| Summary: | The purpose of this work was to investigate the microbial community structure of nitrifying biofilms. Biofilm samples were collected from two reactors operated with distinct retention times. The composition and spatial distribution of nitrifying consortia in biofilms was quantified by fluorescence in situ hybridization (FISH) with rRNA-targeted oligonucleotide probes combined with confocal laser scanning microscopy (CLSM), and digital image analysis. High-resolution analyses os ammonia-oxidizer diversity in both reactors were performed using the gene that encodes the catalytic subunit of the ammonia-monooxygenase enzyme (amoA) as a marker. At least two populations of bete-subclass ammonia-oxidizing bacteria (AOB) were detected in both reactors. As demonstrated by oligonucleotide probing and comparative amoA sequence analysis, one of these populations was closely related to the model organism Nitrosomonas europaea, while the other polpulation surprisingly showed no close relationship with recognized ammonia-oxidizers. Nitrospira-like bacteria was the dominant nitrite-oxidizing bacteria (NOB) in the biofilm reactors studied. According to our results biofilms formed in the two studied reactors with distinct retention time were similar in their microbial diversity and spatial distribution of AOB and NOB populations. Differences occurred, however, in the relative abundance of AOB, which was higher in the reactor operated with shorter retention time. Despite of the fact that the environmental conditions within the reactors represented a common situation, the bacterial diversity was surprisingly diverse.
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