reported that the addition of 10 mg/l Mn2+ leads to a reduction
in citric acid accumulation by 50%. Researches by Clark et al.
(11) and Kisser et al. (35) confirmed the regulatory role of
Mn2+ ions. The anabolism of A. niger is affected by manganese
deficiency and/or limitation by nitrogen and phosphate. The
breakdown of proteins under Mn deficiency results in high
intracellular concentration of ammonium, which contrasts the
citric acid inhibition of phosphofructokinase and hence
glycolysis. Moreover, the combination of high concentrations
of glucose and ammonium represses the synthesis of -
ketoglutarate dehydrogenase, hence inhibiting the catabolism
of citric acid in the Krebs cycle and favoring its
overproduction.