Characterization of and Ethanol Hyper-production by Clostridium thermocellum I-1-B

Abstract

An ethanol hyper-producing clostridial strain, I-1-B, was isolated from Shibi hot spring, Kagoshima prefecture and identified as Clostridium thermocellum based on morphological and physiological properties. The carbohydrates used as energy sources were glucose, fructose, cellobiose, cellulose and esculin. Fermentation products were ethanol, lactate, acetate, formate, carbon dioxide, and hydrogen. The optimum, maximum, and minimum temperature for growth are about 60, 70, and 47°C, respectively. Optimum pH for growth is about 7.5, and growth occurs at starting pH between 6.0 and 9.0. I-1-B strain has strong tolerance for ethanol and hyper ethanol-productivity. Ethanol concentrations causing 50% decrease of growth yield are 27 and 16 g/liter for I-1-B and ATCC27405 of C. thermocellum, respectively. The organism was cultured on a medium containing 80 g/liter cellulose at 60°C for 156h. The culture was fed with a vitamin mixture containing vitamin B₁₂ and mineral salts solution at intervals. In this culture the organism produced 23.6 g/liter (512 mM) ethanol, 8.5 g/liter (94 mM) lactate, 2.9 g/liter (48 mM) acetate, and 0.9 g/liter (20 mM) formate. The molar ratio of ethanol to total acidic products was 3.2. The ethanol productivity of the strain I-1-B is superior to any of the wild and mutant strains of C. thermocellum so far reported.