Extracellular Phosphate Availability Impacts Aspergillus terreus Itaconic Acid Fermentation via Biomass-Specific Product Yield

Itaconic acid (IA) is an important bio-based platform chemical produced via submerged fermentation by the filamentous Ascomycete <i>Aspergillus terreus</i>. In this study, we examined the impact of initial phosphate concentration on IA production from D-glucose and D-xylose in optimized, manganese-limited fermentations. Nine phosphate concentrations ranging from 0.04 to 4 g L^-1 were tested, and representative low (0.04 g L^-1), optimal (0.1 g L^-1), and high (0.8 g L^-1) conditions were analyzed in detail in controlled, 6 L scale bioreactors. Phosphate availability primarily influenced biomass formation and the biomass-to-product ratio rather than directly affecting IA accumulation. Both lower- and higher-than-optimal phosphate concentrations decreased the volumetric and specific IA yields, while the highest productivity was observed at 0.1 g L^-1. Expression of the <i>aoxA</i> gene, encoding the cyanide-resistant alternative oxidase (AOX), and AOX enzymatic activity were inversely correlated with extracellular phosphate concentration, consistent with a role in redox homeostasis under phosphate-limited conditions. In contrast, total respiration rates and pellet-type morphology remained unaffected. These findings indicate that phosphate acts mainly as a secondary modulator of IA fermentation performance through its influence on biomass formation, whereas other metabolic constraints play a more dominant role in controlling IA overflow in <i>A. terreus</i>.