PHA production

Fig. 5. Growth curve and PHAs production during batch fermentation of sweet sorghum by Bacillus arybhattai in 3 L fermentor (Tanamool et al., 2011)...

Although PHAs obtain interest and are widely studied by many researchers, PHAs production is limited by production cost. A major problem to the commercialization of PHAs is much higher production cost than petrochemical-based synthetic plastic (Luengo, 2003). 

Tim Steinbuchel (1990) studied the mechanisms of Alcaligenes spp. and Rhodospirillum rubrum for PHB synthesis by using butyric acid. Later, Steinbuchel (1991) confirmed the results for the potential of PHAs production and the compositions of PHAs. Under various microbial strains used, different contents of PHAs were obtained. 

Table 4. Effect of nitrogen sources on PHAs production (Rhee et al, 1992)...

The fact that short alkanes or alkanoic acids do not support PHA production most likely indicates that P. oleovorans and P. putida cannot polymerize 3HAs shorter than 3HHx [50-52]. The repeating unit compositions of PHAs synthesized by P. oleovorans and P. putida grown with various alkanoic acids are listed in Table 4 [51,52]. Table 4 shows that the PHAs synthesized by different microorganisms grown with the same organic substrate have very similar compositions. 

Fig. 1. General overview of a PHA production process. All PHA production processes consist of a fermentation and a recovery step, followed by polymer processing for specific applications. In many cases the fermentation is divided into two stages, a biomass production and a PHA accumulation stage (for further detail see text). Several methods for recovery of the material have been described, of which solvent-based and non-solvent-based recovery protocols are illustrated...

In order to develop a more efficient PHA production process a two-stage continuous culture system was set up. Biomass was produced in the first phase, while in the second stage poly(3HAMCL) was synthesized in the absence of a nitrogen source. A maximum polymer content of 63 % was reached, at a productivity of 1.06 g 1 1 h. This polymer content is the highest reported for poly(3HAMCL) to date [51,52]. 

Several factors affect the overall economics of PHA production. These include PHA productivity, PHA content, yield of PHA on carbon source, carbon substrate cost, and recovery method employed. Figure 1 shows the production costs of P(3HB) by various P(3HB) contents and P(3HB) productivities [29]. The effect of P(3HB) productivity on the production cost is only related to the cost of the fermentation equipment [18]. However, the P(3HB) content has multiple effects on the volume of the fermentation equipment and the recovery process [17,18]. The increase of P(3HB) yield on carbon source and the use of less expensive carbon substrates reduce the cost of carbon substrate [17, 29]. Development of an efficient recovery method, which will be different for each bacterium employed, is also important to overall economics of PHA production. When the actual fermentation processes employing many different re-... 

Several mutant strains of R. eutropha that were made to possess defective competing metabolic pathways with the PHA biosynthetic pathway were developed for the enhanced PHA production. The isocitrate dehydrogenase leaky mutant of R. eutropha accumulated P(3HB) more favorably at a lower car-bon/nitrogen molar ratio and at a lower carbon concentration than the parent strain [82]. In batch culture, the final cell and P(3HB) concentrations, and P(3HB) yield on glucose were slightly increased. Also, in the P(3HB-co-3HV) biosynthesis, the molar fraction of 3HV and the 3HV yield on propionic acid increased due to the enhanced conversion of propionic acid to 3-hydroxyvaleryl-CoA rather than to acetyl-CoA and C02 in this mutant. Another mutant R. eu-... 

The recombinant strain expressed the Staphylococcus nuclease directed to the periplasm without affecting PHA production. During downstream processing, the viscosity of the recombinant lysate was significantly reduced to facilitate the subsequent purification steps. 

As summarized above, various approaches were taken to alter the PHA production or to facilitate downstream processing by employing recombinant pseudomonads. However, as in the case with recombinant R. eutropha, no study has been carried out on the high cell density culture of recombinant pseudomonads or on the scale-up of fermentation. 

Table 2. Summary of PHA production by various recombinant E. coli strains... 

Other than plants, there have been some examples of PHA production in eucaryotes [113,114]. However, these studies were not carried out for the enhanced or economical production of PHAs, but rather for understanding PHA synthesis and redesigning metabolic pathways in eucaryotes. 

The plant of choice which can be used for PHA production will be influenced by a number of factors. Of prime importance is cost, i. e., in which crop will PHA production be cheapest. The answer to this question is likely to be different depending on the agricultural economics of each country. For example, if one considers oilseed crops, rapeseed may be the best crop for Northern European countries and Canada, sunflower for Southern European countries, and soybean for the USA. Other important factors which may influence the choice of target plant are the nature of the metabolic pathway that needs to modified for synthesis of a particular PHA, the procedure used for PHA purification, and the other uses of the crop besides PHA production. 

The main question is whether synthesis of PHA in plants can succeed in bringing the cost of the polymer down to the range of 0.5 -1 US /kg. Bacterial production of PHA typically relies on a carbon source, such as sucrose or glucose, which is produced from photosynthesis and extracted from plants. Synthesis of PHA directly in plants would, therefore, represent a saving in terms of the number of intermediary steps linking C02 fixation to PHA production. Furthermore, starch is one of the cheapest plant commodity product on the market, at about 0.25 US /kg [86]. It is, thus, likely that the production cost of PHA in plants will be substantially cheaper than bacterial fermentation. The final cost of producing PHA in plants will depend on a number of factors. 

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