Increasing pH leads to more efficient production of polyhydroxyalkanoates from volatile fatty acids using Cupriavidus...

Volatile fatty acids (VFAs) are increasingly recognized as valuable feedstocks for the sustainable production of industrial chemicals in the context of a sustainable bioeconomy. However, their weak acidic character contributes to microbial toxicity. At lower pH values, VFAs predominantly occur in their undissociated form, which can permeate the cell membrane, disturb intracellular metabolism and increase the cellular energy burden. In this study, we investigated the effect of pH on microbial growth and polyhydroxyalkanoate (PHA) production in Cupriavidus necator H16 cultivated on a VFA mixture.To determine the influence of VFA concentrations on growth behaviour, batch experiments were conducted in microbioreactors using different concentrations of a VFA mixture. These experiments revealed multiphasic growth at VFA concentrations > 2.0 g∙L^- 1. Consequently, all subsequent fed-batch experiments were conducted at concentrations below this threshold.To assess pH-dependent effects on growth and PHA production, fed batch experiments were carried out in a 1 L bioreactor at pH values between 6 and 8.5 using a pH-pO₂-stat feeding strategy. Optimal growth performance was achieved at pH 7.5, as indicated by the highest specific growth rate (µ_App = 0.21 ± 0.01 h^- 1) and biomass yield on VFA (Y_X/VFA = 0.58 ± 0.01 g∙g^- 1). Compared to the commonly used pH of 7.0, these results correspond to a 31% increase in the specific growth rate and a 24% increase in the biomass yield c. In contrast, optimal PHA production was observed at pH 8.0, resulting in a PHA concentration of 42.5 g∙L^- 1 and a PHA yield on VFA of 0.53 ± 0.03 g∙g^- 1. Thus, growth rates, biomass yields, and PHA yields decreased at pH values above 8.0, while no growth at all was detected at pH 8.5.These obtained results demonstrate a strong influence of pH on VFA utilization, growth performance, and PHA biosynthesis by C. necator H16. Based on the observed responses, pH 7.5 is recommended for biomass production, whereas pH 8.0 is optimal for PHA synthesis when VFAs are used as the sole carbon source.