Fermentation Performance Assessment of a Genomically Integrated Xylose-Utilizing Recombinant of Zymomonas mobilis 39676

Abstract

In pH-controlled batch fermentations with pure sugar synthetic hard-wood hemicellulose (1% [w/v] glucose and 4% xylose) and corn stover hydrolysate (8% glucose and 3.5% xylose) lacking acetic acid, the xylose-utilizing, tetracycline (Tc)-sensitive, genomically integrated variant of Zymomonas mobilis ATCC 39676 (designated strain C25) exhibited growth and fermentation performance that was inferior to National Renewable Energy Laboratory’s first-generation, Tc-resistant, plasmid-bearing Zymomonas recombinants. With C25, xylose fermentation following glucose exhaustion was markedly slower, and the ethanol yield (based on sugars consumed) was lower, owing primarily to an increase in lactic acid formation. There was an apparent increased sensitivity to acetic acid inhibition with C25 compared with recombinants 39676:pZB4L, CP4:pZB5, and ZM4:pZB5. However, strain C25 performed well in continuous fermentation with nutrient-rich synthetic corn stover medium over the dilution range 0.03–0.06/h, with a maximum process ethanol yield at D = 0.03/h of 0.46 g/g and a maximum ethanol productivity of 3 g/(L-h). With 0.35% (w/v) acetic acid in the medium, the process yield at D = 0.04/h dropped to 0.32 g/g, and the maximum productivity decreased by 50% to 1.5 g/(L-h). Under the same operating conditions, rec Zm ZM4:pZB5 performed better; however, the medium contained 20 mg/L of Tc to constantly maintain selective pressure. The absence of any need for antibiotics and antibiotic resistance genes makes the chromosomal integrant C25 more compatible with current regulatory specifications for biocatalysts in large-scale commercial operations.