Russian researchers show fungi break down iron-based pigments of an XVI-century icon in lab models

Specialists of the Federal Research Center of Biotechnology of the Russian Academy of Sciences studied why mold destroys some areas of old icons while neighboring areas stay intact, pravda.ru reports. The starting point was the XVI-century icon "Descent into Hell" from the Tretyakov Gallery. Two microorganisms were found on its surface: the zygomycete Syncephalastrum contaminatum and the ascomycete Cladosporium sphaerospermum pravda.rupravda.rupravda.ruOpen source ↗.

Pigment composition and fungal damage

To test how the fungi respond to different paints, the researchers made laboratory models using traditional pigments: ochre, cadmium yellow, English red, zinc white and others. The fungi reacted to the chemistry of the paints differently. Earth pigments containing iron oxides and hydroxides (red, ochre and gray-green) were quickly destroyed under the action of the zygomycete. Zinc white and cadmium-based paints were considerably more resistant to biological damage.

To understand how the fungus consumes the paint, the researchers examined its genome. Syncephalastrum contaminatum produces siderophores, which are chelating molecules. According to the study, these molecules bind iron from the pigment structure, extract it from the paint and deliver it into the fungal cells. For this fungus, iron-containing paints serve not only as a substrate but as a necessary resource.

Different strategies of two species

The second fungus, the ascomycete, does not have such tools for obtaining iron, so it destroys ochre more slowly. At the same time, it showed notable tolerance to cadmium, a heavy metal that is usually toxic to most living organisms. This indicates that different mold species use different survival strategies on a painted surface pravda.rupravda.rupravda.ruOpen source ↗.

The results show that the destruction of an icon depends not only on the organic binder, such as egg yolk or wax, but also on the mineral composition of the paints. At this stage, the dependence of the biodegradation rate on specific metals in pigments has been confirmed in laboratory conditions (in vitro) and on models (in situ).

According to the publication, restorers can use these data for targeted protection. Areas with ochre and brown tones are the most vulnerable because of the fungi's "iron diet", and they can be treated with fungicides preventively, before visible mold spots appear.