Polymers, the Light at the End of Dark Fermentation: Production of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) by a Photoheterotrophic Consortium

In this study, the photoheterotrophic consortium C4 was used to produce the copolymer [P(3HB-co-3HV)]. PHA production was enhanced by using response surface methodology (RSM) to determine the effects of different concentrations of acetate and butyrate in mixtures (0.5–3 g L⁻¹), ammonium sulfate and their combinations. This is relevant because PHA accumulation is stimulated by nitrogen limitation. The type and concentration of the substrate determines the monomeric composition and the PHA content (% per cell dry mass (CDM)). The RSM, carbon balance and metabolic behavior analysis results showed that at the lowest ammonium concentration, 0.1 g L⁻¹, and when acetate was in a higher proportion than butyrate, biomass production was favored. In contrast, when the butyrate proportion was high, PHA production increased, reaching a highest production of 58% per CDM. The better conditions were evaluated in a 3-L reactor, and a maximum P(3HB-co-3HV) of 67% was determined. The predominant microbial population consisted of four major species, Macelibacteroides fermentans (37%), Rhodopseudomonas palustris (22%), Acinetobacter sp. (35%), and Clostridium propionicum (2%). Insights into the understanding of copolymer production by photoheterotrophic mixed cultures constitute the basis for developing coupled processes from organic residues. These microorganisms are worth studying since they produce a variety of valuable biotechnological products.