Influence of the properties of granitic rocks on their bioreceptivity to subaerial phototrophic biofilms

Nutrient availability and acid erosion determine the early colonization of limestone by lithobiontic microorganisms

Introduction

Microorganisms, including the pioneer microorganisms that play a role in the early colonization of rock, are extremely important biological factors in rock deterioration. The interaction of microorganisms with limestone leads to biodeterioration, accelerates soil formation, and plays an important role in the restoration of degraded ecosystems that cannot be ignored. However, the process of microbial colonization of sterile limestone in the early stages of ecological succession is unclear, as are the factors that affect the colonization. Acid erosion (both organic and inorganic), nutrient availability, and water availability are thought to be key factors affecting the colonization of lithobiontic microorganisms.

Methods

In this study, organic acid (Oa), inorganic acid (Ia), inorganic acid + nutrient solution (Ia + Nut), nutrient solution (Nut), and rain shade (RS) treatments were applied to sterilized limestone, and the interaction between microorganisms and limestone was investigated using high-throughput sequencing techniques to assess the microorganisms on the limestone after 60 days of natural placement.

Results

The results were as follows: (1) The abundance of fungi was higher than that of bacteria in the early colonization of limestone, and the dominant bacterial phyla were Proteobacteria, Bacteroidota, and Actinobacteriota, while the dominant fungal phyla were Ascomycota, Basidiomycota, and Chytridiomycota. (2) Acid erosion and nutrient availability shaped different microbial communities in different ways, with bacteria being more sensitive to the environmental stresses than fungi, and the higher the acidity (Ia and Oa)/nutrient concentration, the greater the differences in microbial communities compared to the control (based on principal coordinate analysis). (3) Fungal communities were highly resistant to environmental stress and competitive, while bacterial communities were highly resilient to environmental stress and stable.

Discussion

In conclusion, our results indicate that limestone exhibits high bioreceptivity and can be rapidly colonized by microorganisms within 60 days in its natural environment, and both nutrient availability and acid erosion of limestone are important determinants of early microbial colonization.

Environmental and Mineralogical Controls on Biosignature Preservation in Magnesium Carbonate Systems Analogous to Jezero Crater, Mars

Jezero Crater on Mars is a paleolacustrine environment where Mg-carbonates may host evidence of ancient life. To elucidate the environmental and mineralogical controls on biosignature preservation, we examined samples from five terrestrial analogs: Lake Salda (Turkey), Lake Alchichica (Mexico), Qinghai-Tibetan Plateau (China), Mg-carbonate playas (British Columbia, Canada), and a mine with fine-grained ultramafic tailings (Yukon, Canada). The mineralogical compositions of the samples varied, yet were often dominated by either aragonite (CaCO₃) or hydromagnesite [Mg₅(CO₃)₄(OH)₂·4H₂O]. Aragonite-rich samples from Alchichica, Mg-carbonate playas, and the ultramafic mine contained an abundance of entombed microbial biomass, including organic structures that resembled cells, whereas hydromagnesite-rich samples were devoid of microfossils. Aragonite often precipitates subaqueously where microbes thrive, thereby increasing the likelihood of biomass entombment, while hydrated Mg-carbonates typically form by evaporation in subaerial settings where biofilms are less prolific. Magnesite (MgCO₃), the most stable Mg-carbonate, forms extremely slowly, which may limit the capture of biosignatures. Hydrated Mg-carbonates are prone to transformation via coupled dissolution-precipitation reactions that may expose biosignatures to degradation. Although less abundant, aragonite is commonly found in Mg-carbonate environments and is a better medium for biosignature preservation due to its fast precipitation rates and relative stability, as well as its tendency to form subaqueously and lithify. Consequently, we propose that aragonite be considered a valuable exploration target on Mars.

Biodeterioration of stone monuments: Studies on the influence of bioreceptivity on cyanobacterial biofilm growth and on the biocidal efficacy of essential oils in natural hydrogel

An important field of research is devoted to the development of innovative, sustainable, and safe methodologies to counteract biodeterioration of stone monuments due to the growth of microbial communities. However, besides the biocide's efficacy, it is crucial to consider the features of substrates on which biocides must be applied, to define the so-called bioreceptivity of the lithic faces. In this research five different lithotypes, namely Lecce stone, Travertine, Peperino, Serena stone, and Granite, have been used as substrates for the growth of cyanobacterial biofilms. Open porosity, hygroscopic properties, and roughness parameters have been investigated for each lithotype and correlated to the photosynthetic yields of the biofilms colonizing the different stones to propose an easy method to estimate stone bioreceptivity. Different levels of coverage of the stone surfaces have been accomplished in relation to the typology of lithotypes. To develop innovative restoration methodologies against biodeterioration of stone monuments, a hydrogel-biocide system has been optimized by using a polysaccharide dispersion as a matrix where to embed T. vulgaris essential oil (at 0.25 % or 0.1 %) or its main component thymol (at 0.18 % or 0.07 %). The efficacy and the effect of the innovative biocide have been evaluated combining microscopy, photosynthetic measurements, and colorimetric analyses and both the biocides (with T. vulgaris EO or thymol) showed to be highly effective against the cyanobacterial biofilms for at least six months from the treatment without inducing any significant alteration to the lithic surfaces. The efficacy of thymol alone allows to treat colonized surfaces with a single active ingredient, or at least a mixture thereof, much cheaper and reproducible. The results obtained in this work pave the way to develop a sustainable cleaning protocol to counteract the biodeterioration of stone monuments or historic buildings induced by the presence of phototrophic biofilms that endangered their conservation.

Change in composition and potential functional genes of microbial communities on carbonatite rinds with different weathering times

Organisms and time are important factors for rock weathering to form soils. However, weathering time is usually difficult to quantitatively study, and the potential microorganisms involved in rock weathering are difficult to identify qualitatively. Currently, there is no clear conclusion on how ecological strategies of carbonatite weathering rind microorganisms change with weathering time, and how the microbial composition and functional genes involved in element cycling change over two century-scale weathering time. In this study, we selected abandoned carbonate tombstones as the subject and used the date when the tombstones were erected by humans as the onset of weathering. Using metagenome sequencing methods, we investigated the trends in the composition of fungal, bacterial and archaeal communities of carbonate weathering rind and related elemental cycle functional genes during a weathering time of 19 to 213 years. The results showed that: (1) with the increase in weathering time, at the phylum level, microbial taxa gradually shifted from r-strategists (faster turnover rates, higher mortality rates, higher reproduction, lower competition rate) to K-strategists (slower turnover rates, lower mortality rates, lower reproduction, higher competition rate), which correspondingly increased the abundance of functional genes related to C and N cycles. (2) The properties of the parent rock layer determines the colonization and distribution of weathering rind microorganisms (especially prokaryotic microorganisms) and the corresponding functional gene abundance. Our study provides new insights into the weathering process of carbonate rocks.

In Living Color: Pigment-Based Microbial Ecology At the Mineral-Air Interface

Pigment-based color is one of the most important phenotypic traits of biofilms at the mineral-air interface (subaerial biofilms, SABs), because it reflects the physiology of the microbial community. Because color is the hallmark of all SABs, we argue that pigment-based color could convey the mechanisms that drive microbial adaptation and coexistence across different terrestrial environments and link phenotypic traits to community fitness and ecological dynamics. Within this framework, we present the most relevant microbial pigments at the mineral-air interface and discuss some of the evolutionary landscapes that necessitate pigments as adaptive strategies for resource allocation and survivability. We report several pigment features that reflect SAB communities' structure and function, as well as pigment ecology in the context of microbial life-history strategies and coexistence theory. Finally, we conclude the study of pigment-based ecology by presenting its potential application and some of the key challenges in the research.

Community assembly, potential functions and interactions between fungi and microalgae associated with biodeterioration of sandstone at the Beishiku Temple in Northwest China

Fungi, cyanobacteria and algae are specific microbial groups associated with the deterioration and safety of stone monuments. In this study, high-throughput sequencing analysis was used to investigate the diversity, distributions, ecological functions, and interaction patterns of both the fungal and microalgal (including cyanobacteria and algae) communities on sandstone in the Beishiku Temple, located on the ancient Silk Road. The results showed that the core phyla of fungi were affiliated with unclassified Lecanoromycetes, Engyodontium, Knufia, Epicoccum, Endocarpon, and Cladosporium of Ascomycota whereas the phyla of microalgae were dominated by prokaryotic Cyanobacteria and eukaryotic Chlorophyta. The environmental factors of temperature, relative humidity, and light intensity were monitored simultaneously. The structure of the microbial communities was much more strongly shaped by soluble Cl^-, Na⁺, NO₃^- ions than by the light intensity, moisture content or temperature, especially for the weathered sandstone located outside the caves. The co-occurrence network analysis suggested that a more stable community structure was evident outside the caves than inside. The stronger positive connections and coexistence patterns that were detected indicate a strong adaptability of fungi and microalgae to the distinct oligotrophic microhabitats on sandstone. The metacommunity co-occurrence network exhibited the ecological predominance of fungi, and most of the functional fungi in the biofilms outside the caves belonged to the Lichenized group, based on the FUNGuild prediction. These findings highlight the ecology and functions of stone-inhabiting microorganisms to further advance the current understanding and knowledge of sandstone biodeterioration for protection and management.

The Alteration of Giglio Island Granite: Relevance to the Conservation of Monumental Architecture

The research examines the alteration phenomena of Giglio island granite, a rock quarried by Romans from the 3rd century, used for columns in the Italian peninsula and later reemployed in many Christian religious buildings. The study has shown that already in the bedrock there are small percentages of clay minerals. Starting from this condition, the alteration develops by an increase in porosity, which in turn favours the establishment of a slight hydrolysis of the silicates with a decrease in Na, Ca, and K, in accordance with the sericitisation process. The alteration proceeds with a further increase in porosity, apparently not related to a real loss of cohesion, which, however, occurs shortly after, highlighting the necessity of a continuous monitoring of the state of conservation of the material in the architectural heritage.

Revisiting and reanalysing the concept of bioreceptivity 25 years on

2020 marks 25 years since Olivier Guillitte defined the term 'bioreceptivity', to describe the ability of a building material to be colonised by living organisms. Although Guillitte noted in his 1995 paper that several issues required further investigation, to the best of our knowledge the bioreceptivity concept has not been restated, reviewed, reanalysed or updated since then. The present paper provides an opinionated exposition of the status and utility of the bioreceptivity concept for built heritage science and conservation in the light of current knowledge, aimed to stimulate further discussion. A bibliometric analysis highlights the key dimensions of the past 25 years of published research, showing that the term bioreceptivity has been widely used in the field of built cultural heritage. In our reanalysis of the concept, special attention is devoted to the six types of bioreceptivity (primary, secondary, tertiary, intrinsic, extrinsic and semi-extrinsic) articulated by Guillitte in 1995. We propose that field-based studies of bioreceptivity are urgently needed, and that the intrinsic, extrinsic and semi-extrinsic types of bioreceptivity should be dropped, and a new category (quaternary bioreceptivity) added. Additionally, we propose that bioreceptivity in submerged and subsoil environments should also be considered. Bioreceptivity remains an important concept for managing both new build and built heritage, as it provides the key to understanding the drivers and patterns of biological colonisation of building materials.

Influence of Environment on Microbial Colonization of Historic Stone Buildings with Emphasis on Cyanobacteria

Microbial cells that produce biofilms, or patinas, on historic buildings are affected by climatic changes, mainly temperature, rainfall and air pollution, all of which will alter over future decades. This review considers the colonization of stone buildings by microorganisms and the effects that the resultant biofilms have on the degradation of the structure. Conservation scientists require a knowledge of the potential effects of microorganisms, and the subsequent growth of higher organisms such as vascular plants, in order to formulate effective control strategies. The vulnerability of various structural materials (“bioreceptivity”) and the ways in which the environmental factors of temperature, precipitation, wind-driven rain and air pollution influence microbial colonization are discussed. The photosynthetic microorganisms, algae and cyanobacteria, are acknowledged to be the primary colonizers of stone surfaces and many cyanobacterial species are able to survive climate extremes; hence special attention is paid to this group of organisms. Since cyanobacteria require only light and water to grow, can live endolithically and are able to survive most types of stress, they may become even more important as agents of stone cultural property degradation in the future.

Impact of colour on the bioreceptivity of granite to the green alga Apatococcus lobatus: Laboratory and field testing

Recent work has demonstrated that surface colour affects the formation of cyanobacterial subaerial biofilms on polycarbonate coupons and, in turn, influences their bioreceptivity. To explore whether colour is important on other substrates, the influence of colour on the primary bioreceptivity of granite to the terrestrial green alga Apatococcus lobatus (Trebouxiophyceae, Chlorophyta) has been assessed. Two granitoids (Grissal and Rosa Porriño) with the same texture, and very similar chemical composition, open porosity and surface roughness, but different coloration related to feldspars (i.e. greyish and reddish) were used to conduct bioreceptivity studies in parallel field and laboratory tests. Light microscopy, colour spectrophotometry, PAM fluorometry, and optical profilometry were used to evaluate colonisation and its impacts. Short-term results (after 7 and 10 weeks of colonisation by a mono-species biofilm) from both lab and field trials, showed significantly more algae growth on reddish granite (Rosa Porriño) than on greyish granite (Grissal). Also, optical profilometry and light microscopy demonstrated that on both granites algal aggregates developed in hollows. We attribute the roughly double levels of A. lobatus growth on reddish vs greyish granite to differences in the amount of radiant energy absorbed and the higher levels of red wavelength light (known to encourage algal growth) reflected from the reddish surface.

Testing Anti-Biofilm Polymeric Surfaces: Where to Start?

Present day awareness of biofilm colonization on polymeric surfaces has prompted the scientific community to develop an ever-increasing number of new materials with anti-biofilm features. However, compared to the large amount of work put into discovering potent biofilm inhibitors, only a small number of papers deal with their validation, a critical step in the translation of research into practical applications. This is due to the lack of standardized testing methods and/or of well-controlled in vivo studies that show biofilm prevention on polymeric surfaces; furthermore, there has been little correlation with the reduced incidence of material deterioration. Here an overview of the most common methods for studying biofilms and for testing the anti-biofilm properties of new surfaces is provided.

Evaluation of the role of biocolonizations in the conservation state of Machu Picchu (Peru): The Sacred Rock

Machu Picchu Inca sanctuary (Cusco Region, Peru) was constructed on a granitic plateau, better known as Vilcabamba batholith. One of the most important carved granitic rocks from this archaeological site is the Sacred Rock, used by Inca citizens for religious rituals. Due to the location and climatic conditions, different rocks from this archaeological site are affected by biocolonizations. Concretely, the Sacred Rock shows flaking and delamination problems. In this work, a non-destructive multi analytical methodology has been applied to determine the possible role of the biodeteriogens, forming the biological patina on the Sacred Rock, in the previously mentioned conservation problems. Before characterizing the biological patina, a mineralogical characterization of the granitic substrate was conducted using X-ray Diffraction, Raman microscopy (RM) and micro energy dispersive X-ray fluorescence spectrometry. For the identification of the main biodeteriogens in the biofilm, Phase Contrast Microscopy was used. RM also allowed to determine the distribution (imaging) and the penetration (depth profiling) of the biogenic pigments present in the biopatina. Thanks to this study, it was possible to asses that some colonizers are growing on inner areas of the rock, reinforcing their possible assistance in the delamination. Moreover, the in-depth distribution of a wide variety of carotenoids in the patinas allowed to approach the penetration ability of the main biodeteriogens and the diffusion of these biogenic pigments to the inner areas of the rocky substrate.

Biofilm colonization of metamorphic lithotypes of a renaissance cathedral exposed to urban atmosphere

Stone architectural heritage exposed outdoor represents a challenging habitat for biological growths; nevertheless, biocolonization on heritage structure is ubiquitous and represents a major mechanism of alteration. However, the identification of specific microorganisms with known reactivity towards the stone substrate does not necessarily imply that a biodeterioration process is in progress and, in specific conditions, bioprotection effects have been highlighted as a result of colonization. The main objective of the present research is to evaluate the biofilm formation on different lithotypes exposed to similar environmental polluted conditions, and to investigate whether the presence of subaerial biofilms can be associated to an increased magnitude of deterioration of the colonized surfaces with respect to the not colonized ones. In particular, the research examines the extensive biological colonization of the stone surfaces of the façade of the Cathedral of Monza (Italy). Four metamorphic stones widely used in the façade and showing rather different compositional, mineralogical and microstructural features were studied. The state of conservation of the stones was characterized under the mineralogical and compositional point of view by X-ray diffraction and Fourier Transformed infrared analysis. The microstructure of colonized substrates and of reference not colonized ones was studied by means of optical and electron microscopy, to comparatively evaluate the damage extent and weathering patterns in both conservative conditions. The structure and the architecture of biofilms growing on different lithic surfaces were investigated by CLSM in both fluorescence and reflection modes. Captured images were analyzed for 3D reconstructions of biofilm samples. The biovolumes were also calculated to estimate the total biomass. The results indicate that the four lithotypes showed different colonization extents. However, even in presence of extensive biological growth, chemical-physical deterioration mechanisms caused by environmental exposure were largely responsible for deterioration. A relationship between compositional and surface morphological features and biocolonization was also observed.

Exposure to artificial daylight or UV irradiation (A, B or C) prior to chemical cleaning: an effective combination for removing phototrophs from granite

This study evaluated whether exposing samples of granite colonized by a natural biofilm to artificial daylight or UV-A/B/C irradiation for 48 h enhanced removal of the biofilm with a chemical product previously approved for conservation of monuments by the European Biocide Directive. Rodas granite, which is commonly found in stone-built heritage monuments in Galicia (NW Spain), was naturally colonized by a sub-aerial biofilm. The efficacy of the cleaning method was evaluated relative to uncolonized surfaces and colonized control samples without previous irradiation, treated by dry-brushing or with benzalkonium chloride. The effect of UV irradiation in the combined treatment was evident, as comparable cleaning levels were not reached in the controls. Although the biofilm was not totally removed by any of the treatments, UV-B irradiation followed by benzalkonium chloride was potentially useful for cleaning stone, with results comparable to those achieved by UV-C irradiation, which is known to have germicidal effects.

Biofilm architecture on different substrates of an Oculatella subterranea (Cyanobacteria) strain isolated from Pompeii archaeological site (Italy)

The Cyanobacterium Oculatella subterranea Zammit, Billi, Albertano inhabits hypogea and stone caves and is a pioneer of different stone substrata. In this study, a strain isolated from the House of Marco Castricio (Archaeological Park of Pompeii, Italy) was identified by a polyphasic approach and used for an in vitro colonization test to verify the influence of the substrate on the biofilm architecture. Fine structure of O. subterranea microbial mats was revealed as well as filaments orientation toward light source. This aim has been achieved through confocal laser scanner microscope microscopy and computer image analysis. Moreover, bioreceptivity of five different substrates, commonly retrieved in archaeological sites of Campania, was assessed for O. subterranea. Our results show that the three-dimensional structure of O. subterranea microbial mats is poorly affected by physical and geochemical features of substrates: in fact, the porous architecture of its biofilm was preserved, independently of the materials. On the other hand, the area/perimeter ratio relative to the O. subterranea growth on tuff, brick, and porphyry showed significant differences, indicating dissimilar levels of bioreceptivity of the three substrates.

Bioreceptivity index for granitic rocks used as construction material

Bioreceptivity is a fundamental concept in the ornamental stone industry and in the fields of cultural heritage and civil engineering to understand the susceptibility of stone constructions to biological colonisation and subsequent biodeterioration. However, a bioreceptivity index (BI) has not yet been established for any construction material. The aim of the present study is developing a simple, robust and well-founded BI for granitic rocks. For this purpose, a standardised laboratory protocol was used to grow phototrophic biofilms on several varieties of granite. The colonisation was then assessed by chlorophyll fluorescence and colour measurements. Based on the results thus obtained, a BI including two components (BI_growth and BI_colour) is proposed. BI_growth quantifies the extent of the biological growth and BI_colour quantifies the colour change undergone by the stone due to the colonisation, which can be considered the bioreceptivity perceptible to the human eye. The values of BI, BI_growth and BI_colour were fitted to a scale of 0-10, thus enabling qualitative classification of the lithotypes according to their primary bioreceptivity. Eleven varieties of granite commonly used as construction material and with a honed surface finish (one variety with three additional surface finishes: polished, sawn and sanded) were thus assigned the corresponding BI, which represents a new quality factor for the stone industry. The index can therefore be used by end-users as a decision-making tool in the selection of appropriate lithotypes for building and/or ornamental purposes.

Inactivation of Pseudomonas aeruginosa biofilms formed under high shear stress on various hydrophilic and hydrophobic surfaces by a continuous flow of ozonated water

The inactivation of Pseudomonas aeruginosa biofilms grown on glass under high shear stress and exposed to a range of dissolved ozone concentrations (2, 5 and 7 ppm) at 10 and 20 min was investigated. The regression equation, log reduction (biofilm) = 0.64 + 0.59×(C - 2) + 0.33×(T - 10), described the dependence of biofilm inactivation on the dissolved ozone concentration (C, ppm) and contact time (T, min). The predicted D-values were 11.1, 5.7 and 2.2 min at 2, 5 and 7 ppm, respectively. Inactivation of biofilms grown on various surfaces was tested at a single dissolved ozone concentration of 5 ppm and a single exposure time of 20 min. Biofilms grown on plastic materials showed inactivation results similar to that of biofilms on glass, while biofilms grown on ceramics were statistically significantly more difficult to inactivate, suggesting the importance of utilizing non-porous materials in industrial and clinical settings.