A cellulolytic anaerobic bacterium, strain I77R1B, was isolated from a biomat sample of an lcelandic, slightly alkaline, hot spring (78 °C). Strain I77R1B was rod-shaped, non-spore-forming, non-motile and stained Gram-negative at all stages of growth. It grew at 45–82 °C, with an optimum growth temperature around 78 °C. At 70 °C, growth occurred at pH 5·8–8·0, with an optimum near pH 7·0. At the optimum temperature and pH, with 2 g cellobiose I as substrate, strain I77R1B had a generation time of 2 h. During growth on Avicel, strain I77R1B produced acetate, hydrogen and carbon dioxide as major fermentation products together with small amounts of lactic acid and ethanol. The strain fermented many substrates, including cellulose, xylan, starch and pectin, but did not grow with casein peptone, pyruvate, -ribose or yeast extract and did not reduce thiosulfate to HS. The G+C ratio of the cellular DNA was 35 mol%. Comparative 16S rDNA analysis placed strain I77R1B among species of . The closest relative was . Hybridization of total DNA showed 42% hybridization to and 22% hybridization to . A new species, sp. nov. (I77R1B) is proposed.


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