Analysis of Short-Term Responses to Hypoxia During Stirred-Tank Fermentation in Aspergillus oryzae

During fermentation in stirred-tank bioreactors (STBR), filamentous fungi are frequently exposed to hypoxic conditions. However, their responses, especially short-term ones (≤6 h), remain unclear. In this study, we performed a short-term multi-omics profiling in an Aspergillus oryzae hyphal dispersion mutant (AGΔ-GAGΔ) during a controlled transition to hypoxia (a decrease in dissolved oxygen (DO) from 10% to ≤1%) in a 4 L STBR. In transcriptome analysis, the genes encoding mitochondrial respiratory chain Complexes I–III were transiently downregulated at 1 h from DO depletion and were then upregulated, whereas those of Complex IV were upregulated immediately at the onset of hypoxia. In relation to this respiratory remodeling, we also observed an immediate induction of an alternative oxidase (AOX) gene. However, our metabolome data showed no significant change in the ATP level. This result could be explained by the upregulation of the glycolytic genes in hypoxic cultures. Fluorescence imaging revealed a transient increase in intracellular reactive oxygen species (ROS) in hypoxia, and metabolomics data revealed a decrease in the reduced glutathione/oxidized glutathione ratio in hypoxic cultures. Deletion of the AOX gene prolonged the ROS increase. Together, these data indicate that early hypoxia triggers a transient increase in oxidative stress, mitigated by antioxidant systems and mitochondrial respiratory rebalancing including an AOX-mediated bypass.