Ethanol production from monosugars and lignocellulosic biomass by thermophilic bacteria isolated from Icelandic hot springs

Seven strains of thermophilic bacteria were isolated from several Icelandic geothermal areas on various carbohydrates (glucose, xylose, xylan, pectin, cellulose). Phylogenetic studies (16S rRNA) revealed that four of the isolates belong to the genus Thermoanaerobacterium, two to Thermoanaerobacter and one to Paenibacillus. The Thermoanaerobacterium strains had pH optima at low pH’s (pH 5.0 ‐ 6.0), the Thermoanarobacter at slightly acidic to neutral pH‘s (pH 6.0 ‐ 7.0) and the Paenibacillus strain at pH 8.0. Similarly there was a clear distinction of temperature optima between the various genera; Thermoanerobacterium strains had temperature optima close to 60°C, Thermoanaerobacter at 70°C and the Paenibacillus at 50°C. Ethanol tolerance was from low (MIC = 1.6% v/v) for Thermoanaerobacter to moderately high (MIC = 3.2% v/v) for the Thermoanaerobacterium and Paenibacillus strains. Ethanol production capacity on 20 mM of glucose and xylose showed that six of the strains produced between 1.0 to 1.5 mol-EtOH mol -l glucose and 0.4 to 1.3 molEtOH mol -l xylose, respectively. One strain showed much lower yields. Strain AK17 gave the best yields on glucose and xylose with 1.5 mol-EtOH mol -l glucose and 1.1 mol-EtOH mol -l xylose, respectively. Other end products analyzed in the culture broth were acetate and hydrogen but in lower amounts. Growth on 0.75 % (w/v) hydrolysates made from cellulose (Whatman paper), non inked paper, inked paper, glossy paper, saw dust and grass (Phleum pratense) resulted in good ethanol production yields for most of the strains. Strain AK17 produced 43.4 mM of ethanol from cellulose, 21.2 mM from grass, between 14.4 to 23.3 mM from the three types of paper hydrolysates and 3.2 mM from sawdust. Other strains produced less ethanol from biomass hydrolysates but its production was in correlation to lower ethanol production yields from monosugar fermentation. Other end products from hydrolysates were, as in the case of monosugar fermentation, acetate and hydrogen, but in lower amounts.