Abstract The production of organic acids by fungi has profound implications for metal speciation and biogeochemical cycles. Metal-complexing properties of organic acids, e.g. citric and oxalic acid, assists essential metal and anionic (e.g. phosphate) nutrition of fungi, other microorganisms and plants, and affects metal speciation and mobility in the environment, including transfer between terrestrial and aquatic habitats, biocorrosion and weathering. Metal solubilization processes also have potential for metal recovery from contaminated solid wastes, soils and low-grade ores. Such ‘heterotrophic leaching’ can occur by several mechanisms but organic acids occupy a central position in the overall process supplying both protons and metal-complexing organic acid anions, e.g. citrate. Most simple metal oxalates (except those of alkali metals, Fe(III) and Al) are sparingly soluble and precipitate as crystalline or amorphous solids. Calcium oxalate is the most important oxalate in the environment and is ubiquitously associated with free-living, symbiotic and pathogenic fungi. The main forms are the monohydrate (whewellite) and the dihydrate (weddelite) and their formation affects nutritional heterogeneity in soil, especially that of Ca, P, K and Al, while in semi-arid environments, calcium oxalate formation is important in the development of terrestrial subsurface limestones. The formation of insoluble toxic metal oxalates, e.g. Cu, may ensure fungal survival in the presence of elevated metal concentrations.

Abstract This study deals with the design of two hydrophobic hybrid SiO 2 –TiO 2 materials and their application in the field of monument conservation. The nanocomposite consolidants were based on the modification of tetraethoxysilane (TEOS) with the incorporation of both TiO 2 nanoparticles and hydroxyl-terminated polydimethylsiloxane (PDMS). The TiO 2 nanoparticles have been prepared from the hydrolysis of titanium tetraisopropoxide (TTIP) in the presence of oxalic acid as a catalyst. The physico-chemical properties of the designed hydrophobic nanocrystalline SiO 2 –TiO 2 –PDMS composites have been extensively studied before their application as consolidants on a limestone. Overall, the effectiveness of the nanocomposites was evaluated by the comparative characterization of untreated and treated stone specimens. The results of both capillary water absorption and contact angle measurements point out that the TiO 2 –SiO 2 –PDMS nanocomposites functioned as hydrophobic coatings. Furthermore, the results obtained from microdrilling resistance and Fourier Transform Infrared spectroscopy (FTIR) revealed a penetration depth of the nanocomposites within the stone of c. 15 mm. The innovation of these syntheses pertains to the application of hydrophobic semi-transparent and transparent nanocomposites with self-cleaning and strengthening properties, without altering the colour of the stone surface and the water vapour permeability.