Rhamnogalacturonan I is a recalcitrant pectin domain during Clostridium thermocellum-mediated deconstruction of switchgrass biomass

Abstract

Background

Liquid fuels from lignocellulosic feedstocks are required for transition to a sustainable bioeconomy. However, the recalcitrance of carbon-containing feedstock cell walls to deconstruction poses a barrier to cost effective biological conversion of plant biomass to biofuels. One-step consolidated bioprocessing (CBP) in which anaerobic thermophilic bacteria convert lignocellulosic biomass into liquid fuels is a platform for overcoming the recalcitrance of plant biomass.

Results

The amounts of hemicellulosic and pectic polysaccharides, two complex cell wall glycans that contribute to plant biomass recalcitrance and that are partially solubilized during CBP of switchgrass aerial biomass by Clostridium thermocellum were evaluated in the liquor, solid residues and residue washate recovered during a 120-h CBP process. After 120 h, 24% of milled switchgrass was solubilized in the C. thermocellum CBP platform. Higher concentrations of arabinose, xylose, galactose, and glucose accumulated in the CBP-fermentation liquor and washate compared to fermentation controls without C. thermocellum, indicating that C. thermocellum solubilized hemicelluloses, but did not fully metabolize them. After five days of fermentation, the relative amount of rhamnose in the solid residues increased by 16% compared to controls, and CBP solid residues had more than 23% increased reactivity against RG-I reactive monoclonal antibodies, indicating that the pectic polymer rhamnogalacturonan I (RG-I) was not effectively solubilized from switchgrass biomass by C. thermocellum CBP. Similarly, the amount of mannose (Man) in the CBP solid residues increased by 7% and reactivity against galactomannan reactive antibodies increased by greater than 14%, indicating that the hemicellulosic polymer galactomannan was also resistant to degradation by C. thermocellum during CBP fermentation.

Conclusions

These findings show that C. thermocellum is unable to effectively degrade RG-I pectic and galactomannan hemicellulosic components in switchgrass biomass. Targeting these polymers for improved solubilization could enhance the efficiency of conversion of grass biomass to biofuels.