Enhanced cellulose fermentation and end-product synthesis by Clostridium thermocellum with varied nutrient compositions under carbon-excess conditions

Sixteen combinations of seven growth nutrients, namely α-cellulose, yeast extract (YE), urea, CaCl2·2H2O, MgCl2·6H2O, FeSO4·6H2O and vitamins, were studied to improve direct cellulose fermentation by Clostridium thermocellum DSM 1237 under carbon-excess conditions. Varied nutrient compositions improved cellulose fermentation conditions for C. thermocellum and displayed two major types of effects: a general growth enhancement effect and a carbon-flux shifting effect. Different concentrations of the four most influential nutrients (YE, α-cellulose, CaCl2 and MgCl2) resulted in enhanced yields of ethanol or H2. High ethanol yields, high ethanol to acetate (E/A) ratios, and low acetate and H2 yields were obtained when YE, α-cellulose and CaCl2 were present at high concentrations, in combination with low concentrations of magnesium. Vitamins were identified as a relatively less influential nutrient, but high concentrations of vitamins supported enhanced yields of acetate and H2. High urea and YE in combination with low MgCl2 concentrations enhanced cellulose utilization per unit mass of cells and cell-specific yields of both ethanol and H2. Volumetric yields for ethanol and H2 were improved by 2.3-fold (76.5 mM) and 2.04-fold (71.22 mmol per liter), respectively, compared with the basic combination. The highest hydrogen yield (1.64 mol/mol glucose) was obtained in the combination with the lowest ethanol yields while the lowest hydrogen yielding combination had the highest ethanol yield of 1.36 mol/mole glucose, representing 68% of the theoretical maximum for ethanol. The culture conditions determined by this study can be optimized further for enhanced production of either ethanol or H2 by direct cellulose fermentation.

► Effects of nutrients on cellulosic biofuels synthesis by Clostridium thermocellum.
► Four most influential nutrients were yeast extract, cellulose, CaCl2 and MgCl2.
► Varied nutrient compositions enhanced production, growth and shifted carbon-flux.
►  Concentrations of ethanol and H2 improved by 2.3 and 2.04-fold than the reference.
► The highest ethanol yield represented 68% of the theoretical maximum.