Stable Multi-Cycle Production of Bioplastic Poly(3-hydroxybutyrate- co -3-hydroxyvalerate) Using Cyanobacteria Microbiomes

Abstract

This study reports, for the first time, a stable and reproducible semi-continuous bioprocess for the sustained production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) over 98 days, corresponding to 7 consecutive operational repetitions. The process was operated under non-sterile conditions using a cyanobacteria-dominated microbiome and employed acetate and valerate supplementation. Throughout the operation, a consistent 3-hydroxyvalerate (3HV) molar fraction of 8 −12.2 mol % was maintained, with a maximum polymer content of 17.9% of dry cell weight. Notably, reducing the valerate concentration from 100 to 50 mg·L−1 did not compromise PHBV production, indicating the metabolic flexibility of the microbial consortium and highlighting the potential for substrate optimization. Microbial community analysis revealed a strong dominance of Synechocystis sp. (>95%) over Synechococcus sp. (<5%), with the consortium exhibiting resilience to cyclic hypoxic conditions. Transmission electron microscopy, in combination with Nile blue staining, confirmed the intracellular accumulation of PHBV granules within the cyanobacteria cells. Thermal characterization further demonstrated the formation of semi-crystalline PHBV, with characteristic melting transitions spanning 147 −169 °C. Overall, these findings support the potential of cyanobacteria-dominated microbiomes for sustained PHBV production with a relatively stable 3HV molar fraction.