Zymomonas mobilis immobilization in polymeric membranes for improved resistance to lignocellulose-derived inhibitors in bioethanol fermentation

Abstract

Immobilized cell hollow fiber membrane bioreactor (ICHFMB) was designed and operated to mitigate the effect of lignocellulose derived inhibitors during fermentation of synthetic lignocellulosic hydrolysate by Zymomonas mobilis. The individual inhibitors – vanillin, syringaldehyde, 5-hydroxymethylfurfural and 4-hydroxy-3-methoxycinnamaldehyde, exhibited toxicity on suspended cells, whereas simultaneous exposure to multiple inhibitors was detrimental for the cells. In contrast, the immobilized cells showed excellent growth and glucose uptake in the presence of these inhibitors, and the microorganisms achieved 95% of the theoretical ethanol yield under highly inhibitory conditions. ICHFMB performance could be improved significantly by increasing the flow rate of the hydrolysate over the membranes, and by increasing the number of membranes, indicating that diffusion through the membranes was an important factor during the fermentation. Z. mobilis showed gradual acclimatization to the presence of inhibitors as they were slowly released from the membrane pores into suspension, and enhanced glucose uptake rate and ethanol productivity. Cell immobilization within the membranes was also found to be very stable as the ICHFMB performance did not deteriorate during 20 consecutive runs under identical conditions. These results indicate that ICHFMB can be promising in lignocellulosic bioethanol production under inhibitory conditions.