Zymomonas mobilis—Towards Bacterial Biofuel

Abstract

Bioconversion of lignocellulosic hydrolysates to ethanol is a promising solution to energy from renewable non-food sources. While utilisation of all sugars in these hydrolysates by Zymomonas mobilis has been facilitated through genetic modification of this organism, and molecular biology and fermenting capabilities of xylose-utilising Z. mobilis recombinants have been extensively documented, there is considerably less information on fundamental process optimisation and intensification studies to maximise these organisms’ potential so that the advances in molecular biology can be fully realised. In this study, process optimisation using the Z. mobilis 8b recombinant has been conducted by optimising both the process conditions and the process strategy, and process intensification has been examined by coupling a membrane filtration unit to a continuous process. Optimum process conditions on glucose–xylose substrates were quantified as 33.5 °C and pH 6.5 using multiple response optimisation. Under these optimum operating conditions, Z. mobilis 8b exhibited both a superior ethanol yield (94%) and ethanol productivity (1.43 gL⁻¹ h⁻¹) during batch culture on 100 gL⁻¹ sugar (glucose = xylose) when compared with Saccharomyces cerevisiae strains. Continuous operation of the Z. mobilis 8b culture under the optimum conditions enhanced ethanol productivity fivefold to 6.74 gL⁻¹ h⁻¹, the highest yet reported. Importantly, significant enhancement in ethanol productivity (threefold) was also achieved during continuous operation at dilution rates below 0.15 h⁻¹, whilst maintaining the ethanol concentration above the threshold for cost-effective distillation. Cell separation and recycling was facilitated via a membrane unit for process intensification.