Preparation and characterization of poly(hydroxyalkanoate) from the fermentation of Haloferax mediterranei

Abstract

In this study, fed-batch fermentation of Haloferax mediterranei using glucose and yeast extract as carbon and nitrogen source, respectively, was carried out to produce poly(hydroxyalkanoate) (PHA). After fermentation for 117 h, the concentration of H. mediterranei and PHA content reached 85.8 g/l and 48.6%, respectively. 1H- and 13C-NMR spectra proved that the produced PHA was poly(3-hydroxybutyrate-co-3-hydroxyvalerate) P(3HB-co-3HV) co-polymer. However, further fractionation using chloroform/acetone revealed that the produced PHA consisted of at least two compositionally different co-polymers (P1 and P2). One P(3HB-co-3HV) co-polymer (P1, 93.4 wt%) contains 10.7 mol% of 3-HV unit in the chain structure and has a high molecular weight of 569.5 kg/mol. The other one (P2, 6.6 wt%) has a slightly higher 3-HV content, ca. 12.3 mol%, but its molecular weight is relatively low, 78.2 kg/mol. Both fractions exhibit two overlapped melting peaks measured by differential scanning calorimetry when the heating rate is at and below 20°C/min. For example, at a heating rate of 10°C/min, the two melting peaks occur at 134.8°C and 144.3°C for P1, and 131.1°C and 140.6°C for P2. Through observing the variation of relative intensity of these two melting peaks by changing the heating rate, it was proven that the phenomenon is caused by a melt/recrystallization process. Glass-transition temperature, crystallization temperature and thermal degradation behavior of these co-polymers were also discussed.