Industrial-scale microbial fermentation processes often face limitations in mixing and mass transfer, leading to the formation of environmental gradients within the bioreactor. These gradients expose microbes to heterogeneous conditions over time and space. In this study, we evaluated the effects of combined substrate and dissolved oxygen (DO) gradients on the metabolic response of Penicillium chrysogenum at an industrial scale. Three representative heterogeneous environments were simulated in scale-down systems: (1) feed inlet (high glucose, low oxygen (HGLO): CS > 20 mM, DO < 0.012 mM), (2) aeration inlet (high oxygen, low glucose (HOLG): CS < 0.8 mM, DO > 0.2 mM), and (3) global environment (periodic 360 s fluctuation cycle with 45 s of HGLO and 75 s of HOLG conditions). Results showed that prolonged exposure to feed inlet conditions led to a complete loss of penicillin production capacity, accompanied by significant excretion of intracellular metabolites, and this effect was largely irreversible. While, cells randomly walking under the top impeller zone did not lose production capacity but showed signs of premature degeneration due to increased energy demand. When exposed to the global environment, cells finely tuned their metabolism in a periodical manner, with nearly a 50% loss of penicillin productivity. In summary, substrate gradients alone did not cause irreversible effects, but large substrate gradients contributed to reduced productivity. Oxygen gradients, however, not only reduced production but also caused irreversible cellular damage. These findings provide valuable insights for developing scale-up criteria and strain engineering strategies aimed at improving large-scale culture performance. ... Read more

While traveling through different zones in large-scale bioreactors, microbes are most likely subjected to fluctuating dissolved oxygen (DO) conditions at the timescales of global circulation time. In this study, to mimic industrial-scale spatial DO gradients, we present a scale-down setup based on dynamic feast/famine regime (150 s) that leads to repetitive cycles with rapid changes in DO availability in glucose-limited chemostat cultures of Penicillium chrysogenum. Such DO feast/famine regime induced a stable and repetitive pattern with a reproducible metabolic response in time, and the dynamic response of intracellular metabolites featured specific differences in terms of both coverage and magnitude in comparison to other dynamic conditions, for example, substrate feast/famine cycles. Remarkably, intracellular sugar polyols were considerably increased as the hallmark metabolites along with a dynamic and higher redox state nicotinamide adenine dinucleotide hydrogen/nicotinamide adenine dinucleotide of the cytosol. Despite the increased availability of NADPH for penicillin production under the oscillatory DO conditions, this positive effect may be counteracted by the decreased adenosine triphosphate supply. Moreover, it is interesting to note that not only the penicillin productivity was reduced under such oscillating DO conditions, but also that of the unrecyclable byproduct ortho-hydroxyphenyl acetic acid and degeneration of penicillin productivity. Furthermore, dynamic flux profiles showed the most pronounced variations in central carbon metabolism, amino acid (AA) metabolism, energy metabolism and fatty acid metabolism upon the DO oscillation. Taken together, the metabolic responses of P. chrysogenum to DO gradients reported here are important for elucidating metabolic regulation mechanisms, improving bioreactor design and scale-up procedures as well as for constructing robust cell strains to cope with heterogenous industrial culture conditions. ... Read more