Stone Biodeterioration Research Articles

First Genome Sequence of the Microcolonial Black Fungus Saxispiralis lemnorum MUM 23.14: Insights into the Unique Genomic Traits of the Aeminiaceae Family.

Saxispiralis lemnorum MUM 23.14 is an extremotolerant microcolonial black fungus, originally isolated from a biodeteriorated limestone artwork in Portugal. This recently introduced species belongs to the Aeminiaceae family, representing the second member of this monophyletic clade. This fungus exhibits a unique set of characteristics, including xerophily, cold tolerance, high UV radiation tolerance, and an exceptional ability to thrive in NaCl concentrations of up to 30% while also enduring pH levels ranging from 5 to 11. To gain insights into its genomic traits associated with stress resistance mechanisms, specialization, and their potential implications in stone biodeterioration, we conducted a comprehensive genome sequencing and analysis. This draft genome not only marks the first for the Saxispiralis genus but also the second for the Aeminiaceae family. Furthermore, we performed two comparative genomic analyses: one focusing on the closest relative within the Aeminiaceae family, Aeminium ludgeri, and another encompassing the genome of different extremotolerant black fungi. In this study, we successfully achieved high genome completeness for S. lemnorum and confirmed its close phylogenetic relationship to A. ludgeri. Our findings revealed traits contributing to its extremophilic nature and provided insights into potential mechanisms contributing to stone biodeterioration. Many traits are common to both Aeminiaceae species and are shared with other black fungi, while numerous unique traits may be attributed to species-specific characteristics.

A Two-Step Surface Modification Methodology for the Advanced Protection of a Stone Surface.

The biodeterioration of the natural surface on monuments, historical buildings, and even public claddings brings to the attention of researchers and historians the issues of conservation and protection. Natural stones undergo changes in their appearance, being subjected to deterioration due to climatic variations and the destructive action of biological systems interfering with and living on them, leading to ongoing challenges in the protection of the exposed surfaces. Nanotechnology, through silver nanoparticles with strong antimicrobial effects, can provide solutions for protecting natural surfaces using specific coupling agents tailored to each substrate. In this work, surfaces of two common types of natural stone, frequently encountered in landscaping and finishing works, were modified using siloxane coupling agents with thiol groups. Through these agents, silver nanoparticles (AgNPs) were fixed, exhibiting distinct characteristics, and subjected to antimicrobial analysis. This study presents a comparative analysis of the efficiency of coupling agents that can be applied to a natural surface with porous structures, when combined with laboratory-obtained silver nanoparticles, in reducing the formation of microbial biofilms, which are a main trigger for stone biodeterioration.

Unraveling the microbiotas and key genetic contexts identified on stone heritage using illumina and nanopore sequencing platforms

The application of metagenomics uncovering stone-dwelling microbiotas and its functional capabilities are of great significance for early warning of stone monuments deterioration and screening of antimicrobial agents. Here, by harnessing the combination of Nanopore and Illumina sequencing, we investigated the microbial community compositions and potential risks resulting from elemental nitrogen (N) and sulfur (S) metabolism and environmental resistomes. Taxonomic profiling showed that lichenized Ascomycota and Actinobacteria dominate the microbial population in the biofilm, followed by Cyanobacteria, Proteobacteria, and Acidobacteria. Certain microbial groups and gene families were responsible for the biogeochemical N/S cycling, probably contributing to the succession and expansion of stone biodeterioration. High abundant and diverse antibacterial biocide and metal resistance genes were retrieved from assembled genome contigs, including those encoding resistance to copper (copR and copS), zinc (znuC/yebM and zraR/hydH), and quaternary ammonium compounds (gale and vcaM). Conversely, antibiotic resistance genes conferring resistance to multiple-antibiotic, aminoglycoside, and glycopeptide accounted for the relatively low percentage of total microbial metagenome. Binned genomes have further confirmed that bacterial species contained diverse antimicrobial resistance genes and mobile genetic elements, implying the possibility of horizontal gene transfer between bacterial lineages. Overall, our findings expand our knowledge of stone-dwelling microbiome and suggest appropriate treatment for stone biodeterioration.

Risk, hazard, and vulnerability for stone biodeterioration due to higher plants: The methodological proposal of a multifactorial index (RHV)

The growth of higher plants on buildings and stone monuments can cause various biodeterioration phenomena. Along with careful management of higher plants in archeological sites and monumental areas, a comprehensive method for assessing the risk they pose to stone conservation still needs to be developed. We propose, therefore, a multi-factorial index of risk from higher plants which will evaluate relevant threats to stone structures and local conditions of vulnerability (RHV). We suggest associating such threats with the environmental conditions (defined by bioclimate and environmental context) and with the plant growth characteristics (defined by life forms, root systems and stem development features, ecological preference, and physiological characteristics). We also suggest associating vulnerability with edaphic factors (stone typology and colonization microsites) and with conservation status, including management activities. The proposed index was tested at a number of international sites in a bioclimatic gradient varying from a temperate climate (Lucca, Italy) to Mediterranean (Rome, Italy) and sub-Mediterranean xeric (Pasargadae, Iran) and tropical (Angkor, Cambodia) conditions. The results show how useful such an index can be in performing analytical assessments of the risk posed by ruderal plants in cultural heritage sites.

Risk, Hazard, and Vulnerability for Stone Biodeterioration Due to Higher Plants: The Proposal of a Multifactorial Index (Rhv)

Risk, Hazard, and Vulnerability for Stone Biodeterioration Due to Higher Plants: The Proposal of a Multifactorial Index (Rhv)

De-calcification as an important mechanism in (bio)deterioration of sandstone of Angkor monuments in Cambodia

Stone biodeterioration has been categorically attributed to a number of key factors, including physical, chemical, and (micro)biological ones. Though colonization by microorganisms on stone has been reported widely, the specific mechanisms involved for material destruction have only been revealed recently with the biochemical reactions and mechanisms identified, involving nitrogen and sulfur cycling reactions. This article reports a finding on the dissolution and loss of CaCO3 in the destruction of stone over time of exposure, which increases the porosity of the stone to trap atmospheric depositions, and enhance the development and growth of microorganisms because of improved water holding capacity and available nutrients comparing with the fresh stone. This sharp difference in CaCO3 contents between the fresh sandstone from the quarry and those from the Bayon temple in Cambodia illustrates the direct connection between the mineral dissolution reactions and microbial catalyzed contribution to it. Such alteration to the properties of the initial stone is fundamentally important to the colonization and development of microbiome on the stone, especially ammonia-oxidizing archaea (AOA) and bacteria (AOB), and Comammox bacteria as detected. Together with other nitrogen transformation microorganisms, e.g., dissimilatory nitrate reduction to ammonium (DNRA), the microbiome can be sustained and diversified over time. Based on this and other recent results, further verification and experimental studies are needed to confirm this mechanism in a systematic way to advance a better knowledge about stone deterioration under natural conditions, which are basic information for scientific protection and management of world cultural heritage.

Talaromyces saxoxalicus sp. nov., isolated from the limestone walls of the Old Cathedral of Coimbra, Portugal.

Fungi are one of the main agents of stone biodeterioration worldwide, since they strongly interfere with its integrity, aesthetical and structural natural properties. During an experimental survey aimed to isolate fungal species contributing to the biodeterioration of the limestone walls of the Old Cathedral of Coimbra (Portuguese unesco World Heritage site), a Talaromyces species that could not be identified to any currently known species in this genus was isolated. Molecular phylogenetic analysis of the internal transcribed spacer, β-tubulin and RNA polymerase II subunit 2, placed this fungus in Talaromyces sect. Purpurei, while also pointing at its phylogenetic distinction from the remaining species in this section. Thus, a novel species, Talaromyces saxoxalicus sp. nov., is here proposed. Moreover, considering the isolation source of this fungus and in an attempt to understand its contribution to the overall stone monument biodeterioration, the species's in vitro biodeteriorative potential was also evaluated. The results highlighted that the species exhibited an in vitro biodeteriorative ability (calcium oxalate crystal formation), highlighting its potential deteriorative profile.

Potential Use of Carrageenans against the Limestone Proliferation of the Cyanobacterium Parakomarekiella sesnandensis

Stone biodeterioration by cyanobacteria is a common issue in the field of cultural heritage. As they are considered the first stone colonizers, the need to control their growth has increased. In this study, we evaluated the effectiveness of kappa/iota carrageenans from the red seaweed Chondracanthus teedei var. lusitanicus against the limestone proliferation of the cyanobacterium Parakomarekiella sesnandensis, under laboratory conditions. For this purpose, 200 μL of kappa/iota carrageenans (0.01 g mL−1) were applied into the surface of the limestone replicas prior to their inoculation with P. sesnandensis. Results were evaluated after 4 months of incubation through visual inspection, stereomicroscopy, scanning electron microscopy and colorimetric analyses. The gathered data demonstrated that these types of carrageenans have the potential to reduce the colonization of P. sesnandensis. With one sole application, the aesthetical alterations caused by the proliferation of P. sesnandensis were drastically reduced. This study highlights the need to explore marine-based products, particularly those derived from seaweeds with antimicrobial properties, as alternative methods for biocleaning cultural heritage assets.

Microbiome characteristics and the key biochemical reactions identified on stone world cultural heritage under different climate conditions

The surfaces of historical stone monuments are visibly covered with a layer of colonizing microorganisms and their degradation products. In this study, a metadata analysis was conducted using the microbial sequencing data available from NCBI database to determine the diversity, biodeterioration potential and functionality of the stone microbiome on important world cultural heritage sites under four different climatic conditions. The retrieved stone microbial community composition in these metagenomes shows a clear association between climate types of the historical monuments and the diversity and taxonomic composition of the stone microbiomes. Shannon diversity values showed that microbial communities on stone monuments exposed to dry climate were more diverse than those under humid ones. In particular, functions associated with photosynthesis and UV resistance were identified from geographical locations under different climate types. The distribution of key microbial determinants responsible for stone deterioration was linked to survival under extreme environmental conditions and biochemical capabilities and reactions. Among them, biochemical reactions of the microbial nitrogen and sulfur cycles were most predominant. These stone-dwelling microbiomes on historical stone monuments were highly diverse and self-sustaining driven by energy metabolism and biomass accumulation. And metabolic products of the internal geomicrobiological nitrogen cycling on these ancient monuments play a unique role in the biodeterioration of stone monuments. These results highlight the significance of identifying the essential microbial biochemical reactions to advance the understanding of stone biodeterioration for protection management.

From surviving to thriving, the assembly processes of microbial communities in stone biodeterioration: A case study of the West Lake UNESCO World Heritage area in China

Serious concerns regarding stone biodeterioration have been raised due to the loss of aesthetic value and hidden dangers in stone cultural heritages and buildings. Stone biodeterioration involves a complex ecological interplay among organisms, however, the ecological mechanisms (deterministic or stochastic processes) that determine the microbial community on stone remain poorly understood. Here, using both amplicon and shotgun metagenomic sequencing approaches, we comprehensively investigated the biodiversity, assembly, and function of communities (including prokaryotes, fungi, microfauna, and plants) on various types of deteriorating limestone across different habitats in Feilaifeng. By generalizing classic ecological models to stone habitats, we further uncovered and quantified the mechanisms underlying microbial community assembly processes and microbial interactions within the biodeteriorated limestone. Community profiling revealed stable ecosystem functional potential despite high taxonomic variation across different biodeterioration types, suggesting non-random community assembly. Increased niche differentiation occurred in prokaryotes and fungi but not in microfauna and plant during biodeterioration. Certain microbial groups such as nitrifying archaea and bacteria showed wider niche breadth and likely contributing to the initiation, succession and expansion of stone biodeterioration. Consistently, prokaryotes were more strongly structured by selection-based deterministic processes, while micro-eukaryotes were more influenced by dispersal and drift-based stochastic processes. Importantly, microbial coexistence maintains network robustness within stone microbiotas, highlighting mutual cooperation among functional microorganisms. These results provide new insights into microbial community assembly mechanisms in stone ecosystems and may aid in the sustainable conservation of stone materials of interest.

New oxidative alginate-biocide hydrogels against stone biodeterioration

To address the problem of the biodeterioration of stone monuments by developing innovative methodologies, two new alginate hydrogels were obtained by using titanium dioxide as photoactivatable biocide and sodium dichloroisocyanurate as reservoir of hypochlorous acid. The effectiveness of both hydrogels in removing the biological patina on artificially colonized Lecce stones without altering the properties of the treated substrates was demonstrated by measurements of light microscopy, colorimetry, SEM/EDS analyses and 1H-NMR relaxometry. The biocidal activity of the new developed hydrogels has been also tested on natural cyanobacterial biofilms collected in Roman Catacombs and grown on agar medium and on calcareous stones. The results are very promising with the complete inhibition of cyanobacterial photosynthesis and without any side effects on the substrates tested.

Community Composition and Ex Situ Cultivation of Fungi Associated with UNESCO Heritage Monuments in the Bay of Naples

The Bay of Naples, Italy, is renowned for its archaeological heritage. However, this heritage is threatened by the combination of weathering and the biological activity of microorganisms. Fungi are among the major agents of microbial deterioration of cultural heritage since they can cause cracks and lesions in monuments due to the penetrating force of their hyphae. Such biodeterioration may weaken the stone structures and threaten the longevity of these culturally important monuments. To address this, we collected, identified, and maintained in culture filamentous fungi that colonize the external surface of monuments at five important archaeological sites near Naples, namely Cuma, Ercolano, Nola, Oplonti, and Pompei. We isolated a total of 27 fungal taxa, all of which can be cultivated in the laboratory, and form a part of our reference collection. Many of the described fungal taxa we found belong to groups that are involved in stone biodeterioration and can thus be considered as model organisms for in vitro studies. These results emphasize the importance of identifying and cultivating fungal stock cultures for non-invasive studies on biodeterioration. Our newly developed reference collection represents a useful resource that is available to other researchers to rapidly identify potentially hazardous fungi on other monuments.

Effect of Inorganic Carbon Concentration on the Development of Subaerial Phototrophic Biofilms on Granite

Organisms living at the stone–air interface are expected to be affected by changes in the atmospheric composition due to greenhouse gases emissions. Increased CO2 concentrations may particularly affect phototrophic microorganisms that colonize stone cultural heritage and form subaerial biofilms. However, little is known about the effects of the environmental changes on microorganisms that colonize stone and the consequences for cultural heritage conservation. In the present study, we investigated how an increase in inorganic carbon concentration affected the development of a subaerial biofilm composed by the cyanobacterium Synechocystis sp. PCC 6803 grown on granite. For this purpose, we established two experiments on biofilm formation, with and without addition of inorganic carbon to the growth medium. Higher concentrations of carbon promoted biofilm growth and increased the concentrations of the photosynthetic pigments chlorophyll a and carotenoids on granite surface, potentially exacerbating the aesthetic impact of these biofilms on stone-made cultural heritage. However, the extracellular polysaccharides produced were not significantly affected by carbon availability, so that physical stone biodeterioration might not be increased by the cyanobacterial matrix. The findings provide valuable data on how the existing global change scenario might affect organisms inhabiting stone cultural heritage and encourage to develop new sustainable treatments and methodologies to prevent biodeterioration and thus preserve stone cultural heritage.

Enzymatic Activity as a Measure of Total Microbial Activity on Historical Stone

Stones of historical monuments exposed to the open air deteriorate over the course of time depending on physical, chemical, and biological factors acting in co-association. Among the biological factors, microorganisms play a key role in the deterioration process of stones. Detecting the level of microbial activity on stones is an essential step in diagnostic and monitoring studies of stone biodeterioration, and aids in controlling the performance of treatments applied to the stones. Therefore, this study aimed to develop a practical and rapid method for the determination of microbial activity on historical stones and use this method on the Mount Nemrut monuments (MNMs) (Adiyaman, Turkey). For that purpose, the fluorescein diacetate (FDA) hydrolysis method, frequently employed for soil environments, was adapted for the estimation and assessment of total microbial activity to understand whether microorganisms posed a potential risk for the biodeterioration of the limestones and sandstones of the MNMs. The traditional plate count method was also applied simultaneously to the same stone samples to compare and assist in the interpretation of the results of the FDA hydrolysis method, which relies on the quantitative determination of bacterial and fungal colonies in nutrient agar and malt extract agar medium, respectively. The results of the FDA hydrolysis and plate count methods showed consistency. The total microbial activity determined by the FDA hydrolysis method was low for both types of stone samples. In addition, the plate count method showed low bacterial and fungal counts on all of the samples. This revealed that microbial activity did not play an important role in the stone deterioration process on the MNMs, although different lichen species were frequently observed on both the sandstones and the limestones. Hence, further investigation must be undertaken for determination of their long-term behavior and effects on the stones of the MNMs. On the other hand, the results of the FDA hydrolysis and plate count methods showed correlation. Lower bacterial counts were observed when lower enzymatic activity was observed in the stone samples, and likewise, higher bacterial counts were observed when higher enzymatic activity was observed. Consequently, the application of the FDA hydrolysis method was determined to be reliable for the estimation of total microbial activity on historical stones. The method had obvious advantages in terms of its rapid measurement rate and sensitivity, even on small samples.

Nutrient cycling potential within microbial communities on culturally important stoneworks.

SummaryPrevious studies on microbes associated with deterioration of cultural heritage (CH) stoneworks have revealed a diverse microbiota adapted to stresses such as low nutrients, aridity and high salinity, temperatures and radiation. However, the function of these pioneer microbial communities is still unclear. This study examines bacterial and archaeal diversity in exfoliated and dark encrustation sandstone from Portchester Castle (UK) by 16S rRNA and functional gene analyses. Bacterial and archaeal communities from the exfoliated sites were distinctly different from the dark encrustation. Detected genera were linked to extreme environmental conditions, various potential functional roles and degradation abilities. From these data it was possible to reconstruct almost complete nitrogen and sulfur cycles, as well as autotrophic carbon fixation and mineral transformation processes. Analysis of RNA showed that many of the detected genera in these nutrient cycles were probably active in situ. Thus, CH stonework microbial communities are highly diverse and potentially self‐sustaining ecosystems capable of cycling carbon, nitrogen and sulfur as well as the stone biodeterioration processes that lead to alterations such as exfoliation and corrosion. These results highlight the importance of diversity and internal recycling capacity in the development of microbial communities in harsh and low energy systems.

Deterioration-Associated Microbiome of Stone Monuments: Structure, Variation, and Assembly.

Research on the microbial communities that colonize stone monuments may provide a new understanding of stone biodeterioration and microbe-induced carbonate precipitation. This work investigated the seasonal variation of microbial communities in 2016 and 2017, as well as its effects on stone monuments. We determined the bacterial and fungal compositions of 12 samples from four well-separated geographic locations by using 16S rRNA and internal transcribed spacer gene amplicon sequencing. Cyanobacteria and Ascomycota were the predominant bacterial and fungal phyla, respectively, and differences in species abundance among our 12 samples and 2 years showed no consistent temporal or spatial trends. Alpha diversity, estimated by Shannon and Simpson indices, revealed that an increase or decrease in bacterial diversity corresponded to a decrease or increase in the fungal community from 2016 to 2017. Large-scale association analysis identified potential bacteria and fungi correlated with stone deterioration. Functional prediction revealed specific pathways and microbiota associated with stone deterioration. Moreover, a culture-dependent technique was used to identify microbial isolates involved in biodeterioration and carbonatogenesis; 64% of 85 bacterial isolates caused precipitation of carbonates in biomineralization assays. Imaging techniques including scanning electron microscopy with energy-dispersive spectroscopy, X-ray diffraction, and fluorescence imaging identified CaCO3 crystals as calcite and vaterite. Although CaCO3 precipitation induced by bacteria often has esthetically deleterious impacts on stone monuments, this process may potentially serve as a novel, environmentally friendly bacterial self-inoculation approach to the conservation of stone.IMPORTANCE Comprehensive analyses of the microbiomes associated with the deterioration of stone monuments may contribute to the understanding of mechanisms of deterioration, as well as to the identification of potentially beneficial or undesirable microbial communities and their genomic pathways. In our study, we demonstrated that Cyanobacteria was the predominant bacterial phylum and exhibited an increase from 2016 to 2017, while Proteobacteria showed a decreasing trend. Apart from esthetic deterioration caused by cyanobacteria and fungi, white plaque, which is composed mainly of CaCO3 and is probably induced by Crossiella and Cyanobacteria, was also considered to be another threat to stone monuments. We showed that there was no significant correlation between microbial population variation and geographic location. Specific functional genes and pathways were also enriched in particular bacterial species. The CaCO3 precipitation induced by an indigenous community of carbonatogenic bacteria also provides a self-inoculation approach for the conservation of stone.

Preliminary investigation of combined laser and microwave treatment for stone biodeterioration

In this work, the potential applications of surface laser removal of biological crusts from Carrara marble artefacts and in-depth microwave selective heating for treatment of possible endolithic growths have been evaluated. The investigations were carried out on seriously deteriorated marble fragments from the monumental tombs of the English Cemetery in Florence, Italy. The second harmonic (532 nm) of a Q-switched Nd:YAG laser was used for cleaning while a microwave system emitting at 2.45 GHz was tested for in-depth sterilization. As is well known, microwaves are strongly absorbed by water via dipolar energy dissipation, which permits selective heating of endolithic organisms. The effects of laser and microwave treatments were characterized using scanning electron microscopy (SEM) along with chlorophyll fluorescence with pulsed amplitude modulated imaging. The latter has allowed the quantification of the impairment effects to photosystem II of epilithic phototrophic organism residues by mapping the maximum quantum yield, while the damage to endolithic growths was evaluated by means of SEM following osmium tetroxide staining of cytoplasmic lipids. The results provide an early picture of the development and application perspectives of the combined laser and microwave treatments in the conservation of biodeteriorated stone artefacts.

The spatial organization and microbial community structure of an epilithic biofilm.

Microbial biofilms are common on lithic surfaces, including stone buildings. However, the ecology of these communities is poorly understood. Few studies have focused on the spatial characteristics of lithobiontic biofilms, despite the fact that spatial structure has been demonstrated to influence ecosystem function (and hence biodegradation) and community diversity. Furthermore, relatively few studies have utilized molecular techniques to characterize these communities, even though molecular methods have revealed unexpected microbial diversity in other habitats. This study investigated (1) the spatial structure and (2) the taxonomic composition of an epilithic biofilm using molecular techniques, namely amplicon pyrosequencing and terminal restriction fragment length polymorphism. Dispersion indices and Mantel correlograms were used to test for the presence of spatial structure in the biofilm. Diversity metrics and rank-abundance distributions (RADs) were also generated. The study revealed spatial structure on a centimetre scale in eukaryotic microbes (fungi and algae), but not the bacteria. Fungal and bacterial communities were highly diverse; algal communities much less so. The RADs were characterized by a distinctive 'hollow' (concave up) profile and long tails of rare taxa. These findings have implications for understanding the ecology of epilithic biofilms and the spatial heterogeneity of stone biodeterioration.

Testing the antibacterial activity of doped TiO2 for preventing biodeterioration of cultural heritage building materials

Application of biocide products, alone or in combination with protective or consolidants, is a common practice to prevent further colonization on monuments surface. TiO2 is as one of the most fascinating materials characterized by chemical stability, non-toxicity, high photo-reactivity, broad-spectrum antibiosis and cheapness, and it has been used extensively as biocide against various microorganisms: bacteria, fungi and viruses. Its problem is that its reactivity is due to a restricted wavelength in the UV region and thus its application can be limited in certain circumstances (e.g. indoors). Recently, alkaline-earth metal ions have been considered as promising, efficient and economic approaches for doping the TiO2 because they have a larger photocatalytic spectrum.M–TiO2 thus may be more effective in the protection and maintenance of stone surfaces than pure TiO2 in reducing the biodeterioration processes. However before any application on monument surfaces, the products must be tested not only for their biocide efficacy but also for being harmless to cultural heritage material.To fulfil this requirement, a set of experiments has been carried out testing M-doped TiO2 compounds for their efficiency as photocatalysts, for the chromatic changes induced by their application on stone surfaces and for their antibacterial activity against selected Gram positive and Gram negative bacteria.Our results showed that TiO2 doped with Ag alone or in combination Fe3+ or with Sr has antibacterial activity and could be successfully applied in field to prevent stone biodeterioration.

A multianalytical approach to investigate stone biodeterioration at a UNESCO world heritage site: the volcanic rock-hewn churches of Lalibela, Northern Ethiopia

A multianalytical approach combining Optical Microscopy (OM), Backscattered Variable Pressure Scanning Electron Microscopy + Energy Dispersive X-ray Spectroscopy (VP-BSEM + EDS), Powder X-ray Diffractometry (PXRD), Raman Spectroscopy, and Microbiological techniques has been applied to characterize decay products and processes occurring at the surface of two rock-hewn churches (Bete Gyorgis and Bete Amanuel) at the UNESCO’s World Heritage site of Lalibela, Northern Ethiopia. The two churches were carved into volcanic scoria deposits of basaltic composition. In their geological history, the Lalibela volcanic rocks underwent late to post-magmatic hydrothermal alteration together with partial laterization and are therefore characterized by a decay-prone highly vesicular microtexture with late stage to post-magmatic precipitation of secondary mineral phases (calcite–zeolite–smectite). The main objective of the study was to gain a better insight into the weathering products and mechanisms affecting the surface of the stone monuments and to assess the relative contribution of natural “geological” weathering processes versus biological/salt attack in stone decay at this unique heritage site. Results indicate that while the main cause of bulk rock deterioration and structural failure could be related to the stone inherited “geological” features, biological attack by micro- (bacteria) and/or macro- (lichens) organisms is currently responsible for severe stone surface physical and chemical weathering leading to significant weakening of the stone texture and to material loss at the surface of the churches walls. A prompt and careful removal of the biological patinas with the correct biocidal treatment is therefore recommended.