Production of polyhydroxyalkanoates and astaxanthin from lignocellulosic biomass in high cell density membrane bioreactor

Abstract

Microbial-derived bioplastics are receiving more attention and are a potential option for ecological sustainability. Lignocellulosic biomass is the ultimate resource for the production of second-generation biofuels and biochemicals. In this study, lignocellulosic biomass (corn cob) was first pretreated and hydrolyzed to produce fermentable sugars. Then, polyhydroxyalkanoates (PHA) and astaxanthin were produced through batch and cell retention cultures of two strains, Bacillus megaterium ALA2 and Paracoccus sp. LL1 using corn cob hydrolysate as a carbon source. Higher cell and PHA concentrations (101.7 g/L and 57.6 g/L, respectively) with a PHA productivity of 1.07 g/L/h were obtained by B. megaterium ALA2, but Paracoccus sp. LL1 could co-produce PHA and astaxanthin. PHA and astaxanthin concentrations were significantly increased by cell retention culture of Paracoccus sp. LL1 in high cell density membrane bioreactor compared to batch culture (9.04 and 13.1-folds, respectively). In addition, PHA and astaxanthin productivities by cell retention culture of Paracoccus sp. LL1 increased to 0.579 g/L/h and 0.385 mg/L/h, respectively, which were 4.02 and 10.1-folds higher than those of batch culture. Based on gas chromatography, 1H nuclear magnetic resonance, and Fourier transform infrared analysis, B. megaterium ALA2 produced a homopolymer of poly(3-hydroxybutyrate), whereas poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with 7.84 mol% of 3-hydroxyvalerate was accumulated by Paracoccus sp. LL1. This study demonstrates that bioplastic PHA and high value-added astaxanthin can be economically produced from lignocellulosic biomass as an inexpensive renewable resource. This is the first report on the enhanced co-production of PHA and astaxanthin from lignocellulosic biomass in a membrane bioreactor.