Pressure Dependent Changes in CO₂-Concentrations Modulate the 3-Hydroxyvalerate Fraction in Terpolymeric...

Polyhydroxyalkanoates (PHAs), especially their copolymeric forms, such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-4-hydroxyvalerate) (PHBVV), have promising properties for replacing petrochemical-derived plastics. However, the research focuses mostly on producing PHA from a specific substrate or waste-stream, while the reproducibility of a precise polymer composition is seldom targeted. This study systematically focuses on the influence of pressure conditions on the production of terpolymeric PHBVV by Cupriavidus necator NCIMB 11599 using levulinic acid (LA). Three different process setups were evaluated to assess the impact of hydrostatic pressure (Δp_Hydro) and applied pressure (Δp_Vessel) compared to the process at ambient pressure. Differences were detected regarding carbon utilization (Y_X/Gluc & Y_PHA/LA) and polymer composition. The Y_X/Gluc increased in the pilot scale compared to the technical scale from 0.31 ± 0.01 g_X g_Gluc ^-1 to 0.41 ± 0.01 g_X g_Gluc ^-1. Conversely, as Δp_Vessel increased, the Y_PHA/LA decreased from 0.35 ± 0.02 g_PHA g_LA ^-1 to 0.23 ± 0.02 g_PHA g_LA ^-1, while the corresponding 3HV contents increased from 48 ± 1%_(w/w) to 69 ± 2%_(w/w). These changes were consistent with pressure-dependent variations in CO₂ concentration and a potentially increased carbon fixation via the energy-intensive CBB cycle. This indicates that the polymer composition and cell vitality can be adjusted via the CO₂ concentration, a factor that should be taken into account when further scaling up of PHA production processes.