Anaerobic Alkali-thermophiles, a Novel Group of Extremophiles
Juergen WIEGEL*
Department of Microbiology and Center for Biological Resource Recovery, University of Georgia, Athens, GA 30602-2605, USA
We have recently isolated several different species and genera which are able to grow at alkaline pH (above pH 9.0) and at temperatures above 55℃. All of these isolates --extremophiles in respect to both temperature and pH range for growth-- belong phylogenetically to various novel genera or species from the Gram-type positive clostridial/bacillus subphylum. All the isolated alkalithermophiles are characterized by relatively short doubling times with a low of 10 min exhibited by Thermobrachium celere growing optimally at 66℃ and pH66C 8.5. This bacterium grows in a temperature range from 37 to 75℃ and pH66C 5.0 to 9.3. When the pH of the medium was measured at 25℃ instead of at the growth temperature the pH- optimum increased to pH25C 9.2. The most alkaliphilic species is the peptidolytic and glucolytic Clostridium paradoxum growing at alkaline pH values of up to pH60C 10.3 (pH25C 11.3). At the end of the growth phase about 99% of the cells sporulates and became highly motile. The internal pH of this bacterium is more acidic than the external pH and changes with the external pH. The largest difference (ΔpH) was 1.1 pH units and was observed at the optimal growth pH55C of 9.3. The difference became close to zero at pHmin and pHmax. C. paradoxum and C. thermoalkaliphilum were isolated from mesobiotic sewage sludge containing up to 104 CPU/ml sludge incubated at pH25C 10.5 and 55℃. The non-sporulating C. hermoalkaliphilum grows at pH25C10 between 27 and 57.5℃; its pH25C range was 7.0 to 11.0. Growing on glucose both alkalithermophiles produced mainly acetate, lactate, CO2, H2, and an unidentified product. The peptidolytic Anaerobranca horikoshii is the first anaerobic thermophile described with a branched morphology. It was isolated from hot springs in Yellowstone National Park, grows between 34 and 66℃ in the pH range of 6.9 to 10.3, and forms mainly acetate, H2, and CO2 from casein. From the African Lake Bogoria we isolated the lipolytic Thermosyntropha lipolytica. It is the first isolated syntrophic thermophile (clustering in the 16S rDNA tree with Syntrophomonas and Syntrophospora). It does not utilize glycerol but grows in the presence of sulfate reducers or methanogens on long chain fatty acids. The growth range is from pH60C7.2 to 9.5 and from 52 to 70℃. The species produces constitutively an temperature stable alkaline lipolase.
The diversity of these alkalithermophiles isolates suggests that more novel and unusual alkalithermophiles should be isolable. Whether anaerobic (eu)bacterial alkalithermophiles exist which can grow at more alkaline pH values and/or at even higher temperatures remains to be seen.
