Ambar SUSILORUKMI Diversity and Physiology of Syntrophic Substrate-Oxidizing Anaerobes in Methanogenic Ecosystems. Akiyoshi OHASHI, Hideki HARADA Syntrophic bacteria are among the most important microbial populations in the degradation of complex organic compounds in methanogenic ecosystems. The diversity and physiology of syntrophic substrate-oxidizing anaerobes were studied by applying conventional cultivation techniques combined with rRNA-based molecular approaches. For cultivation syntrophic bacteria, various anaerobic samples were used as inoculum in primary enrichments with ethanol, benzoate and propionate as the substrate. Based on 16S rDNA clone analysis, some clones showed close relation with known bacteria to date as syntroph, such as Desulfovibrio sp. Nonetheless, several clones seemed to indicate novel bacterial lineages that have never cultivated and isolated so far, such as clones related with the genus Geobacter in two mesophilic ethanol enrichments, clones representing a deeply branched lineage of the phylum Firmicutes in a thermophilic ethanol enrichment culture, and clones related with the genus Desulfobulbus in a mesophilic propionate degrading anaerobes. Specific DNA probes were designed and applied for determination the dominant clone in the cultures by FISH analyses, suggesting the probe-positive cells were the dominant microbes in the cultures. Through a molecular-directed isolation strategy, two novel syntrophic, ethanol-oxidizing bacteria and a novel benzoate-degrading bacterium were successfully isolated. This study strongly suggested that the strategy employing conventional techniques combined with 16S rRNA-based approaches is advantageous to determine the diversity of recalcitrant microbes like syntrophic microorganisms and to attempt at subsequent isolation of targeted cells.