Fungal transformation of mineral substrata of biodeteriorated medieval murals in Saint Sophia's cathedral, Kyiv, Ukraine.

Microbial activity following invasion of human-made structures and artifacts can have profound social and economic consequences including the permanent loss of cultural heritage. The unique frescoes in the 11th century Saint Sophia's Cathedral (Kyiv, Ukraine) have recently suffered from dark-spot biodeterioration. The aim of this work was to elucidate the microbial nature of biodeterioration and the biogeochemical processes occurring in the areas of the dark spots. Culture-independent approaches including scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), micro-X-ray diffraction and real-time quantitative polymerase chain reaction (qPCR) analysis were used in this study. SEM and qPCR data demonstrated that the main agents of fresco biodeterioration were mycelial fungi, with bacteria unlikely to play a major role in the development of the dark spots. SEM-EDS results showed that fungi colonization of the dark spotted areas resulted in mechanical and chemical weathering involving dissolution of mineral components of the plaster (mainly calcite) and displacement of mineral grains, which compromise the stability of the plaster or fresco. SEM-EDS also detected fungal biomineralization of secondary mycogenic minerals: calcium malate, hydrated aluminium and ferric phosphates. Biomineralization of calcium malate by fungi, as found in this study, is a rare biogeochemical phenomenon, possibly linked to the presence of calcite and nitrogen limitation. • 11th century frescoes in Saint Sophia's Cathedral (Kyiv, Ukraine) were affected by dark spot biodeterioration. • The main agents causing fresco defacement were filamentous fungi. • Fungi acted on fresco plaster through biomechanical and biochemical mechanisms. • Fungal activity resulted in dissolution of mineral components of the plaster, e.g. calcite. • Calcium malate, hydrated aluminum and ferric phosphate minerals were formed by fungi. [ABSTRACT FROM AUTHOR]