| Summary: | The diversity of thermophilic hydrogen- and ethanol-producing anaerobes in various geothermal areas has been largely unresolved. The aim of this study was to investigate the phylogenetic relationship between ethanol and hydrogen-producing anaerobes at temperatures varying from 50 to 75°C. Extensive enrichments on various carbon substrates (glucose, xylose, cellulose [Whatman paper and cellulose powder], pectin and xylan) were done from 48 samples collected from the geothermal area in Grensdalur (Hveragerði, SW Iceland). Enrichments were regarded as positive if they showed good ethanol and/or hydrogen production. After repeated enrichments and end point dilutions, partial 16S rRNA from 59 samples were analysed, as well as end product formations from glucose and xylose. At moderate temperatures (50 to 60°C) most of the bacteria were phylogenetically most closely related to the genera of Caloramator, Clostridium and Thermoanaerobacterium, whereas at higher temperatures (70 to 75°C) Caldicellulosiruptor and Thermoanarobacter dominated the microbial flora. At lower temperatures the main flow of carbohydrate fermentation was towards ethanol and butyrate, whereas at higher temperatures, acetate and hydrogen (and CO2) were the main end products formed. Thus, a clear correlation between phylogenetic relationship and end product formation was observed in this ecological survey of thermophilic anaerobes in the geothermal area investigated. The study indicates the presence of bacteria in geothermal springs that could potentially be used for hydrogen and ethanol production from organic matter. Fjölbreytileiki hitakærra, loftfirrtra etanól- og vetnisframleiðandi baktería í margvíslegum jarðhitasvæðum hefur ekki verið kannaður til hlítar. Markmið þessarar rannsóknar var að rannsaka fjölbreytileika slíkra baktería við mismunandi hitastig (50-75°C), bæði hvað varðar lífeðlisfræði og skyldleika þeirra. Ítarlegar auðgunartilraunir á margvíslegum hvarfefnum (glúkósa, xýlósa, sellulósa [Whatman pappír og sellulósa duft], pektíni ...
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