ИСТИНА

Fungi are essential members of lithobiont communities playing one of the leading roles in mineral substrates deterioration (Gadd, 2017). During our survey of fungi colonizing mineral building materials (i.e., limestone and plaster) inside the cultural heritage sites in Russia, we have isolated verticillium-like micromycete that we have described as Lecanicillium gracile V.B. Ponizovskaya, A.A. Grum-Grzhim., Georgieva & Bilanenko (Cordycipitaceae, Ascomycota). The fungus was obtained in high amounts (104–105 CFU/g) that reflected its active development on these substrates. The development of fungi over a period of time may be determined by a combination of various factors (Warscheid and Braams, 2000). Among them water availability and external pH are the most important climatic conditions on indoor mineral substrates (Verdier et al., 2014; Li et al., 2020). We have attempted to evaluate mechanisms of functioning of L. gracile on mineral materials inside cultural monuments and to reveal its possible participation in biodeterioration process. For this purpose, dependence of the growth rate of L. gracile on parameters of water activity (aw) and pH of substrate were studied. Then, composition of metabolites in the mycelium and in the culture fluid of L. gracile was characterized. Finally, quantitative analysis of the extracellular polymer matrix (EPS) excreted by the fungus was performed. Growth tests at different aw values have revealed that L. gracile grew best at maximum aw studied (about 0.99 aw), dramatically reduced growth rate at 0.95 aw and did not grow at 0.9 aw. Then, L. gracile successfully developed at extremely wide pH range (i.e., pH 4–10) and was alkalitolerant. We believe that this character gives to L. gracile an advantage in colonization of limestone and plaster and proliferating there for a long period in the conditions of long-lasted material humidification as pH of these materials inside cultural heritage sites may vary from slightly acidic to alkaline (pH 6–9) (Verdier et al., 2014; Ponizovskaya et al., 2019). While developing on stone materials, mycelium penetration into the substrate causes mechanical destabilization of the mineral structure (Warscheid, Braams, 2000). In the exponential growth phase, the diversity of metabolites in the mycelium was low. In the stationary growth phase, L. gracile possessed high enzymatic activity as the diversity of metabolites in its mycelium was high. It is known that members of Lecanicillium genus are entomogenous and fungicolous (Zare & Gams 2001). It is proposed that verticillium-like species could decompose chitin in insect remains and fungi on indoor mineral substrates (Gorbushina & Petersen 2000). Studies of the culture fluid have shown that L. gracile excreted EPS and changed pH of the medium to an alkaline. EPS contribute to biofilm formation, can protect from adverse environmental factors (Grum-Grzhimaylo et al., 2016), make biocidal treatment ineffective (Sand, 1997) and destabilize mineral matrix (Gadd, 2007). Moreover, under alkaline conditions EPS excretion leads to secondary calcite precipitation in the case of fungal development on Ca-containing materials (i.e. limestone, marble) (Sazanova et al., 2020).