Continuous fermentation studies with xylos-utilizing recombinant Zymomonas mobilis

Area of Science:

• Biotechnology and Bioengineering
• Industrial Microbiology
• Renewable Energy

Background:

• Recombinant Zymomonas 39676:pZB4L was previously studied in pH-stat batch fermentations for ethanol production.
• Optimizing continuous fermentation performance is crucial for cost-effective bioethanol production from lignocellulosic feedstocks.

Purpose of the Study:

• To evaluate the continuous cofermentation performance of a prehydrolysate-adapted recombinant Zymomonas strain.
• To assess the impact of substituting yeast extract with corn steep liquor (CSL) and magnesium on fermentation efficiency and cost.
• To determine the effects of varying sugar ratios and acetic acid concentrations on maximum dilution rate (Dmax) and ethanol productivity.

Main Methods:

• Continuous cofermentation experiments were conducted using a dilute-acid prehydrolysate-adapted recombinant Zymomonas strain.
• Yeast extract was replaced with corn steep liquor (CSL) and magnesium, and different sugar compositions were tested.
• Maximum dilution rate (Dmax) was determined based on 80% sugar utilization, and Pirt plot analysis was used to assess sugar utilization kinetics under inhibitory conditions.

Main Results:

• Substitution of yeast extract with 1% CSL and 2 mM Mg did not negatively impact cofermentation performance.
• Glucose utilization was complete due to higher transporter affinity, while xylose remained in the effluent, achieving 94% of theoretical ethanol yield.
• Increasing glucose concentration and decreasing xylose concentration shifted Dmax from 0.07/h to 0.08/h; acetic acid significantly reduced Dmax and volumetric ethanol productivity.

Conclusions:

• Corn steep liquor is a cost-effective alternative to yeast extract for continuous ethanol fermentation.
• The recombinant Zymomonas strain exhibits efficient glucose and xylose cofermentation, achieving high theoretical ethanol yields.
• Acetic acid poses a significant challenge to continuous fermentation performance, increasing the maintenance coefficient and reducing productivity.