A kinetic model for polyhydroxyalkanoates production by Cupriavidus necator using dark fermentation effluents

Polyhydroxyalkanoates (PHAs) represent a family of biodegradable polymers that are precursors of bioplastics, offering a sustainable alternative to plastics derived from fossil fuels, within a circular economy framework. This study investigates the production of PHAs by the Cupriavidus necator bacterium, using as carbon sources synthetic media with volatile fatty acids (VFAs) and real effluents from dark fermentation (DFE) of seaweed wastes, which contain this type of acids. Experimental results indicate that, when acids are fermented singly, propionic acid yields a lower polymer quantity than acetic or butyric acids. However, when VFAs are supplied in mixtures, synergistic effects are observed. The kinetic models actually published fail to account for the increasing alkalinity of the medium during the VFA consumption, which frequently leads to biomass growth inhibition, and do not incorporate pH-dependent growth equations or consider the deleterious impact of alkaline conditions on the polymer stability. Thus, a novel kinetic model is proposed, based on the Monod equation, but integrating the inhibitory effects of pH in the biomass growth and the alkaline polymer degradation, with only nine fitting parameters. The new model allow for the robust prediction of the PHA production, the substrate consumption and the biomass growth, in the experimental conditions applied, which are typical of the PHA fermentations of DFE. The overall regression coefficient of theoretical curves (R²) is over 0.92, thus facilitating the designing and optimization of the industrial-scale processes.