title: Consolidated bioprocessing of Populus using Clostridium (Ruminiclostridium) thermocellum: a case study on the impact of lignin composition and structure.
abstract: BACKGROUND
  Higher ratios of syringyl-to-guaiacyl (S/G) lignin components of Populus were shown to improve sugar release by enzymatic hydrolysis using commercial blends. Cellulolytic microbes are often robust biomass hydrolyzers and may offer cost advantages; however, it is unknown whether their activity can also be significantly influenced by the ratio of different monolignol types in Populus biomass. Hydrolysis and fermentation of autoclaved, but otherwise not pretreated Populus trichocarpa by Clostridium thermocellum ATCC 27405 was compared using feedstocks that had similar carbohydrate and total lignin contents but differed in S/G ratios.
  
  
  RESULTS
  Populus with an S/G ratio of 2.1 was converted more rapidly and to a greater extent compared to similar biomass that had a ratio of 1.2. For either microbes or commercial enzymes, an approximate 50 % relative difference in total solids solubilization was measured for both biomasses, which suggests that the differences and limitations in the microbial breakdown of lignocellulose may be largely from the enzymatic hydrolytic process. Surprisingly, the reduction in glucan content per gram solid in the residual microbially processed biomass was similar (17-18 %) irrespective of S/G ratio, pointing to a similar mechanism of solubilization that proceeded at different rates. Fermentation metabolome testing did not reveal the release of known biomass-derived alcohol and aldehyde inhibitors that could explain observed differences in microbial hydrolytic activity. Biomass-derived p-hydroxybenzoic acid was up to nine-fold higher in low S/G ratio biomass fermentations, but was not found to be inhibitory in subsequent test fermentations. Cellulose crystallinity and degree of polymerization did not vary between Populus lines and had minor changes after fermentation. However, lignin molecular weights and cellulose accessibility determined by Simons' staining were positively correlated to the S/G content.
  
  
  CONCLUSIONS
  Higher S/G ratios in Populus biomass lead to longer and more linear lignin chains and greater access to surface cellulosic content by microbe-bound enzymatic complexes. Substrate access limitation is suggested as a primary bottleneck in solubilization of minimally processed Populus, which has important implications for microbial deconstruction of lignocellulose biomass. Our findings will allow others to examine different Populus lines and to test if similar observations are possible for other plant species.
authors: Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA ; BioEnergy Sciences Center, Bioscience Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 USA.
authors: Alexandru
authors: A
authors: Dumitrache
authors: Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA ; BioEnergy Sciences Center, Bioscience Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 USA ; Renewable Bioproducts Institute, Georgia Institute of Technology, Atlanta, GA 30332 USA ; UT-ORNL Joint Institute for Biological Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors: Hannah
authors: H
authors: Akinosho
authors: Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA ; BioEnergy Sciences Center, Bioscience Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 USA.
authors: Miguel
authors: M
authors: Rodriguez
authors: BioEnergy Sciences Center, Bioscience Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 USA ; Renewable Bioproducts Institute, Georgia Institute of Technology, Atlanta, GA 30332 USA ; School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332 USA.
authors: Xianzhi
authors: X
authors: Meng
authors: Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA ; BioEnergy Sciences Center, Bioscience Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 USA ; UT-ORNL Joint Institute for Biological Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors: Chang Geun
authors: CG
authors: Yoo
authors: Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA ; BioEnergy Sciences Center, Bioscience Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 USA.
authors: Jace
authors: J
authors: Natzke
authors: Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA ; BioEnergy Sciences Center, Bioscience Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 USA.
authors: Nancy L
authors: NL
authors: Engle
authors: National Renewable Energy Laboratory, US Department of Energy, Golden, CO 80401 USA.
authors: Robert W
authors: RW
authors: Sykes
authors: Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA ; BioEnergy Sciences Center, Bioscience Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 USA.
authors: Timothy J
authors: TJ
authors: Tschaplinski
authors: Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA ; BioEnergy Sciences Center, Bioscience Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 USA.
authors: Wellington
authors: W
authors: Muchero
authors: Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA ; BioEnergy Sciences Center, Bioscience Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 USA ; UT-ORNL Joint Institute for Biological Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA ; Department of Chemical and Biomolecular Engineering, Department of Forestry, Wildlife, and Fisheries, University of Tennessee, Knoxville, TN 37996 USA.
authors: Arthur J
authors: AJ
authors: Ragauskas
authors: Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA ; BioEnergy Sciences Center, Bioscience Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 USA.
authors: Brian H
authors: BH
authors: Davison
authors: Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA ; BioEnergy Sciences Center, Bioscience Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 USA.
authors: Steven D
authors: SD
authors: Brown
keywords: Clostridium thermocellum; Consolidated bioprocessing; Guaiacyl; Lignin; Molecular weight; Populus; S/G ratio; Syringyl