Response coefficient analysis of a fed-batch bioreactor to dissolved oxygen perturbation in complementary cultures during PHB production

Abstract

BackgroundAlthough the production of poly-β-hydroxybutyrate (PHB) has many biological, energetic and environmental advantages over chemically synthesized polymers, synthetic polymers continue to be produced industrially since the productivities of fermentation processes fr PHB are not yet economically competitive. Improvement of a PHB fermentation requires good understanding and optimization under the realistic conditions of large bioreactors.Laboratory-scale studies have shown that co-cultures of Ralstonia eutropha and Lactobacillus delbrueckii generate better fermentation efficiencies than R. eutropha alone. In large bioreactors, incomplete dispersioin and perturbations in the dissolved oxygen (DO) concentration, both of which affect the fermentation, have to be considered. This study analyzes the effect of DO fluctuations on bioreactor performance for both ideal and optimally dispersed broths.ResultsResponse coefficient analysis was employed to obtain quantitative information on the effect of DO perturbations on different variables. Three values of the Peclet number (Pe) cheracterized three levels of dispersion: Pe = 0.01 for nearly complete dispersion, Pe = 20 for optimum dispersion and Pe = 60 for insufficient dispersion. The response coefficients (RCs) of the pairs of bacterial concentrations and the main substrates, glucose and ammonium chloride, showed contrasting variations with time. Lactate, a critical intermediate, and PHB had similar RC profiles but those of lactate were one to two orders of magnitude larger than other RCs. Significantly, the optimum Pe also resulted in the largest RCs, suggesting a balance between productivity and reactor stability.ConclusionSince R. eutropha requires oxygen for its growth whereas L. delbrueckii does not, fluctuations in the DO concentartion have a strong influence on the fermentation. Apart from this, the mechanism of PHB biosynthesis indicates that control of lactate is a critical determinant of fermentation efficiency. The RC profiles indicate that, under non-ideal conditions, a compromise may be required between PHB formation and reactor stability, especially in the latter half of the process.

Cognitive optimization of microbial PHB production in an optimally dispersed bioreactor by single and mixed cultures

“…For a fermentation with only R. eutropha, the optimum configuration was 4-2-3-2, where the numbers denote the input, recurrent, hidden and output neurons [39,41]. With a combination of R. eutropha and L. delbrueckii, the optimized topology was 4-3-4-2.…”

“…Thus, DO has a far more central role in mixed culture fermentations than in single culture. Tohyama et al's [23] model obviously considered this fact, and it has been reinforced recently [41] in a response coefficient analysis with respect to DO.…”

Transient Response Analysis of the Eukaryotic Chemosensory System to Intra-Cellular Fluctuations

“…However, recent biological literature 23,24 favors the term "response coefficient", a contextually more relevant expression since it quantifies the response of a system to changes in certain conditions.) Previous applications 13,14,25 have demonstrated its validity and usefulness for cellular dynamics.…”