An iterative approach to improve xylose consumption by Clostridium autoethanogenum: From substrate concentration to pH adjustment

Abstract

Clostridium autoethanogenum is an autotrophic acetogen also able to utilize xylose as a carbon source. Here, we evaluated this organism's industrially relevant capability since the transformation of sugars derived from lignocellulosic materials to biofuels is of great interest. Initially, the organism was challenged with different initial xylose levels to determine xylose utilization and maximum inhibitory concentrations for C. autoethanogenum when grown in batch cultures. A critical factor that could limit xylose utilization is culture pH, and this factor was separately examined as a means to provide C. autoethanogenum with more suitable environmental conditions. Finally, optimum pH conditions for growth were tested to assess how efficiently initial xylose concentrations could be fully utilized. C. autoethanogenum specific growth rate was not impacted when cells were exposed to initial xylose concentrations up to 30 g L−1, while cultures amended with concentrations as high as 45 g L−1 showed a delay in growth and in formation of final fermentation product concentrations. Intermittent adjustment of the culture pH to 5.8 allowed for complete substrate utilization when step-wise fed-batch xylose additions were performed. Under these conditions the cultures showed a unique biphasic growth which enhanced biomass and end product formation. In C. autoethanogenum cultures exposed to an initial xylose concentration as high as 30 g L−1, pH adjustment to 5.8 supported full utilization of all available xylose and generation of up to 2.4 g L−1 ethanol and 9.5 g L−1 acetate. This work demonstrated that C. autoethanogenum can effectively biotransform xylose at concentrations typically present in lignocellulosic hydrolysate streams.