Biosynthesis of polyhydroxyalkanoate bioplastics using waste-derived volatile fatty acids

The widespread environmental and health issues associated with petroleum-derived plastics have prompted the search for sustainable alternatives, positioning polyhydroxyalkanoates (PHAs) as a particularly promising class of biocompatible and fully biodegradable polyesters. Despite their favourable properties, high production costs, mainly stemming from the expensive conventional carbon substrates, currently impede the industrial-scale up of PHAs. This review highlights that waste-derived volatile fatty acids (VFAs) serve as strategically viable precursors for PHA biosynthesis, not only for cost reduction but for their unique metabolic advantages. Utilising VFAs shortens the microbial metabolic pathway, leading to high substrate-to-PHA conversion efficiencies, with yields reaching up to 91% dry cell weight in optimised systems. Crucially, the key strength of this pathway is that control of VFA composition (e.g., the propionate-to-acetate ratio) enables tailoring of the polymer monomer profile and, consequently, its mechanical properties for different applications. However, this review highlights the central bottleneck to industrialisation as the trade-off between this metabolic efficiency and the inherent microbial toxicity of concentrated VFAs. A comparative analysis against conventional substrates highlights the significant environmental and economic advantages of the VFA pathway, including lower cumulative energy demand and gate-free incentives associated with traditional carbon sources.