Production of different types of PHA from various carbon sources.
Even though more than 300 different PHA-producing microorganisms have been identified, only a few natural producers are able to produce high concentrations of PHA with high productivity. Strains such as C. necator, D. acidovorans, C. violaceum, Bacilus spp. and some methylotrophs are known for scl-PHA production while P. oleovorans, P. putida and P. aeruginosa can biosynthesize mcl-PHA. Members of the Aeromonas genera (e.g., A. caviae) possess unique and intrinsic abilities to produce scl-mcl-PHA copolymers. Additionally, their robust growth and simple substrate requirements make these bacteria potential hosts for the industrial production of biopolyesters.
The carbon feedstock used in PHA biosythesis is one of the key factors determining the cost of PHA production and the type of PHA produced. The cost of carbon substrate has been estimated to be approximately 28 – 50% of the total production cost. Sugars such as glucose and fructose are among the most common carbon sources; large-scale production of the P(3HB-co-3HV) copolymer has been attempted using a mixture of glucose and propionic acid. Another PHA copolymer, P(3HB-co-4HB), which is an ideal material for biomedical applications, can be produced in the presence of sugars or precursors such as 4-chlorobutyrate, 1,4-butanediol, γ-butyrolactone and 4-hydroxybutyric acid. Meanwhile, plant oils and fatty acid derivatives are relatively cheaper and renewable carbon sources that can generate higher yields of polymer. These oil-based substrates are also suitable for the production of scl-mcl PHA copolymers like P(3HB-co-3HHx), and mcl-PHA using Pseudomonas spp.