Effect of Zymomonas mobilis and Pichia stipitis presence/absence strategies in a two-stage process on bioethanol production from glucose-xylose mixture

In this study, bioethanol production was compared via a novel two-stage fermentation using Zymomonas mobilis and/or Pichia stipitis with low inoculum size (5%). A synthetic glucose (30 g/l)/xylose (20 g/l) medium simulated the real hydrolyzed lignocellulose, and the inhibitory effects of different compounds and high sugar and bioethanol concentrations were restricted to find strains synergistic interactions. Higher ethanol yield and sugar conversion alongside lower time and cost were also considered. Xylose was entirely consumed by single P. stipitis only, but glucose was completely fermented in all strategies. Maximum glucose and xylose conversion efficiencies were obtained by single Z. mobilis and P. stipitis cultures, respectively. Sequential culture provided the same glucose conversion and consumption rate (100% to 1.01 g g /l.h) and yield (0.50 g e /g g ) as single Z. mobilis and the nearest xylose conversion efficiency (30% to 0.14 g x /l.h to 0.16 g e /g x ) to single P. stipitis (47% to 0.34 g x /l.h to 0.27 g e /g x ). However, this culture was inefficient since Z. mobilis was removed. By individually immobilizing the strains on stable calcium alginate beads, the sequential-co-immobilized strategy resulted in higher xylose conversion, yield, and overall productivity of 83%, 0.45 g e /g x , and 0.30 g e /l.h than the sequential-co culture (16%, 0.31 g e /g x , and 0.14 g e /l.h), respectively. This strategy provides synergistic interactions by allocating a part of glucose to rapid growth of P. stipitis and utilizing intermediate metabolites of xylose fermentation by Z. mobilis to enhance overall efficiency. The sequential-co-immobilized strategy was also examined in 2 L bioreactor where xylose uptake rate, xylose conversion, and ethanol productivity increased 0.04 g x /l.h, 11%, and 0.27 g e /l.h compared to the sequential-co, respectively. The sequential-co-immobilized culture was found as an efficient strategy in production of second-generation bioethanol from lignocellulose. Graphical abstract