Transporter Engineering for Enhancing Citric Acid Production in Aspergillus niger

The efficient industrial production of citric acid by A. niger requires overcoming the limitations of substrate uptake and citrate export on the citrate synthesis efficiency. This study addresses these obstacles using a transporter engineering strategy, modifying the endogenous high-affinity glucose transporter MstF and citrate exporter CexA. The “push–pull” strategy was used to improve citric acid production by increasing glucose import and citrate export. A single overexpression of mstF improved citric acid production, reaching 179.35 g/L in the H7 strain. However, cexA high expression impaired dense mycelium pellet formation and affected the expression of key genes, resulting in reduced citric acid production. For balancing intracellular accumulation and secretion of citrate, simultaneous overexpression of mstF and cexA increased citric acid production and efficiency. In a 30 L fermenter, strain A5 achieved a citric acid titer of 185.91 g/L, a productivity of 3.21 g/h/L, and a shortened fermentation cycle. Collectively, these results provide a reference for the industrial production of citric acid and other organic acids.