Fermentation Performance of Field-Grown Natural Variant and Transgenic Feedstocks

Populus trichocarpa and switchgrass are dedicated bioenergy feedstocks and their inherent recalcitrance to bioconversion is one of the major economic hurdles for the production of biofuels. Implications of improvements in these feedstocks through use of natural variant and transgenic selection will be discussed with the goal of moving towards an improved phenotype which maintains normal growth characteristics and has increased fermentation yield. Microbial bioconversion assays of the feedstocks were performed with a traditional enzymatic hydrolysis and yeast fermentation as well as a consolidated bioprocessing platform using thermophilic bacterial species producing their own biomass-degrading enzymes. We examined Populus trichocarpa variants from three common garden field sites, which have a mutation that leads to reduced lignin content. We also examined field-grown transgenic switchgrass lines with reduced lignin content due to genetic modifications in the lignin biosynthetic pathway. The results showed superior conversion yields for the transgenic switchgrass and the Populus natural variants compared to their controls. In addition, we performed fermentations on the top lines at increased solids loading up to 50 g/L on a biomass dry basis. We concluded that transgenic switchgrass and Populus natural variants have reduced recalcitrance and are a valuable resource for producing economical biofuels. We also determined that when characterizing new biomass sources, in vitro assays such as sugar release should be supplemented with in vivo fermentation tests which we have shown to detect inhibitory compounds.