Aspergillus niger is the leading workhorse for the production of citric acid, an organic acid, that is extensively used in food and pharmaceutical processes. However, the carbon yield of this acetyl-CoA derived product is capped to 67%. The decarboxylation of pyruvate, the precursor of acetyl-CoA, loses a carbon equivalent in form of a CO2 molecule. In addition, another enzymatic step in the oxidative pentose phosphate pathway requires the emission of CO2 when the metabolite ribulose-5-phosphate is formed. We aim to design a carbon conserving pathway and increase acetyl-CoA alternatively to achieve a more carbon efficient and sustainable production of the target metabolite citric acid. Therefore, the enzymatic step that connects glycolysis and the pentose phosphate pathway is engineered and downregulated to avoid excessive CO2 emissions. On the other hand, putative phosphoketolases that convert sugars to acetyl phosphate, the precursor of acetyl-CoA, are screened for their enzymatic activity on diverse sugar substrates. The hypothesis to be tested is that by combining the downregulation of the pentose phosphate pathway as well as the overexpression of an acetyl phosphate generating phosphoketolase leads to a reduction of CO2 loss and hence, a more carbon efficient biocatalyis of citric acid in A. niger.