Impact of overflow vs. limitation of propionic acid on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) biosynthesis

Abstract

The 3HV content of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(3HB-co-3HV)), a biobased and biodegradable polyester, must be controlled for improved thermomechanical properties. Propionic acid (PA) is the main acid allowing 3HV monomer biosynthesis which is present in volatile fatty acid mixes produced from agro-industrial coproducts acidogenesis. With the strain Cupriavidus necator, we investigated the impact of carbon-limiting vs. carbon-overflow conditions thanks to fed-batch cultures, with sequential or continuous feeding of propionic acid as the sole carbon source, and phosphorus deficiency conditions. Regardless of the C-limiting continuous specific feeding rate imposed (i.e., below the determined critical specific feeding rate (qPAcrit) of 0.24 Cmol.Cmol−1.h−1), a regular incorporation of the 3HV in the copolymer at a content of 23 ± 4 mol% was observed with a high molecular weight (≈ 1000 kDa) and a remarkably low polydispersity index (≈ 1.5). This feeding strategy maximized both the productivity (up to 0.78 g.L−1.h−1) and conversion yield of propionic acid into P(3HB-co-3HV) (up to 0.59 Cmol.Cmol−1) if operated close to qPAcrit. With C-overflow sequential feeding (i.e., above qPAcrit), the residual propionic acid concentration has been highlighted as a key parameter in 3HV content improvement (≥ 44 mol%) even though it occurred at the expense of the copolymer yield and productivity.