Marina Basaglia

Acidogenic fermentation of organic wastes represents a strategic platform for the co-production of H₂, CO₂, and volatile fatty acids (VFAs), which are potential key intermediates for cost-effective polyhydroxyalkanoate (PHAs) biosynthesis. This typically relies on carbon sources that are too expensive and hinder the commercialization of PHAs. This study provides metagenomic insights into the microbial dynamics underpinning the acidogenic conversion of waste melon under increasing organic loading rates (OLRs). Metabarcoding revealed that Megasphaera dominated the community, with its abundance rising markedly from 5 to 20 gCOD/L, accompanied by relevant contributions from Solobacterium, Prevotella, and Clostridium. These taxa were associated with the formation of acetic, propionic, and butyric acids and with enhanced hydrogenogenesis. Higher OLRs, up to 20 gCOD/L, promoted hydrogen-producing species while suppressing lactic acid bacteria, thereby improving H₂ and VFAs yields up to 26.7% v/v and 13 gCOD/L, respectively. By linking microbial shifts to metabolic outputs, this work advances the understanding of acidogenic pathways essential for integrating dark fermentation-derived H₂, CO₂, and VFAs into sustainable PHAs production systems.

Wine production generates significant quantities of by-products each year, among which surplus grape must is notable for its high content of fermentable sugars, organic acids, and polyphenols. As wine consumption declines and grape must surpluses grow, identifying sustainable valorization strategies becomes increasingly critical. This study investigates for the first time the potential use of red and white grape musts as substrates for polyhydroxyalkanoates (PHAs) production by two well-characterized bacterial strains, Cupriavidus necator DSM 545 and Hydrogenophaga pseudoflava DSM 1034, under both batch and fed-batch fermentation regimes. Both musts supported microbial growth and PHAs accumulation. In batch cultures, C. necator DSM 545 achieved a PHB content of up to 61.5% of cell dry weight (CDW), while H. pseudoflava DSM 1034 reached 67.9% PHB on grape must, with yields and biomass comparable to or exceeding those obtained with synthetic sugar-based media. Under fed-batch conditions with red must, C. necator DSM 545 sustained growth and PHB production across multiple feeding feeding periods, outperforming the control medium. Conversely, H. pseudoflava DSM 1034 displayed initial growth but failed to increase biomass over time, suggesting that this strain may be poorly suited for use in fed-bacth-based applications, likely due to nutrient depletion or the accumulation of inhibitory compounds. Overall, these results proved that grape musts are promising feedstocks for sustainable PHAs production. Their integration into circular economy frameworks offers a valuable opportunity for waste recovery and the development of bioplastics within the agri-food industry, especially in light of the increasing grape must surpluses recently experienced worldwide.