We studied the differences in a microbial community structure with respect to the water pollution level and seasonal changes. The determination of phylogenetic groups of Bacteria and Archaea was done using fluorescent in situ hybridization (FISH). The total number of microorganisms was determined by direct counting of DAPI (4',6-diamidino-2-phenylindole) stained samples using a fluorescence microscope. Our results showed that the microbial community structure was significantly dependent on the level of water pollution, both in absolute microbial counts and in relative abundance of phylogenetic groups. For surface water with anthropogenic pollution, the microbial community with significant proportion of Betaproteobacteria and Cytophaga-Flavobacterium was characteristic. Gammaproteobacteria were significant in municipal waste water. In microbial communities with low numbers of microorganisms (e.g. non-polluted water and some industrial waste water) represented the significant component groups Alphaproteobacteria and Archaea. The impact of seasonal changes on the microbial distribution was not significant.

T G Masaryk Water Research Institute Public Research Institution Branch Brno Mojmirovo nam 16 612 00 Brno Czech Republic

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