A review of the nature, role and control of lithobionts on stone cultural heritage: weighing-up and managing biodeterioration and bioprotection.

Lithobionts (rock-dwelling organisms) have been recognized as agents of aesthetic and physico-chemical deterioration of stonework. In consequence, their removal from cultural heritage stone surfaces (CHSS) is widely considered a necessary step in conservation interventions. On the other hand, lithobiontic communities, including microbial biofilms ('biological patinas'), can help integrate CHSS with their environmental setting and enhance biodiversity. Moreover, in some cases bioprotective effects have been reported and even interpreted as potential biotechnological solutions for conservation. This paper reviews the plethora of traditional and innovative methodologies to characterize lithobionts on CHSS in terms of biodiversity, interaction with the stone substrate and impacts on durability. In order to develop the best management and conservation strategies for CHSS, such diagnosis should be acquired on a case-by-case basis, as generalized approaches are unlikely to be suitable for all lithobionts, lithologies, environmental and cultural contexts or types of stonework. Strategies to control biodeteriogenic lithobionts on CHSS should similarly be based on experimental evaluation of their efficacy, including long-term monitoring of the effects on bioreceptivity, and of their environmental safety. This review examines what is known about the efficacy of control methods based on traditional-commercial biocides, as well as those based on innovative application of substances of plant and microbial origin, and physical techniques. A framework for providing a balanced scientific assessment of the role of lithobionts on CHSS and integrating this knowledge into management and conservation decision-making is presented.

Protecting books from mould damage by decreasing paper bioreceptivity to fungal attack using decoloured cell-free supernatant of Lysobacter enzymogenes C3.

AIMS: To evaluate whether decoloured cell-free supernatant of Lysobacter enzymogenes C3 can decrease paper bioreceptivity to fungal attack. METHODS AND RESULTS: To prepare colourless C3 supernatant, single-factor design and uniform design were applied. The optimum conditions with high decolouration degree and low antifungal activity loss were achieved as follows: carbon granule content 1·6% (M/V), temperature 27°C, decolouring time 1·2 h and pH 8·0. An agar plate bioassay was used to assess the antifungal activity of the decoloured supernatant against the fungal isolates obtained from contaminated books, and strong suppression was observed. Small-sacle laboratory test was further introduced, in which common book papers were artificially inoculated with the fungal isolates, and then sprayed uniformly with decoloured supernatant or water. The results showed that, after treatment, the paper showed a significantly low extent of fungal colonization and high tensile strength, and maintained the same colour before and after treatment. CONCLUSION: These results suggest that the decoloured C3 supernatant inhibits fungal growth on types of paper commonly used in books. SIGNIFICANCE AND IMPACT OF THE STUDY: Decoloured C3 supernatant could be used as a preventive agent to protect books and other paper-based items against fungal growth.

Laboratory grown subaerial biofilms on granite: application to the study of bioreceptivity.

Simulated environmental colonisation of granite was induced under laboratory conditions in order to develop an experimental protocol for studying bioreceptivity. The experimental set-up proved suitable for producing subaerial biofilms by inoculating granite blocks with planktonic multi-species phototrophic cultures derived from natural biofilms. The ability of four different cultures to form biofilms was monitored over a three-month growth period via colour measurements, quantification of photosynthetic pigments and EPS, and CLSM observations. One of the cultures under study, which comprised several taxa including Bryophyta, Charophyta, Chlorophyta and Cyanobacteria, was particularly suitable as an inoculum, mainly because of its microbial richness, its rapid adaptability to the substratum and its high colonisation capacity. The use of this culture as an inoculum in the proposed experimental set-up to produce subaerial biofilms under laboratory conditions will contribute to standardising the protocols involved, thus enabling more objective assessment of the bioreceptivity of granite in further experiments.

An integrated approach for assessing the bioreceptivity of glazed tiles to phototrophic microorganisms.

A laboratory-based methodology was designed to assess the bioreceptivity of glazed tiles. The experimental set-up consisted of multiple steps: manufacturing of pristine and artificially aged glazed tiles, enrichment of phototrophic microorganisms, inoculation of phototrophs on glazed tiles, incubation under optimal conditions and quantification of biomass. In addition, tile intrinsic properties were assessed to determine which material properties contributed to tile bioreceptivity. Biofilm growth and biomass were appraised by digital image analysis, colorimetry and chlorophyll a analysis. SEM, micro-Raman and micro-particle induced X-ray emission analyses were carried out to investigate the biodeteriorating potential of phototrophic microorganisms on the glazed tiles. This practical and multidisciplinary approach showed that the accelerated colonization conditions allowed different types of tile bioreceptivity to be distinguished and to be related to precise characteristics of the material. Aged tiles showed higher bioreceptivity than pristine tiles due to their higher capillarity and permeability. Moreover, biophysical deterioration caused by chasmoendolithic growth was observed on colonized tile surfaces.

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.

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.

Lithobiontic recolonization following cleaning and preservative treatments on the rock engravings of Valle Camonica, Italy: A 54-months monitoring.

Both the indirect control of microclimate conditions and the direct application of preservative products to contrast stone bioreceptivity may contribute to limit lithobiontic recolonization of cultural heritage surfaces after cleaning interventions. However, the priority deserved by these different preventive approaches has still been poorly evaluated, particularly in outdoor environments. This work dealt with the engraved sandstone surfaces of the National Park of Rock Engravings of Naquane (Italy, UNESCO WHS), widely colonized by lichens, mosses and a dark cyanobacterial biofilm, and thus requiring frequent cleaning interventions to preserve their legibility for visitors and scholars. In particular, post-cleaning recolonization by the different lithobionts was seasonally monitored along 54 months in different zones of an engraved outcrop, primarily differing in levels of shading, on parcels exposed to nine different conservative treatments. These included (or not) a pre-cleaning devitalization of lithobionts and the post-cleaning application of biocidal (benzalkonium chloride, plant essential oils, usnic acid) and other restoration products (nanocrystalline anatase, polysiloxane-based water repellent, ethyl-silicate-based consolidant). The combination of surface image analyses, fluorimetric and colorimetric measurements showed that mosses and the cyanobacterial biofilm rapidly recolonized all the parcels in the more shaded zone, irrespective of conservative treatments. In the other areas, recolonization significantly differed depending on the treatment. The post-cleaning application of biocides determined the best results through two vegetative seasons, but only nanocrystalline anatase and the polysiloxane-based water repellent maintained the surfaces lighter than uncleaned controls along the whole monitoring period. Recolonization primarily proceeded by the uncleaned surfaces surrounding the parcels and, at least in the examined case of lichens, did not show substantial shifts in community composition, although some nitrophytic species increased their frequency. In conclusion, the effectiveness of preservative treatments to prevent a rapid recolonization of heritage stone surfaces appeared subordinate to the presence of microenvironmental conditions less favourable to lithobionts.

Damaging and protective interactions of lichens and biofilms on ceramic dolia and sculptures of the International Museum of Ceramics, Faenza, Italy.

Although ceramic objects are an important part of the worldwide cultural heritage, few investigations on the effects of lithobiontic growth on their outdoor conservation are available in the literature. Many aspects of the interaction between lithobionts and stones are still unknown or strongly debated, as in the case of equilibria between biodeterioration and bioprotection. This paper describes research on the colonization by lithobionts on outdoor ceramic Roman dolia and contemporary sculptures of the International Museum of Ceramics, Faenza (Italy). Accordingly, the study i) characterized the mineralogical composition and petrographic structure of the artworks, ii) performed porosimetric measurements, iii) identified lichen and microbial diversity, iv) elucidated the interaction of the lithobionts with the substrates. Moreover, v) the measurements of variability in stone surface hardness and in water absorption of colonized and uncolonized areas were collected to assess damaging and/or protective effects by the lithobionts. The investigation showed how the biological colonization depends on physical properties of the substrates as well on climatic conditions of environments in which the ceramic artworks are located. The results indicated that lichens Protoparmeliopsis muralis and Lecanora campestris may have a bioprotective effect on ceramics with high total porosity and pores with very small diameters, as they poorly penetrate the substrate, do not negatively affect surface hardness and are able to reduce the amount of absorbed water limiting the water ingress. By contrast, Verrucaria nigrescens, here widely found in association with rock-dwelling fungi, deeply penetrate terracotta causing substrate disaggregation, with negative consequences on surface hardness and water absorption. Accordingly, a careful evaluation of the negative and positive effects of lichens must be carried out before deciding their removal. Regarding biofilms, their barrier efficacy is related to their thickness and composition. Even if thin, they can impact negatively on substrates enhancing the water absorption in comparison to uncolonized parts.

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.

Surface colour: An overlooked aspect in the study of cyanobacterial biofilm formation.

Cyanobacteria can grow as biofilms, communities that colonize surfaces and that play a fundamental role in the ecology of many diverse habitats and in the conversion of industrial production to green platforms. Although biofilm growth is known to be significantly affected by several characteristics, the effect of colour surface is an overlooked aspect that has not yet been investigated. In this study, we describe the effect of colour hues (white, red, blue and black) on the growth of cyanobacterial biofilms on air-exposed substrates. We measured growth, architecture, pigment production and levels of ATP and reactive oxygen species in cyanobacterial biofilms formed on different coloured substrates. The study findings demonstrate, for the first time, that the colour of a surface affects biofilm formation at the air-solid interface (with more biomass accumulating on white and red substrates than on blue and black substrates) and also alters the biofilm architecture. In addition, the roles of chromatic adaptation, phototrophic cells and reactive oxygen species as intermediates between colour sensing and biofilm response are discussed. Our results support the importance of colour as a new factor that favours surface colonization by cyanobacteria and its contribution to biofilm formation.

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.

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 (BIgrowth and BIcolour) is proposed. BIgrowth quantifies the extent of the biological growth and BIcolour 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, BIgrowth and BIcolour 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.

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

As any stone substrate is susceptible to biological colonisation, the choice of lithotype used for construction is a key strategy for preventing biodeterioration. For this purpose, a comprehensive evaluation of the primary bioreceptivity to phototrophic biofilms of eleven varieties of granitic rocks, commonly used as building material, was carried out. Blocks were inoculated with a multi-species phototrophic culture and subjected to standardised growth conditions for three months. Biofilm formation was assessed by chlorophyll (chl) fluorescence, colour measurements and extracellular polymeric substances (EPS) quantification. Relationships between the biofilm growth indicators and the properties of the different rocks studied were then analysed. Results showed that the bioreceptivity of the granites is more strongly affected by the physical characteristics of the stones than by their chemical and mineralogical properties, possibly because of the similar composition of the rocks studied. Growth of phototrophic biofilms was enhanced by high open porosity, capillary water content and surface roughness, and the bioreceptivity of weathered granites was higher than that of sound granites. The results obtained can therefore help in the selection of appropriate lithotypes for building purposes. The amounts of EPS produced by subaerial biofilms primarily depended on the requirements and/or characteristics of the biofilm-forming microorganisms, rather than on the bioreceptivity of the substratum, and microorganisms produce the amounts of EPS required at the initial stage of establishment on the stone surface, independently of the subsequent biomass development. These findings are especially important from the point of view of biodeterioration, in which the EPS matrix plays a central role.

Optimising the bioreceptivity of porous glass tiles based on colonization by the alga Chlorella vulgaris.

Green façades on buildings can mitigate greenhouse gas emissions. An option to obtain green facades is through the natural colonisation of construction materials. This can be achieved by engineering bioreceptive materials. Bioreceptivity is the susceptibility of a material to be colonised by living organisms. The aim of this research was to develop tiles made by sintering granular waste glass that were optimised for bioreceptivity of organisms capable of photosynthesis. Tiles were produced by pressing recycled soda-lime glass with a controlled particle size distribution and sintering compacted samples at temperatures between 680 and 740°C. The primary bioreceptivity of the tiles was evaluated by quantifying colonisation by the algae Chlorella vulgaris (C. vulgaris), which was selected as a model photosynthetic micro-organism. Concentrations of C. vulgaris were measured using chlorophyll-a extraction. Relationships between bioreceptivity and the properties of the porous glass tile, including porosity, sorptivity, translucency and pH are reported. Capillary porosity and water sorptivity were the key factors influencing the bioreceptivity of porous glass. Maximum C. vulgaris growth and colonisation was obtained for tiles sintered at 700°C, with chlorophyll-a concentrations reaching up to 11.1±0.4µg/cm(2) of tile. Bioreceptivity was positively correlated with sorptivity and porosity and negatively correlated with light transmittance. The research demonstrates that the microstructure of porous glass, determined by the processing conditions, significantly influences bioreceptivity. Porous glass tiles with high bioreceptivity that are colonised by photosynthetic algae have the potential to form carbon-negative façades for buildings and green infrastructure.

Predicting the long-term durability of hemp-lime renders in inland and coastal areas using Mediterranean, Tropical and Semi-arid climatic simulations.

Hemp-based composites are eco-friendly building materials as they improve energy efficiency in buildings and entail low waste production and pollutant emissions during their manufacturing process. Nevertheless, the organic nature of hemp enhances the bio-receptivity of the material, with likely negative consequences for its long-term performance in the building. The main purpose of this study was to study the response at macro- and micro-scale of hemp-lime renders subjected to weathering simulations in an environmental cabinet (one year was condensed in twelve days), so as to predict their long-term durability in coastal and inland areas with Mediterranean, Tropical and Semi-arid climates, also in relation with the lime type used. The simulated climatic conditions caused almost unnoticeable mass, volume and colour changes in hemp-lime renders. No efflorescence or physical breakdown was detected in samples subjected to NaCl, because the salt mainly precipitates on the surface of samples and is washed away by the rain. Although there was no visible microbial colonisation, alkaliphilic fungi (mainly Penicillium and Aspergillus) and bacteria (mainly Bacillus and Micrococcus) were isolated in all samples. Microbial growth and diversification were higher under Tropical climate, due to heavier rainfall. The influence of the bacterial activity on the hardening of samples has also been discussed here and related with the formation and stabilisation of vaterite in hemp-lime mixes. This study has demonstrated that hemp-lime renders show good durability towards a wide range of environmental conditions and factors. However, it might be useful to take some specific preventive and maintenance measures to reduce the bio-receptivity of this material, thus ensuring a longer durability on site.

Evaluation of natural colonisation of cementitious materials: effect of bioreceptivity and environmental conditions.

Incorporation of living organisms, such as photosynthetic organisms, on the structure envelope has become a priority in the area of architecture and construction due to aesthetical, economic and ecological advantages. Important research efforts are made to achieve further improvements, such as for the development of cementitious materials with an enhanced bioreceptivity to stimulate biological growth. Previously, the study of the bioreceptivity of cementitious materials has been carried out mainly under laboratory conditions although field-scale experiments may present different results. This work aims at analysing the colonisation of cementitious materials with different levels of bioreceptivity by placing them in three different environmental conditions. Specimens did not present visual colonisation, which indicates that environmental conditions have a greater impact than intrinsic properties of the material at this stage. Therefore, it appears that in addition to an optimized bioreceptivity of the concrete (i.e., composition, porosity and roughness), extra measures are indispensable for a rapid development of biological growth on concrete surfaces. An analysis of the colonisation in terms of genus and quantity of the most representative microorganisms found on the specimens for each location was carried out and related to weather conditions, such as monthly average temperature and total precipitation, and air quality in terms of NOx, SO2, CO and O3. OPC-based specimens presented a higher colonisation regarding both biodiversity and quantity. However, results obtained in a previous experimental programme under laboratory conditions suggested a higher suitability of Magnesium Phosphate Cement-based (MPC-based) specimens for algal growth. Consequently, carefully considering the environment and the relationships between the different organisms present in an environment is vital for successfully using a cementitious material as a substrate for biological growth.

Bioreceptivity evaluation of cementitious materials designed to stimulate biological growth.

Ordinary Portland cement (OPC), the most used binder in construction, presents some disadvantages in terms of pollution (CO2 emissions) and visual impact. For this reason, green roofs and façades have gain considerable attention in the last decade as a way to integrate nature in cities. These systems, however, suffer from high initial and maintenance costs. An alternative strategy to obtain green facades is the direct natural colonisation of the cementitious construction materials constituting the wall, a phenomenon governed by the bioreceptivity of such material. This work aims at assessing the suitability of magnesium phosphate cement (MPC) materials to allow a rapid natural colonisation taking carbonated OPC samples as a reference material. For that, the aggregate size, the w/c ratio and the amount of cement paste of mortars made of both binders were modified. The assessment of the different bioreceptivities was conducted by means of an accelerated algal fouling test. MPC samples exhibited a faster fouling compared to OPC samples, which could be mainly attributed to the lower pH of the MPC binder. In addition to the binder, the fouling rate was governed by the roughness and the porosity of the material. MPC mortar with moderate porosity and roughness appears to be the most feasible material to be used for the development of green concrete walls.