High-yield citric acid overflow requires a combination of unusual cul- ture conditions
of which the deficiency of manganese(II) ions in the growth medium is particularly
critical: concentrations >5 μg/L (= 5 ppb) reduces final citric acid yield by some
25%. In this study we demon- strated that under high-yield citric acid producing conditions
even a brand-new bioreactor releases more manganese(II) ions into the cul- ture broth
than the threshold level. The leaching of manganese(II) ions from metal surfaces does
not limit citric acid accumulation as long as it occurs in the late stages of the
fermentation. On the other hand, manganese deficiency in the first 48 hours of the
cultivation appears critical for citric acid overflow. The results imply that once
A. niger cit- ric acid overflow commences due to the special cultivation conditions,
it continues irrespective of the changing environment in the bioreac- tor. We thus
hypothetized that the genes involved in the response to manganese(II) ion deficiency
are expressed at an early stage of the cultivation. To test this hypothesis we analyzed
the transcriptome of three parallel A. niger citric acid fermentations at 24, 48 and
72 hours and identified genes which display an at least 2-fold up- or downreg- ulation
in dependence on the availability of manganese(II) ions. The expression profile of
cexA, encoding a citrate exporter that secretes citric acid from the cytosol into
the culture broth, suggests that it plays a major role in the mechanism of the manganese
effect.
