Organic acids induce tolerance to zinc- and copper-exposed fungi under various growth conditions.

Heavy metals, Zn and Cu, in high concentration (2 mM for Zn and 0.5 mM for Cu) have some inhibiting effect on the growth of Aspergillus niger and Penicillium citrinum. Toxic effects of these metals considerably depend on cultivation conditions including nitrogen sources, pH of nutrient media, and its consistency (presence or absence of agar). In general, nitrate media provides less inhibiting effect on fungal growth under heavy metal exposure than ammonium-containing media. Adding of Zn in nitrate media induces oxalic acid production by fungi. Importance of oxalic acid production in detoxification of heavy metals is confirmed by the formation of Zn-containing crystals in fungal cultures. Cu bringing to the cultural media had no stimulating effect on oxalic acid production as well as no copper-containing crystals were observed. But proceeding from essential increase in oxalic acid production during a long-term fungi adaptation to Cu, it may be proposed that oxalic acid plays some functional role in Cu tolerance of fungi as well. It may be concluded that the role of organic acids and oxalate, in particular, in fungi tolerance and adaptation to heavy metals can be determined by the nature of the metal and its ability to form stable complexes with an acid anion. Stimulating effect of metals on acid production is not universal for all species of fungi and largely depends on metal concentration, nitrogen form in a medium, and other cultivation conditions.