Biodeterioration aspects of the 16th century icon “Deesis tier of thirteen figures” by the fungus Iodophanus sp. STG-150: a range of materials for biodegradation and selection of target antiseptics

• Iodophanus sp. STG-150 is capable to degrade egg tempera paints. • Some mineral pigments protect tempera paints from biodeterioration. • The antiseptic benzalkonium chloride does not protect paints from studied biodamage. • Novel effective antiseptics have been found against Iodophanus sp. STG-150. Cultural heritage objects—particularly, paintings—consist of materials that can be deteriorated by specialized microorganisms. In this regard, it is imperative to both delineate their biodegradation capacity as well as select target antiseptics that can prevent this process, while remaining inert towards the painting materials. This study characterized the fungus Iodophanus sp. STG-150, belonging to the Pezizaceae family, which was isolated from bio-lesion sites of the 16th century painting “Deesis Tier of 13 Figures”—a unique habitat that has not been described before. To analyse their ability to degrade painting materials, cells of the STG-150 strain were inoculated onto crafted mock layers that were coloured with individual painting materials, including basis, adhesives, egg yolk emulsion and tempera paints, watercolour, and varnishes. STG-150 was able to actively grow on basis (pavoloka), adhesives (sturgeon glue and mordant), egg yolk emulsion, and ochre-pigment. Contrastingly, moderate growth was observed on watercolour black and varnishes (linseed oil, acrylic varnish). The addition of cobalt green, strontian yellow, cadmium red, and grey-green to the egg yolk resulted in significant antifungal resistance. Furthermore, the addition of zinc white and burnt sienna completely inhibited STG-150 growth. Using scanning electron microscopy (SEM), it was observed that STG-150 produced two morphological forms of mycelium on the egg yolk emulsion and ochre-pigment—vegetative and aerial with conidia. All other art materials revealed only vegetative mycelium. Fourier-transform infrared spectroscopy (FTIR) also detected areas of STG-150 growth at the periphery of the inoculation site that were invisible to the eye. Mineral pigments as an antiseptic cannot be used to protect paintings from biodegradation, as they introduce unwanted colouration and degrade the original colour. Consequently, a targeted selection of new generation antiseptics were developed for the protection of paintings against STG-150: (i) nucleoside derivatives: N 4 -dodecyl-5-methyl-2′-deoxycytidine (Ala 54), N 4 -dodecyl-5-methylcytidine (Ala 106), 3′-amino- N 4 -dodecyl-5-methyl-2′,3′-dideoxycytidine (SOV4); (ii) sulphur-containing heterocyclic compounds: 3,5-dinitropyridin-2-yl thiocyanate – (М1), 4-nitro-2,1,3-benzothiadiazol-5-yl thiocyanate (M2), 3-cyano-5-nitropyridin-2-yl thiomorpholine-4-carbodithioate (M3), and ethyl 1‑hydroxy-6-thioxo-1,6-dihydropyridine-3-carboxylate (М4); as well as (iii) H -phosphinic amino acid analogues: 1-Aminoethyl- H -phosphinic acid (Ala-pH), l -amino-2-methylpropyl- H -phosphinic acid (Val-pH), 1-amino-3-methylthiopropyl- H -phosphinic acid (Met-pH), and l -amino-2-carboxyethyl- H -phosphinic acid (Asp-α-pH). Their effects completely suppressed the growth of Iodophanus sp. STG-150. Further exploration of the antifungal activity and compatibility of these successful biocides with paints and varnishes should be undertaken in future studies on mock layers.