[Filterable microbial forms in the Rybinsk water reservoir]

Molecular identification of the filterable forms of microorganisms in the water of the Rybinsk reservoir, one of the largest open water bodies in European Russia, was carried out. The number of ultrasmall microbial cells passing through 0.22 μm filters was 10(4) cells/mL. These were represented by both bacteria and archaea. Most bacterial 16S rRNA gene sequences retrieved from filtered water belonged to the Betaproteobacteria and exhibited high similarity (99.0-99.5%) to thos of bacteria of the genus Polynucleobacter. The archaeal 16S rRNA gene clone library was composed of the sequences of members of the Euryarchaeota, including the orders Methanobacteriales and Methanomicrobiales, as well as of two archaeal groups (LDS and RC-V) with no characterized representatives. The species composition of filterable bacteria from reservoir water wast different from that revealed previously in bogs and small lakes at catchment areas; The pool of filterable archaea in the reservoir exhibited, however, significant similarity to that for boggy catchment areas and was characterized by perdominance of the clade LDS. Available data indicate that this archaeal group is typical of the northern freshwater ecosystems, and the organisms of this group are represented by ultrasmall cells.