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 104 cells/mL. These were represented by both bacteria and archaea. Most bacterial 16S rRNA gene sequences retrieved from filtered water affiliated with the Betaproteobacteria and exhibited high similarity (99.0–99.5%) to those of bacteria of the genus Polynucleobacter. The archaeal 16S rRNA gene clone library was composed of the sequences from members of the Euryarchaeota, including the orders Methanobacteriales and Methanomicrobiales, as well as two archaeal groups (LDS and RC-V) with no characterized representatives. The species composition of filterable bacteria from reservoir water was different from that revealed previously in bogs and small lakes at catchment areas. By contrast, the pool of filterable archaea in the reservoir exhibited significant similarity to that at boggy catchment areas and was characterized by predominance 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.