| Summary: | Microbial fermentations are potential producers of sustainable energy carriers. In this study, 68 samples were used for the isolation of ethanol and hydrogen producing bacteria from various carbon substrates from geothermal springs in Iceland. 16S rRNA analysis revealed that most of low temperature (50°C) enrichments indicated the presence of bacteria belonging to Thermoanaerobacterium, Caloramator and Clostridium. At higher temperatures (60°C) Thermoanaerobacterium and Thermoanaerobacter became more dominant and at 70 and 75°C only members of the genera Thermoanaerobacter and Caldicellulosiruptor were present. Low temperature enrichments (50°C) produced mainly ethanol, acetate and butyrate from monosugar fermentation and two enrichments produced more than 1.3 mol-EtOH/mol-glucose and two enrichments produced more than 1.0 mol-EtOH/mol xylose. Most of the enrichments that directed their carbon flow to ethanol belong to Caloramator, Paenibacillus and Thermoanaerobacterium. At higher temperatures, several enrichments produced more than 1.0 mol-EtOH/mol-glucose and xylose, especially at 60°C, mostly belonging to Thermoanaerobacter and Thermoanaerobacterium. Butyrate was only detected in one of the 18 enrichments at 60°C and not at higher temperatures. Only one enrichment of a total of 16 at 70 and 75°C produced more than 1.0 mol- EtOH/mol glucose but the carbon flow was directed more towards acetate and hydrogen formation. Seven cultures were investigated further with respect to phylogeny and physiology. Six of the strains belonged to Thermoanaerobacterium and one to Paenibacillus. Basic characteristics concerning Topt and pHopt were performed on all seven strains, indicating that the Thermoanaerobacterium strain grow best at around 60°C and at low pH between 5 and 6. The Paenibacillus strain was a moderate thermophile (Topt; 50°C) and had a pHopt at 8.0. Paenibacillus was the fastest growing bacterium with the shortest generation time of 0.39 h (pHopt) and 0.71 h (Topt). The Thermoanaerobacterium strains showed ...
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