Abstract
A propionate-oxidizing microorganism in the methanogenic granular sludge from a thermophilic (55°C) upflow anaerobic sludge blanket (UASB) process was studied by cultivation and in situ hybridization analysis. To isolate the propionate-oxidizing anaerobe, primary enrichment was made with propionate as the sole carbon source under anaerobic conditions at 55°C. After several attempts to purify the propionate-oxidizing microorganism, we finally isolated a thermophilic, syntrophic bacterium that was able to oxidize propionate in syntrophic coculture with Methanothermobacter thermautotrophicus (“Methanobacterium thermoautotrophicum”) strain ΔH. This syntrophic propionate-oxidizing bacterium, designated as strain SI, oxidized propionate as well as ethanol, lactate, 1-butanol, 1-pentanol, 1,3-propanediol, 1-propanol and ethylene glycol in syntrophic coculture. In addition, strain SI fermented pyruvate and fumarate in pure culture. The spatial distribution of strain SI in sludge granules was investigated by fluorescence in situ hybridization (FISH) with a 16S rRNA-targeted oligonucleotide probe specific for strain SI in combination with confocal laser scanning microscopy. Using these methods, strain SI was shown to be present in the middle and inner layers of the thermophilic granules and it was juxtaposed with hydrogenotrophic methanogens. In addition, strain SI-type cells were found to account for 1.1% of the total cells in the sludge by the FISH-direct count method. These results indicate that strain SI may be responsible for propionate oxidation in situ as one of the significant populations in sludge granules.
