Semi-continuous non-sterile production of medium chain-length polyhydroxyalkanoates from fatty acids

Medium chain-length polyhydroxyalkanoates (mcl-PHAs) are efficiently synthesized from fatty acids. It was hypothesized that under non-axenic conditions, the anti-microbial properties of fatty acids would reduce growth of microbial competitors and allow Pseudomonas putida LS46 to dominate the population resulting in non-sterile mcl-PHA production. Non-sterile batch and fed-batch cultures produced similar final mcl-PHA content, monomer composition, and productivity during 24–27 h cultivations when compared to sterile control cultures for initial octanoic acid concentrations ranging from 5 to 40 mM at an initial pH of 6.5. In the absence of a P. putida inoculum, growth was eventually observed in non-sterile medium after a lag period of up to 120 h, the length of which was dependent on the octanoic acid concentration. The efficacy of prolonged non-sterile cultivation was tested using a sequential batch reactor (SBR). After 28 days (672 h), 1.62 g L⁻¹ of total biomass was observed, of which the mcl-PHA content was 47%. This resulted in a PHA titer of 0.77 g L⁻¹, and a volumetric PHA productivity of 32 mg L⁻¹ h⁻¹. The polymer composition remained stable throughout at 87.0 ± 2.7% C₈ monomers, and short-chain length (scl-) monomers were not detected. This study is the first of its kind to report that purely mcl-PHAs were produced after prolonged periods in a non-sterile environment and demonstrate that medium chain-length fatty acids exert a strong selective pressure toward organisms that synthesize mcl-PHA. This suggests an opportunity for mcl-PHA production in open continuous cultivation, which could reduce both fixed and operating costs.