Effect of substrate co-feeding with short-chain volatile fatty acids (VFAs) on PHBV production by Halomonas alkalicola EXT

Polyhydroxyalkanoates (PHAs) are biodegradable biopolymers with strong potential as sustainable alternatives to conventional plastics. In this study, we explored how different synthetic, short-chain length volatile fatty acids (VFAs) namely acetate, butyrate, and propionate influence the growth, substrate utilization, and PHA production of Halomonas alkalicola, a haloalkaliphilic bacterium isolated from a Kenyan soda lake. Cultures were grown on glucose-rich media supplemented with low levels of VFAs (0.1 %, w/v), and the kinetics of biomass growth, glucose consumption, and PHA biosynthesis modeled using logistic and Luedeking–Piret equations. We found that H. alkalicola showed relatively low tolerance to VFAs. For PHA production, cultures with acetate produced the highest PHA titer (1.472 g/L), followed by butyrate (1.405 g/L) and propionate (1.27 g/L). Acetate and butyrate co-fed cultures showed no significant difference in PHA accumulation while propionate led to a significant decline in production. While VFAs did not enhance overall PHA production, propionate led to significant increase in the 3-hydroxyvalerate (3 HV) fraction of the polymer (18.36 %), compared to 7.15 % in control cultures, suggesting a promising route for tailoring polymer composition toward production of PBHV with high 3 HV contents. Meanwhile, glucose consumption followed similar trends across all treatments, suggesting that VFAs at low concentration had minimal impact on primary metabolism. Our kinetic analysis further revealed that acetate supported the highest biomass accumulation (3.07 g/L), with μₘₐₓ values remaining comparable across treatments (0.11–0.12 h⁻¹). This work provides insights into optimizing co-substrate feeding strategies for haloalkaliphilic PHA producers, with potential applications in designing higher-performance bioplastics.