The improvement of glucose/xylose fermentation by Clostridium acetobutylicum using calcium carbonate

Batch fermentation of 60 g/l glucose/xylose mixture by Clostridium acetobutylicum ATCC 824 was investigated on complex culture medium. Different proportions of mixtures, ranged between 10 and 50 g of each sugar/l, were fermented during pH control at 4.8 (optimum pH for solventogenesis) or during CaCO3 addition. Using xylose-pregrown cells and pH control, an important amount of xylose was left over at the end of the fermentation when the glucose concentration was higher than that of xylose. The addition of 10 g of CaCO3/l (to prevent the pH dropping below 4.8) increased xylose uptake: a substantial decrease of residual xylose was observed when xylose-pregrown cells as well as glucose-pregrown cells were used as inoculum for all the mixture proportions studied. MgCO3 (Mg2+-containing compound) and CaCl2 (Ca2+-containing compound) reduced residual xylose only during pH control at 4.8 by NaOH addition. As butanol is the major limiting factor of xylose uptake in C. acetobutylicum, fermentations were carried out with or without CaCO3 in butanol-containing media or in iron deficient media (under iron limitation, butanol synthesis occurred early and could inhibit xylose uptake). Results showed that an excess of CaCO3 could increase butanol tolerance which resulted in an increase in xylose utilization. This positive effect seem to be specific to Ca2+- or Mg2+-containing compounds, going beyond the buffering effect of carbonate.