Enhancing Bio-acetone Synthesis With ABE Fermentation Integrating With C. Acetobutylicum/S. Cerevisiae Co-Culturing With Acetate Addition

The products of acetone-butanol-ethanol(ABE fermentation) fermentation are mixtures of acetone, butanol and ethanol, with the molar ratio of 3:6:1. Acetone and butanol are important platform chemicals and powerful liquid fuel/additive. Almost all of acetone production is relied on fossil resources utilization. The studies on ABE fermentation are mostly focused on enhancing butanol concentration or butanol/acetone ratio, and little attention is paid on acetone biosynthesis. However, the economics and flexibility of ABE fermentation strongly rely on evaluating acetone as a valuable platform chemical or bio-fuel, enhancing acetone biosynthesis could be considered as an alternative measure to improve ABE fermentation performance. This thesis focused on investigating the mechanism and feasibility of improving ABE fermentation performance using a novel process integrating C. acetobutylicum/S. cerevisiae co-culturing with acetate addition, aiming at enhancing acetone biosynthesis and its concentration, while maintaining butanol concentration at normal level. The primary research contents and conclusions were summarized as follows:(1) Small amount acetate addition in ABE fermentation could reduce NADH regeneration rate in C. acetobutylicum and promote intracellular accumulation of the amino acids favorable for C. acetobutylicum survival and butanol synthesis. Concentrations of acetone and butanol, acetone/butanol ratio could reach 7.34 g·L-1, 12.18 g·L-1 and 0.6, with the increments of 25%, 4.7% and 20% as compared with those of control.(2) The mechanism of ABE fermentation performance improvement using the novel process integrating the co-culturing system with small amount acetate addition(4 g·L-broth-1) was studied. The results indicated that the proposed process could increase glucose utilization ability of C. acetobutylicum; control NADH regeneration rate at moderately low level and relieve the cellular energy load to direct more carbon source into acetone synthesis route without sacrificing butanol production; and enhance the tolerance of C. acetobutylicum against high butanol environment by intracellular accumulating favorable amino acids. Final acetone and butanol concentrations could reach the levels of 8.27 g·L-1 and 13.91 g·L-1 simultaneously with the increments of 41% and 20% as compared with those of control.(3) The availability of further enhancing acetone synthesis by modifying the above strategy via reducing initial glucose concentration in the medium and consecutively adding acetate(a total amount of 10 g·L-1) was investigated. In this case, concentrations of acetone and butanol, acetone/butanol ratio reached higher levels of 9.28 g·L-1, 13.44 g·L-1 and 0.69.(4) The feasibility of using acetate fermentation supernatant to replace synthetic acetate to reduce cost of the proposed ABE fermentation process was investigated. In this case, concentrations of acetone and butanol, acetone/butanol ratio also reached the levels of 8.55 g·L-1, 14.23 g·L-1 and 0.60, with the increments of 46%, 22% and 20% as compared with those of control. The results indicated that utilizing acetate fermentation supernatant could also improve ABE fermentation performance.