Organic acid accumulation is probably the best-known example of primary metabolic
overflow. Both bacteria and fungi are capable of producing various organic acids in
large amounts under certain condi- tions, but in terms of productivity – and consequently,
of commercial importance – fungal platforms are unparalleled. For high product yield,
chemical composition of the growth medium is crucial in providing the necessary conditions,
of which the concentrations of four of the first- row transition metal elements, manganese
(Mn2+), iron (Fe2+), copper (Cu2+) and zinc (Zn2+) stand out. Three of them – Mn,
Fe, Cu – provide the necessary redox and catalytic activity for many important biologi-
cal process-es. They possess a stable +2 oxidation state and can gen- erate many additional
stable states, which allows them to change their oxidation states in biological reactions.
Manganese concentrations > 5 μg/L decrease the final yield of citric acid in A. niger
and itaconic acid in A. terreus, respectively. Various methods have therefore been
patented or published to remove the surplus manganese ions from the growth medium,
but a more convenient strategy is to counteract their effect. Both for A. niger citric-
and A. terreus itaconic acid fermenta- tions, low product yield in the presence of
high Mn-concentrations can be counteracted by increasing the copper concentration
in the culture broth. We recently described that the ratio of copper to manganese
– rather than their respective absolute concentration – modulates itaconic acid production
yield on D-glucose and D-fructose. In this study we demonstrate that the high-affinity
Mn2+ transport in A. niger is inhibited – in addition to copper – also by Zn2+ roughly
to the same extent.
