Citric acid fermentation in stainless steel bioreactors is highly sensitive to the presence of manganese ions (Mn²⁺), which may unintentionally leach from reactor surfaces under acidic conditions. Even trace amounts of Mn²⁺ can significantly alter the physiology and productivity of Aspergillus niger. The primary aim of this thesis was to investigate how varying Mn²⁺ concentrations, different supplementation time points, and pH-dependent metal leaching influence the performance of citric acid fermentation, with particular focus on the A. niger NRRL 2270 strain. The results demonstrated that manganese-limited conditions (5 µg/L Mn²⁺) consistently supported the highest citric acid yields, as evidenced by elevated Yp/s values and continuous citrate accumulation. In contrast, the addition of external Mn²⁺, especially during the early stages of fermentation, significantly reduced productivity. Although later Mn²⁺ supplementation (48–172 h) partially alleviated this inhibitory effect, none of the concentrations tested were able to match the performance observed under manganese-limited conditions. The pH-controlled leaching experiments revealed a strong pH dependence of Mn²⁺ release from stainless steel components. Below pH 2.0, leaching intensified markedly, and Mn²⁺ concentrations increased to levels known to inhibit citric acid production. This finding is particularly relevant from a technological perspective, as the continuous acidification during fermentation naturally increases the risk of metal contamination over time. Overall, the results indicate that the stability and productivity of citric acid fermentation on an industrial scale depend heavily on minimizing manganese contamination, maintaining stringent pH control, and ensuring proper surface treatment of bioreactors. The study highlights that manganese affects the fermentation process through a combination of morphological, metabolic, and kinetic mechanisms, collectively influencing final product yield. These findings contribute to the technological optimization of citric acid production and provide a foundation for future improvements aimed at reducing Mn²⁺ contamination and increasing fermentation robustness.