From Winery Residues to Multicomponent Bioplastics: Phototrophic Production of Polyhydroxyalkanoates from Waste-Derived Carboxylic Acids

The valorization of organic waste streams into biodegradable polymers offers a sustainable alternative to petrochemical plastics while addressing environmental pollution. In this work, polyhydroxyalkanoates (PHAs) were produced by a mixed phototrophic culture enriched in Rhodopseudomonas palustris using a complex carbon feedstock composed of medium-chain carboxylic acids (MCCAs) derived from winery residues. Under nutrient-limited and alkaline conditions, a maximum biomass concentration of 1617 ± 17 mg L–1 was obtained in the batch cultivation, with a PHA recovery yield of 41.45 ± 3.5% from independent extractions of the recovered biomass. Comprehensive physicochemical characterization revealed a multicomponent PHA comprising 3-hydroxybutyrate (78 mol %), 4-hydroxybutyrate (5 mol %), 3-hydroxyvalerate (10 mol %), and 3-hydroxyhexanoate (7 mol %). Notably, 1H and 13C NMR spectroscopy provided structural evidence for 3-hydroxyhexanoate incorporation, which had not been identified in previous GC-based analyses of PHAs produced using the same inoculum and substrate system, highlighting the value of complementary spectroscopic approaches for resolving complex PHA compositions. Thermal and X-ray diffraction analyses demonstrated a semicrystalline structure, while dual melting transitions at 136 and 164 °C were consistent with heterogeneous chain architecture. These results demonstrate that waste-derived MCCAs can be directly assimilated into PHA backbones by phototrophic mixed cultures, expanding the compositional diversity of biopolymers accessible from environmental feedstocks. This work establishes a direct link among waste-stream composition, microbial metabolism, and PHA structure while advancing integrated strategies for circular biopolymer production.