Conversion of rice husks to polyhydroxyalkanoates (PHA) via a three-step process: optimized alkaline pretreatment, enzymatic hydrolysis, and biosynthesis by Burkholderia cepacia USM (JCM 15050)

BACKGROUND: Rice husks (RH) are agricultural residues with abundant storage of cellulose and hemicellulose, making them a potential feedstock for polyhydroxyalkanoate (PHA) production. In this study, optimization of pretreatment with alkali under various conditions was performed before enzymatic hydrolysis using Celluclast 1.5 L (EC 3.2.1.4) and Novozyme 188 (EC 3.2.1.21). The hydrolysate was fed to two strains, Burkholderia cepacia USM (JCM 15050) and Cupriavidus necator NSDG-GG, an engineered strain of Cupriavidus necator H16, to evaluate their PHA production. RESULTS: Pretreatment of RH using 1.0 mol L⁻¹ potassium hydroxide (KOH) at high temperature and pressure (HTP) (121 °C, 0.1 MPa) gave maximum sugar yield of up to 87% (per total carbohydrate content) after optimized enzymatic hydrolysis, whereby the undiluted hydrolysate contained approximately 20 g L⁻¹ total reducing sugars (TRS). B. cepacia USM utilized the hydrolysate more efficiently compared with C. necator NSDG-GG, with a maximum cell dry weight (CDW) of 4.9 g L⁻¹ and 40 wt% PHA at shake-flask scale. The CDW and PHA content of B. cepacia USM cultivated in a 5 L fermentor were 7.8 g L⁻¹ and 50%, respectively. The decrease in total phenolics at the end of fermentation suggested that B. cepacia USM was able to metabolize phenolic compounds. CONCLUSION: Through optimized alkali pretreatment and enzymatic hydrolysis, RH has the potential to be converted to PHA by B. cepacia USM, thus valorizing this agricultural by-product.