Cyanobacterial production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) from carbon dioxide or a single organic substrate: improved polymer elongation with an extremely high 3-hydroxyvalerate mole proportion

Abstract

The bioplastic poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) changes material properties upon altering mole ratio (mol%) of the two monomers: 3-hydroxybutyrate (HB) and 3-hydroxyvalerate (HV). Biotechnology for producing PHBV with a wide range of HV mol% using low amount of organic substrate is required. Here, 20 evolutionarily divergent cyanobacteria were examined for PHBV production. Under photoautotrophy with carbon dioxide (CO2) supply, one strain produced PHBV. But under heterotrophy using a medium containing only one type of organic substrate, 4, 15, and 13 strains were found to accumulate PHBV when supplied with acetate (C2H3O2), propionate (C3H5O2), and valerate (C5H9O2), respectively. Hence, these cyanobacterial PHBV productions required only CO2 or a single type of organic substrate. The obtained PHBV showed a broad range of HV mole ratios from 4 to 96 mol%. The cells producing PHBV with high HV mole ratios (31–96 mol%) exhibited low cellular acetyl-CoA levels. The PHBV with a 96.2 mol% HV and 3.8 mol% HB from Nostoc microscopicum elongated at 98 ± 15% (length/length), a 19.6-fold higher elongation than that of commercial poly-3-hydroxybutyrate containing 100 mol% HB. The results demonstrated the economical utility of cyanobacterial conversion of CO2 and a single organic substrate to synthesize PHBV with improved elongation quality.