Plant Oils and Products of Their Hydrolysis as Substrates for Polyhydroxyalkanoate Synthesis

Plant oils could provide a sustainable source of carbon for polyhydroxyalkanoate production as they are both renewable and inexpensive. No study to our knowledge has undertaken a comparative study of the use of major European and global commodity plants oils and products of their hydrolysis as substrates for medium chain length polyhydroxyalkanoate (mcl-PHA) production. There have been several studies which have investigated the use of plant oils and their hydrolysis products for short chain length PHA (scl-PHA) production, therefore, in this study, we have focused specifically on mcl-PHA-producing organisms. A comparison between direct growth on oils and the products of their hydrolysis is described here for several mcl-PHA-producing Pseudomonas strains. Pseudomonas putida KT2440, CA-3, 0016, Pseudomonas chlororaphis 555 were screened for their ability to utilize a range of common plant oils (olive, sunflower, rapeseed, and palm) and their hydrolysis products as sole sources of carbon and energy for growth and PHA accumulation. When the oils were supplied in shaken flask experiments, P putida CA-3 and P putida KT2440 showed little or no growth, while P putida 0016 reached a cell dry weight of between 0.33 and 0.56 g L-1, and accumulated mcl-PHA to between 12 and 25 % of CDW, P chlororaphis 555 reached a cell dry weight of between 0.67 and 0.86 g L-1, and accumulated mcl-PHA to between 27 and 34 CDW in 48 h. In contrast, when the hydrolyzed fatty acid mixtures were supplied, all 4 strains tested grew and accumulated mcl-PHA. P putida CA-3 and 0016 achieved the highest biomass (1.02 - 1.06 g L-1) with the majority of the hydrolyzed plant oil fatty acids, however P chlororaphis 555 accumulated similar levels of PHA as these two strains. Despite being the strain of choice for mcl-PHA accumulation, for the majority of studies, P putida KT2440 achieved less biomass and accumulated less PHA than other strains tested with the majority of oil-derived fatty acids. It is important to note that both biomass and PHA levels varied significantly across strain and hydrolyzed oil type. Due to the fact that P chlororaphis 555 was able to grow and accumulate PHA from both plant oils and hydrolyzed oil fatty acids, it was selected for bioreactor trials to try to achieve high cell density and high PHA productivity using rapeseed oil and hydrolyzed rapeseed oil fatty acids. Rapeseed oil (RO) and its hydrolysis product (HROFA) were chosen for these experiments because P. chlororaphis 555 accumulated approximately 30 % mcl-PHA from both substrates, and as this oil can be produced globally, it would offer less barriers to scale-up than Palm oil. The mcl-PHA volumetric productivity with RO as the substrate was 0.53 g L-1 h(-1) after 25 h with a yield of 0.22 g PHA g(-1) oil, while the volumetric productivity with HROFA as the substrate was 0.54 g L-1 h(-1) after 25 h with again a lower yield of 0.15 g PHA g(-1) HROFA. Thus, under the fermentation conditions tested, BROFA was an inferior substrate for PHA production when compared to RO.