Citric acid is an organic molecule with the IUPAC name 2-hydroxypropane-1,2,3-tricarboxylic acid. It is utilized in a wide variety of industries, leading to an increase in demand for it. Thus, optimizing its production is important, and scientists are always trying to find the most efficient way of production. Citric acid is primarily produced through fermentation, using the fungus Aspergillus niger as the main microorganism in this process. The pH value is one of the many factors that influence the production of citric acid. In my diploma thesis, I analyze the tolerance of Aspergillus niger fungus at different pH values. The same experiment was conducted twice: once in the presence of manganese ions, and the other time under manganese ion deficiency. Based on the obtained results, the presence of manganese at low pH values results in higher quantities of citric acid, a lower usage of D-glucose, and the presence of pellet morphology. These results are very promising as they open up a new door in science that could serve as the beginning of major innovations.