Primary bioreceptivity of granitic rocks to phototrophic biofilms. Development of a bioreceptivity index

  1. Vázquez Nion, Daniel
Dirixida por:
  1. Beatriz Prieto Lamas Director
  2. Benita María Silva Hermo Co-director

Universidade de defensa: Universidade de Santiago de Compostela

Fecha de defensa: 25 de novembro de 2016

  1. Ana Zélia Miller Presidente/a
  2. Graciela Paz Bermúdez Secretario/a
  3. Nuria Sánchez Delgado Vogal
  1. Departamento de Edafoloxía e Química Agrícola

Tipo: Tese


The main objective of this PhD thesis was to undertake a comprehensive study of the primary bioreceptivity to phototrophic biofilms in various types of granitic rock commonly used as building material and/or ornamental stone, thus enabling development of a bioreceptivity index. The microbial communities forming the subaerial biofilms that grow naturally on several granite historic buildings in Santiago de Compostela (NW Spain) were characterised using next-generation sequencing (Pacific Biosciences) techniques for environmental barcoding. This revealed complex microbial communities mainly comprising species of Chlorophyta (green algae) and Ascomycota (fungi) that are commonly associated with rocky substrata. The estimated species richness and diversity were higher for the fungal assemblages than for algae, and fungal samples were more heterogeneous. The data supported the assumption that subaerial biofilms are ecosystems with relatively low algal diversity and that many of the species common to the biofilms are ubiquitous. Multi-species phototrophic cultures derived from biofilms comprising common pioneer colonisers of granite rocks were found to be capable of forming subaerial biofilms on granite in a clearly defined, laboratory-based experimental protocol. One of the cultures proved particularly suitable for bioreceptivity studies, mainly owing to its microbial richness, rapid adaptability to the substratum and high capacity for colonization. An inoculum capable of inducing environmental-like colonisation of granite and a standardised laboratory protocol for biofilm formation (both essential for correct assessment of bioreceptivity) were successfully developed. A method for extracting EPS from subaerial biofilms growing on stone surfaces was also designed and optimized. Application of this method revealed that the amounts of EPS produced by subaerial biofilms mainly depended on the requirements and/or characteristics of the biofilm-forming microorganisms rather than on the bioreceptivity of the substratum. Moreover, microorganisms were found to produce the amounts of EPS they require at the initial stage of establishment on the stone surface, independently of subsequent biomass development. Finally, a comprehensive evaluation of the primary bioreceptivity of several varieties of granite was carried out. Chlorophyll fluorescence analysis enabled estimation of the phototrophic biomass of subaerial biofilms formed and assessment of their photosynthetic performance. Colour measurements enabled quantification of the aesthetic impact of biofilm growth on granite surfaces, which was more closely correlated with the initial lightness of the stone than with the final biomass of the biofilm. The bioreceptivity of the granites was more strongly affected by the physical characteristics of the stones than by their chemical composition. Growth of phototrophic biofilms was strongly 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 findings of the study enabled us to develop a robust and well-founded bioreceptivity index (BI) for granitic rocks. The proposed BI has two components: BIgrowth, which quantifies the extent of the biological growth, and BIcolour, which quantifies the colour change undergone by the stone due to the colonisation and which can be considered the bioreceptivity perceptible by 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. The index can therefore be used as a decision-making tool for selection of appropriate lithotypes for building and/or ornamental purposes.