Engineering Strategies for Improving Microbial Utilization of Thermally Depolymerized Biomass

Thermochemical processing is an attractive means of depolymerizing biomass into sugar and aromatic monomers for biological upgrading to fuels and chemicals. However, microbial utilization of these compounds is limited at several levels. Most of the commonly-used microbial biocatalysts lack the metabolic pathways needed for utilization of the most abundant anhydrosugars, levoglucosan and cellobiosan. Even though these anhydrosugars comprise a substantial fraction of the global carbon cycle, microbial utilization of these sugars has remain largely undercharacterized until recently. We will discuss efforts to characterize these pathways and to engineer production organisms for utilization of these sugars. A second challenge is the presence of trace inhibitory compounds in the depolymerized biomass. This problem can be addressed by chemical treatment of the sugars to remove the inhibitors, or by engineering of the microbial biocatalyst for improved tolerance – both approaches will be described. Finally, we will discuss efforts to improve microbial utilization of aromatic monomers produced from the lignin fraction.