A Comprehensive study on PHB biosynthesis and biodegradation through kinetic modelling

Polyhydroxyalkanoates (PHAs) are microbial bioplastics that are fully biodegradable, biocompatible and can be produced by renewable feedstocks through fermentation. These are all desirable attributes for the replacement of current fossil-based plastics. Strong mathematical models describing bioprocesses are invaluable tools that can be used for enhancing bioprocess understanding as well as optimization. In this study, polyhydroxybutyrate (PHB), by Cupriavidus necator DSM 545 was produced using glycerol and ammonium sulphate (AS) as the sole carbon and nitrogen sources, respectively. In addition, a kinetic bioprocess model was developed. The kinetic parameters of the model were calibrated with five fermentation experiments with different initial conditions (e.g. variable glycerol and AS concentrations) in order to properly establish the inhibition regions and provide a generalized model as much as possible. The model was successfully validated by three independent experiments, two with extreme and one with moderate initial conditions. In addition, the analysis of the data provided valuable metabolic insights for this bioprocess. It was shown that the PHB storage capacity of cells strongly correlates with the initial nitrogen concentration in the medium, meaning that efforts for maximizing biomass production in a two-stage bioprocess strategy should be carefully designed so that the nitrogen concentration does not irreversibly compromise the maximum PHB accumulation potential. Furthermore, a preliminary kinetic analysis of the biodegradation of solvent casted PHB films was performed.