11-71: Improving a recombinant Zymomonas mobilis strain through continuous adaptation on dilute acid pretreated corn stover hydrolysate

Dilute acid pretreatment of lignocellulosic biomass produces a hemicellulosic sugar stream (hydrolysate) that contains many potent fermentation inhibitors.  Successful conversion of these biomass-derived sugars in the presence of these compounds is a challenge for many microorganisms, particularly in highly concentrated hydrolysates. High hydrolysate concentrations are desired to reduce production cost. One strategy for improving microorganism tolerance to high hydrolysate concentration is through an adaption.  For this purpose, a recombinant glucose-xylose fermenting bacterium Zymomonas mobilis 8b, was improved through continuous adaptation on a dilute acid pretreated corn stover hydrolysate. Pretreated corn stover was produced in a pilot scale pretreatment reactor at 30% total solids and the liquor (hydrolysate) was removed by centrifugation and diluted to approximately 55% of its original strength. Adaption was performed on hydrolysate neutralized to pH 5.8 with ammonium hydroxide and on “conditioned’ hydrolysate. Conditioned hydrolysate is produced by raising it pH to 8.5 with ammonium hydroxide, holding at room temperature for 30 min, and then lowering the pH to 5.8 with sulfuric acid.  A total of 20 sub-strains were isolated from this work. Three of the adapted strains performed better than the parent strain. The improved strains demonstrate higher glucose and xylose conversion yields and faster fermentation rates.