Effect of iron on the activity of aconitate hydratase and synthesis of citric acid by Aspergillus niger.

Two strains of A. niger were cultivated on beet molasses medium by the surface method, and the effects of iron on the mycelium formation, the rate of sugar utilization, the activity of aconitate hydratase, and synthesis of citric acid were determined. Among various sources of di- and trivalent iron, ferrous sulphate was chosen for studies, which most effectively limited the synthesis of organic iron, ferrous sulphate was chosen for studies, which most effectively limited the synthesis of organic acids. The molasses solutions were purified with potassium ferrocyanide and FeSO4 at a concentration of 20 mg/100 ml (in terms of Fe2+) was added. It was found in both strains that such an amount of Fe2+, additionally introduced into the molasses medium, only slightly effected the synthesis of the mycelium biomass and the rate and extent of sugar utilization, when at the same time a high increase in the activity of aconitate hydratase occurred. Citric acid production, however, was strongly inhibited by this Fe2+-concentration in case of A. niger 23 (about 94%), whereas A. niger Lcz showed a relatively small decrease (about 20%).