Effect of syngas fermentation effluents containing volatile fatty acids on polyhydroxyalkanoates production

Syngas fermentation offers a promising route for valorizing carbon-rich waste streams, with volatile fatty acids (VFAs) serving as key intermediates for biopolymer production such as polyhydroxyalkanoates (PHA). This study evaluates a novel two-stage, axenic fermentation strategy integrating syngas-based VFA production with subsequent PHA synthesis. The feasibility of using a canola meal-derived protein hydrolysate (CanXtract, CX), a low-value agricultural byproduct, as a nutrient source was also examined. In the first stage, Clostridium carboxidivorans was cultivated on syngas supplemented with CX to produce VFAs, primarily acetic acid (AA) and butyric acid (BA). CX concentrations ≥ 75 % significantly enhanced growth rates and acid profiles compared to yeast extract. Elevated CO partial pressure further improved microbial growth and VFA production, while periodic headspace replacement with fresh syngas increased overall yields. Maximum VFA productivity reached 0.71 g/L.d, with a total acid concentration of 3.21 g/L. In the second stage, the VFA-rich fermentation effluent was used by Cupriavidus necator for PHA synthesis. A central composite design identified 5 g/L AA and 2.5 g/L BA as optimal for biomass formation and PHA accumulation. BA showed the highest inhibitory effect, whereas lactic acid exhibited no toxicity. Due to low carbon and phosphate levels, PHA production was limited when the effluent was used alone; however, supplementation with phosphate salts, CX, and additional VFAs significantly improved outcomes. Maximum PHA productivity reached 1.11 g/L/d, with PHA content up to 76 % of dry cell mass. Overall, this integrated platform demonstrates the potential of syngas and agro-industrial byproducts for sustainable PHA production.