Potential sustained production of polyhydroxyalkanoate copolymers using cyanobacteria microbiomes

The copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is considered one of the most promising PHA candidates for bioplastics production, exhibiting superior physical properties than the homopolymer poly(3-hydroxybutyrate) (PHB). In this study, for the first time, PHBV was successfully produced under non-sterile conditions with Synechocystis sp. and Synechococcus sp. as the dominant cyanobacteria in microbiomes. A semi-continuous cultivation approach was employed, alternating between growth and accumulation phases over an extended period of 56 days. The process included valerate supplementation as an external carbon precursor, and the working volume was 2.5 L. Additionally, four independent repetitions of PHBV production and recovery were conducted. This methodology contrasts with conventional experiments, which typically involve only a single repetition, focus on isolated strains, and are limited to small-scale, short-term flask experiments. The maximum PHBV content reached 10.7%dcw, the highest achieved proportion of the monomer 3-hydroxyvalerate (HV) was 57.4%. Moreover, fluorescence microscopy images staining with Nile Blue positive validated that PHBV biosynthesis in cyanobacterial cells. Image quantification revealed that Synechococcus sp., produced larger PHBV granules (avg. 0.80 ± 0.13 μm) than Synechocystis sp. (avg. 0.52 ± 0.13 μm). Excessive valerate concentration at the start of the growth phases caused a certain inhibition, impairing both biomass productivity and nitrogen assimilation. Thus, optimizing valerate dosage is critical to balancing microbial metabolism and maximizing PHBV yield. This work demonstrates the potential feasibility of sustained and cyclic PHBV production and extraction, and establishing a scalable, eco-friendly biomanufacturing platform.