Biochemical studies of citric acid production and accumulation by Aspergillus niger mutants

The biochemical rationale for the inhibition of citric acid fermentation by Aspergillus niger in the presence of Mn2+ ions has been investigated using high citric acid-yielding, Mn2+ ion-sensitive as well as Mn2+ ion-tolerant mutant strains of A. niger. In the presence of Mn2+ (1.5 mg/l), citric acid production by the Mn2+ ion-sensitive strain (KCU 520) was reduced by about 75% with no apparent effect on citric acid yield by the Mn2+ ion-tolerant mutant strain (GS-III) of A. niger. The significantly increased level of the Mn2+ ion-requiring NADP(+)-isocitrate dehydrogenase activity in KCU 520 cells and the lack of effect on the activity level of the enzyme in GS-III mutant cells by Mn2+ ions during fermentation seem to be responsible for the Mn2+ ion inhibition of citric acid production by the KCU 520 strain and the high citric acid yield by the mutant strain GS-III of A. niger even in the presence of Mn2+.