High-Yield Synthesis of Poly(3-hydroxybutyrate- co-3-hydroxyvalerate) Copolymers in a Mixed Microbial Culture: Effect of Substrate Switching and F/M Ratio

The accumulation capacity of a mixed microbial culture (MMC) is affected if the substrate used in the accumulation experiments differs from the one used in the enrichment. For this reason, the effect of substrate switching was studied to determine the versatility of an MMC enriched in a mixture of volatile fatty acids (VFA_mix) to overcome this problem. The MMC was enriched using a VFA_mix composed of 48.3:24.3:7.3:14.7 CmM acetic (HAc), propionic (HPr), butyric (HBu), and valeric (HVa) acids, respectively. The accumulation capacity was tested using single VFAs (HAc, HPr, HBu, and HVa), as well as the VFA_mix used in the enrichment. The accumulation capacities were 52.8 ± 4.7, 48.8 ± 4.3, 45.2 ± 3.0, 48.4 ± 1.0, and 54.5 ± 8.0 wt% for HAc, HPr, HBu, HVa, and VFA_mix, respectively, with polymer compositions of 50.6 ± 15.1, 0.4 ± 0.1, 63.2 ± 1.5, 0.3 ± 0.0, and 2.0 ± 0.7 g 3-HB/g 3-HV, following the same order. The average yields were 0.84 ± 0.08, 0.76 ± 0.09, 0.74 ± 0.02, 0.70 ± 0.01, and 0.68 ± 0.09 Cmol_PHA/Cmol_VFA for HVa, VFA_mix, HAc, HBu, and HPr, respectively. The feed-to-microorganism (F/M) ratio showed that values of 1-7 Cmol_VFA/(Cmol_X) in the accumulation experiments led to the optimal yields. Based on the results obtained, it seems feasible to enrich an MMC able to produce tailor-made biopolymers from different VFAs at high yields.