blends were successful at a 50% dilution for one hour and half. This integrative study advanced our knowledge on taxonomic and metabolic profiles of biofilms on ancient stuccoes and highlighted the potential impacts of these techniques in the field of cultural heritage conservation.

Long-term monitoring of the hypogeal Etruscan Tomba degli Scudi, Tarquinia, Italy. Early detection of black spots, investigation of fungal community, and evaluation of their biodeterioration potential

AIMS

Hypogeal environments with cultural heritage interest pose a real challenge for their preservation and conservation. The ancient Etruscan Necropolis of Tarquinia, Italy, consists of 200 tombs decorated with extraordinary mural paintings, of great artistic and historical value. Since the beginning of the restoration campaign in 2016, a regular microbiological survey has been performed in the Tomba degli Scudi. The aim of this study was to investigate the nature of an expansion of black spots on the pictorial layers recently observed.

METHODS AND RESULTS

To determine the origin of the black spots in the atrium chamber of the Tomba degli Scudi, the fungal community was sampled using various techniques: cellulose discs, swabs, and nylon membranes and investigated by a multi-analytical approach. The obtained results suggest that the identified fungal strains (e.g. Gliomastix murorum and Pseudogymnoascus pannorum) are common to many subterranean environments around the world, such as Lascaux cave.

CONCLUSIONS

The continuous and long-term monitoring made it possible to detect alterations at an early stage and assess the harmfulness of different fungal strains. This work is a demonstration of the effectiveness of prevention and monitoring actions within these fragile and valuable environments.

Biomineral deposits and coatings on stone monuments as biodeterioration fingerprints

Biominerals deposition processes, also called biomineralisation, are intimately related to biodeterioration on stone surfaces. They include complex processes not always completely well understood. The study of biominerals implies the identification of organisms, their molecular mechanisms, and organism/rock/atmosphere interactions. Sampling restrictions of monument stones difficult the biominerals study and the in situ demonstrating of biodeterioration processes. Multidisciplinary works are required to understand the whole process. Thus, studies in heritage buildings have taken advantage of previous knowledge acquired thanks to laboratory experiments, investigations carried out on rock outcrops and within caves from some years ago. With the extrapolation of such knowledge to heritage buildings and the advances in laboratory techniques, there has been a huge increase of knowledge regarding biomineralisation and biodeterioration processes in stone monuments during the last 20 years. These advances have opened new debates about the implications on conservation interventions, and the organism's role in stone conservation and decay. This is a review of the existing studies of biominerals formation, biodeterioration on laboratory experiments, rocks, caves, and their application to building stones of monuments.

Geology defines microbiome structure and composition in nunataks and valleys of the Sør Rondane Mountains, East Antarctica

Understanding the relation between terrestrial microorganisms and edaphic factors in the Antarctic can provide insights into their potential response to environmental changes. Here we examined the composition of bacterial and micro-eukaryotic communities using amplicon sequencing of rRNA genes in 105 soil samples from the Sør Rondane Mountains (East Antarctica), differing in bedrock or substrate type and associated physicochemical conditions. Although the two most widespread taxa (Acidobacteriota and Chlorophyta) were relatively abundant in each sample, multivariate analysis and co-occurrence networks revealed pronounced differences in community structure depending on substrate type. In moraine substrates, Actinomycetota and Cercozoa were the most abundant bacterial and eukaryotic phyla, whereas on gneiss, granite and marble substrates, Cyanobacteriota and Metazoa were the dominant bacterial and eukaryotic taxa. However, at lower taxonomic level, a distinct differentiation was observed within the Cyanobacteriota phylum depending on substrate type, with granite being dominated by the Nostocaceae family and marble by the Chroococcidiopsaceae family. Surprisingly, metazoans were relatively abundant according to the 18S rRNA dataset, even in samples from the most arid sites, such as moraines in Austkampane and Widerøefjellet ("Dry Valley"). Overall, our study shows that different substrate types support distinct microbial communities, and that mineral soil diversity is a major determinant of terrestrial microbial diversity in inland Antarctic nunataks and valleys.

Unraveling disparate roles of organisms, from plants to bacteria, and viruses on built cultural heritage

The different organisms, ranging from plants to bacteria, and viruses that dwell on built cultural heritage can be passive or active participants in conservation processes. For the active participants, particular attention is generally given to organisms that play a positive role in bioprotection, bioprecipitation, bioconsolidation, bioremediation, biocleaning, and biological control and to those involved in providing ecosystem services, such as reducing temperature, pollution, and noise in urban areas. The organisms can also evolve or mutate in response to changes, becoming tolerant and resistant to biocidal treatments or acquiring certain capacities, such as water repellency or resistance to ultraviolet radiation. Our understanding of the capacities and roles of these active organisms is constantly evolving as bioprotection/biodeterioration, and biotreatment studies are conducted and new techniques for characterizing species are developed. This brief review article aims to shed light on interesting research that has been abandoned as well as on recent (some ongoing) studies opening up new scopes of research involving a wide variety of organisms and viruses, which are likely to receive more attention in the coming years. KEY POINTS: • Organisms and viruses can be active or passive players in heritage conservation • Biotreatment and ecosystem service studies involving organisms and viruses are shown • Green deal, health, ecosystem services, and global change may shape future research.