Daniel Vázquez-Nion

Spectrophotometric color measurement for early detection and monitoring of greening on granite buildings

This paper addresses the detection and monitoring of the development of epilithic phototrophic biofilms on the granite façade of an institutional building in Santiago de Compostela (NW Spain), and reports a case study of preventive conservation. The results provide a basis for establishing criteria for the early detection of phototrophic colonization (greening) and for monitoring its development on granite buildings by the use of color changes recorded with a portable spectrophotometer and represented in the CIELAB color space. The results show that parameter b* (associated with changes of yellowness-blueness) provides the earliest indication of colonization and varies most over time, so that it is most important in determining the total color change. The limit of perception of the greening on a granite surface was also established in a psycho-physical experiment, as Δb*: +0.59 CIELAB units that correspond, in the present study, to 6.3 μg of biomass dry weight cm−2 and (8.43 ± 0.24) × 10−3 μg of extracted chlorophyll a cm−2.

Subaerial biofilms on granitic historic buildings: microbial diversity and development of phototrophic multi-species cultures.

Abstract Microbial communities of natural subaerial biofilms developed on granitic historic buildings of a World Heritage Site (Santiago de Compostela, NW Spain) were characterized and cultured in liquid BG11 medium. Environmental barcoding through next-generation sequencing (Pacific Biosciences) revealed that the biofilms were mainly composed of species of Chlorophyta (green algae) and Ascomycota (fungi) commonly associated with rock substrata. Richness and diversity were higher for the fungal than for the algal assemblages and fungi showed higher heterogeneity among samples. Cultures derived from natural biofilms showed the establishment of stable microbial communities mainly composed of Chlorophyta and Cyanobacteria. Although most taxa found in these cultures were not common in the original biofilms, they are likely common pioneer colonizers of building stone surfaces, including granite. Stable phototrophic multi-species cultures of known microbial diversity were thus obtained and their reliability to emulate natural colonization on granite should be confirmed in further experiments.

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.

The susceptibility of weathered versus unweathered schist to biological colonization in the Côa Valley Archaeological Park (north-east Portugal)

This study addresses the primary and secondary bioreceptivity of schist used as a support for prehistoric rock art in the Côa Valley Archaeological Park (north-east Portugal) and provides some parameters that can be related to the risk of biologically induced schist weathering. Samples of freshly quarried and naturally weathered schist were characterized in terms of their intrinsic properties and maintained in controlled environmental conditions after inoculation with biofilm-forming cyanobacteria. The physical properties of the studied schist, as well as its abrasion pH, all varied according to the weathering degree of the samples and so did its susceptibility to colonization by biofilm-forming cyanobacteria. Complete separation between weathered and unweathered schist samples in terms of laboratory-induced photosynthetic biomass was obtained by measuring total colour change in the CIE (International Commission on Illumination) L*a*b* colour space. Weathered schist was more bioreceptive than unweathered schist, associated with increased open porosity, water saturation, capillary water and capillarity coefficient and decreased abrasion pH. In the future, it might be possible to determine the susceptibility of schist surfaces to biological colonization through evaluation of colour differences associated with the different weathering degrees presented by those surfaces prior to colonization.

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

Abstract 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.

Acidification of mixtures of granite powder and compost for reuse in plant production

ABSTRACT We evaluated the utilization of granite powder—a residue of stone cutting and polishing resulting from its preparation for construction and ornamental use—in the preparation of nursery potting mixtures, within a general objective of waste recycling and reuse. Granite powder was blended with two composts of different origins and properties: one elaborated from the organic fraction of municipal solid waste (MSW), and another from pine bark. Since all materials presented pH values over 8, which are excessive for growing most vegetal species, the use of iron sulphate to acidify them has been assessed. Three doses of iron sulphate (14, 69, and 97 g kg−1, equivalent to 0.1, 0.5, and 0.7 meq H+ g−1) were added to the granitic powder and mixtures with 25 or 75% (v/v) of each compost, which were incubated in the laboratory during 30 days. Doses of iron sulphate above 0.1 meq H+ g−1 resulted in excessive electrical conductivity (>2 dS m−1) in all the samples, and too low pH values (<5) in most of them. The productivity of granite powder/compost mixtures acidified with 0.1 meq H+ g−1 was tested in a greenhouse experiment with ryegrass, where it was observed that plant productivity increased with respect to a control commercial substrate based on peat, in particular when the MSW compost was employed. On the basis of the physicochemical properties of the mixtures and the results of the greenhouse experiment, the use of granite powder mixed with 75% of MSW compost (v/v), acidified with 14 g kg−1 of iron sulphate (0.1 meq H+ g−1) is recommended.

Response surface optimization of a method for extracting extracellular polymeric substances (EPS) from subaerial biofilms on rocky substrata

AbstractThe aim of the present study was to optimize a protocol for extracting extracellular polymeric substances (EPS) from biofilms on rocky substrata, as the EPS matrix is considered key to understanding the biofilm mode of life. For this purpose, we tested the extraction efficacy of NaOH and H2SO4 at different concentrations, temperatures and times for obtaining EPS from multi-species subaerial biofilms grown on granite blocks under laboratory conditions. Two experimental designs (Box-Behnken design and full factorial design) were used in testing each extractant. The extraction efficiency was determined by analysing the carbohydrate, protein and DNA contents of the extracts obtained. H2SO4 proved unsuitable as an extractant as it caused excessive cell lysis. However, response surface optimization of NaOH-mediated extraction enabled cell lysis to be minimized. Confirmation experiments were performed under the optimal conditions established and a protocol for extracting EPS is proposed, yielding the first quantitative data on EPS extracted from subaerial biofilms developed on rocky substrata. Graphical abstractDevelopment of a method for extracting EPS from subaerial biofilms on rocky substrata

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.