[Formation of organic acids by fungi isolated from the surface of stone monuments]

Capacity of the fungi isolated from the surface of stone monuments for acid formation was studied in cultures under various carbon sources and cultivation conditions. The composition of organic nutrients was adjusted according to the results of investigation of the surface layers from the monuments in urban environment. The primary soil formed at the surface of the stone monuments under urban conditions was shown to contain a variety of carbon and nitrogen sources and is a rich substrate for fungal growth. Oxalic acid was produced by fungi grown on media with various concentrations of sugars, sugar alcohols, and organic acids. Malic, citric, fumaric, and succinic acids were identified only at elevated carbohydrate concentrations, mostly in liquid cultures. Oxalic acid was the dominant among the acids produced by Aspergillus niger at all experimental setups. Unlike A. niger, the relative content of oxalic acid produced by Penicillium citrinum decreased at high carbohydrate concentrations.