Abstract
 Microbial community structure of a landfill leachate-treating nitrifying reactor was investigated by using molecular cloning and cultivation techniques. The most dominant phylum in the community was the Proteobacteria, which covers 65% of the clone library. In addition, candidate division TM7, OD1, and TM6 were also found as major groups. Clones close to the microorganisms capable of degrading persistent organic matters and of oxidizing ammonia and nitrite in marine environments were retrieved. Total 312 strains were isolated from agar- and gellan gum-based media, in which the isolates close to PAH degraders were obtained. Furthermore, phylogenetically novel microorganisms were also isolated.