This thesis investigates the role of the second alternative oxidase gene, aoxB, in Aspergillus niger, one of the most researched filamentous fungi for industrial citric acid production. The hypothesis states that a wild-type strain with functional aoxB and aoxA is a more productive strain than a mutant strain with a non-functional aoxB. A series of fermentation experiments, morphology experiments and growth characterization experiments in submerged and solid state revealed that aoxB presence does not significantly impact productivity under acid production settings or submerged fermentation morphology but does impact metabolic diversity in low carbon conditions. The aoxB mutant strain exhibited varying colony morphology, reduced radial growth on solid state media, and increased biomass generation at the cost of citric acid. Therefore, aoxB does not play a significant role in citric acid production but instead in continued generation and survival under stressful conditions. These results bear significance to those studying mitochondrial respiratory diversity as well as applicability to biotechnological endeavours in the field.