Ethanol Fermentation And Flocculating Mechanism Of Zymomonas Mobilis

While Saccharomyces cerevisiae produces ethanol through the Embden-Meyerhof-Parnas (EMP) pathway, Zymomonas mobilis metabolizes glucose via the Entner-Doudoroff (ED) pathway with50%less ATP generated, and thus less biomass would accumulate during fermentation for improved ethanol yield. Moreover, cells of Z. mobilis are smaller with higher specific surface area than that of S. cerevisiae, which would uptake glucose faster, and consequently improve ethanol productivity of the fermentation system.In this project, ethanol fermentation was performed by the two species under comparable conditions to validate these speculations. Compared to S. cerevisiae ADY, increases of3.5%in ethanol yield and58.1%in ethanol productivity were observed in ethanol fermentation by Z. mobilis ZM4from low gravity (LG) medium with-100g L1glucose. Although increase in ethanol yield was3.3%for ethanol fermentation by Z mobilis ZM4from high gravity (HG) medium with-200g L-1glucose, almost the same as that previously observed with the LG fermentation system, improvement in ethanol productivity depended on processes applied to the fermentation system, and an improvement of77.8%was observed under the fed-batch fermentation condition.Not only can biomass be recovered by sedimentation when microbial cells flocculate, but most importantly their tolerance to environmental stresses might be improved. Therefore, ethanol fermentation by the flocculating strain ZM401was explored. Although no significant difference was observed in ethanol yield and productivity, tolerance to inhibitory byproducts released from the degradation of hemicelluloses and lignin during biomass pretreatment, particularly acetic acid and vanillin, was improved with ZM401, and29-29.9g L-1more ethanol were produced when10.5g L-1acetic acid and1.0g L-1vanillin were added respectively. Furthermore, the bacterial flocs were online characterized by the focused beam reflectance measurement system, and impact of physical and chemical factors on the flocculation was investigated, indicating that extracellular polymeric substances (EPS), particularly polysaccharides such as cellulose might be responsible for this phenomenon, which not only benefit to developing engineering strategies to control the flocculating process, but also to exploring genes and their expression for the EPS biosynthesis.The genome of ZM401was sequenced, and the genome annotation detected58single nucleotide polymorphisms (SNPs) compare to the non-flocculationg strain ZM4, in which28SNPs (48.3%) localize in the coding regions, and30SNPs (51.7%) fall into the noncoding regions. Deletion of8-bp was found in the intergenic sequence of a hypothetical protein ZMO1010, which might be a part of the repetitive sequences. Synteny analysis of the ZM401 and ZM4genomes showed92variations of nucleotides, which further lays a foundation for exploring molecular mechanism underlying the flocculation of the bacterial species,In conclusion, Z. mobilis, particularly its flocculating strain, is superior to S. cerevisiae as a host to be engineered for fuel ethanol production from lignocellulosic biomass.