High ethanol tolerance of the thermophilic anaerobic ethanol producer Thermoanaerobacter BG1L1

The low ethanol tolerance of thermophilic anaerobic bacteria, generally less than 2% (v/v) ethanol, is one of the main limiting factors for their potential use for second generation fuel ethanol production. In this work, the tolerance of thermophilic anaerobic bacterium Thermoanaerobacter BG 1L1 to exogenously added ethanol was studied in a continuous immobilized reactor system at a growth temperature of 70 degrees C. Ethanol tolerance was evaluated based on inhibition of fermentative performance e.g.. inhibition of substrate conversion. At the highest ethanol concentration tested (8.3% v/v), the strain was able to convert 42% of the xylose initially present, indicating that this ethanol concentration is not the upper limit tolerated by the strain. Long-term strain adaptation to high ethanol concentrations (6 - 8.3%) resulted in an improvement of xylose conversion by 25% at an ethanol concentration of 5% v/v, which is the concentration required in practice for economically efficient product recovery. For all ethanol concentrations tested, relatively high and stable ethanol yields (0.40 - 0.42 g/g) were seen. The strain demonstrated a remarkable, ethanol tolerance, which is the second highest displayed by thermophilic anaerobic bacteria known to the authors. This appears to be the first study of the ethanol tolerance of these microorganisms in a continuous immobilized reactor system.