Multi-scale Interpretating Natural Lignocellulosic Biomass Recalcitrance

Lignocellulosic plant cell walls are composed of crystalline cellulose nanofibrils embedded in an amorphous matrix of cross-linked lignin and hemicelluloses that impedes enzyme and microbial accessibility. The complex cell wall micro-structure and heterogeneous distribution of cell wall components are believed to be the natural biomass recalcitrance. In order to increase overall process efficiency and reduce costs, pretreatment of biomass is necessary to improve the accessibility of biomass polysaccharides to enzymatic hydrolysis. While various pretreatments have been employed, a detailed cellular level understanding of the pretreatment process is lacking due in part to complexity of the biomass composition and structure as well as interference from traditional processing chemicals that sometimes alter the material under study. In the present work, microscopic and microspectroscopic techniques were used to investigate cell wall micro-structural and topochemical information at cellular and sub-cellular. The conclusions obtained were as follows:In the developing xylem tissue of Populus tomentosa, prior to fiber lignification, the cellulosic polysaccharides have been deposited. The fiber lignification started from cell corner middle lamella (CCML), and then extends into secondary wall (Sw), The lignification level for the newly formed fiber Sw was higher than that of the previous latewood fiber.TEM images exhibited that the Populus nigra fiber wall was typically differentiated into three layers:middle lamella (ML), primary wall (P) and secondary wall (S1, S2and S3), and the staining intensities represented differing lignin concentrations. The striated appearance in the fiber S2indicated the orientation of cellulose microfibirls.Compared to Populus nigra opposite wood (OW), the tension wood (TW) fiber displayed an additional gelatinous layer (GL) with higher cellulose and less lignin. Meanwhile, the cellulose enriched GL in TW had much more abundant porosity than that of OW. Moreover, the microfibrils in the TW fiber S2and GL were stretched during TW formation and the microfibrils still keep the tensional deformation even after the fibers are transversally cut.Anatomical observations indicated that Forsythia suspensa was diffuse-porous wood. Helical thickenings and alternate intervessel pits were present on vessel cell wall. Confocal images (488nm) revealed a high level of lignin autofluorescence in the cell corner middle lamella (CCML), with lower levels of fluorescence in the compound middle lamella (CML) and S2region. The results from SEM-EDXA demonstrated that lignin concentration ratio in different regions of fibre wall was1.3(CCML):1.1(CML):1(S2).The inhomogeneity in cell wall components (cellulose and lignin) among different cells and within morphologically distinct cell wall layers was observed in Cornus Alba. As the significant precursors of lignin biosynthesis, the pattern of coniferyl alcohol and aldehyde (joint abbreviation Lignin-CAA for both structures) distribution in fiber ceJl wall was also identified by Raman images, with higher concentration occurring in the fiber secondary wall (Sw) where there was the highest cellulose concentration. Moreover, noteworthy was the observation that higher concentration of lignin and very minor amounts of cellulose and pectin was visualized in the pit membrane (PM) areas between fibers.A great degree of inhomogeneity in the layering structure of Miscanthus sinensis sclerenchymatic fiber (Sf) secondary wall (Sw) was visualized, while the Sw of xylem vessel can not clearly be divided into sub-layers. Moreover, we proposed an architectural model of protoxylem vessel (Pxv) composed of two layers:an outermost Pw composed of a meshwork of Mfs, and inner Sw containing regularly parallel Mfs.In herbaceous biomass Miscanthus sinensis lignin mainly accumulated within the secondary wall of epidermis, metaxylem vessel, sclrenchyma fiber (Sf) and middle lamella (ML). And higher concentration of hydroxycinnamic acids (HCA) was located at the secondary wall of Sf, parenchyma (Par), metaxylem vessel (Mxv) and ML. Moreover, a clear accompanied trend between lignin and HCA distribution within morphologically distinct cell wall layers of Sf and Par was observed.During ionic liquids ([Emim][OAc]) pretreatment of Pinus bungeana Zucc. opposite wood (OW), swelling occurred primarily in the secondary wall (Sw) adjacent to compound middle lamella (CML) and the ILs had little effect on the Sw adjacent to cell corner middle lamella (CCML). The time series of lignin Raman images showed that lignin decreased significantly from the Sw adjacent to Cml in OW while for the Sw of the CW tracheids there was no significant change of the lignin signal intensity within the same pretreatment time. Moreover, after washing with distilled water (80℃) OW tracheids displayed obvious decrease in lignin, carbohydrates and cellulose concentration.