[Picocyanobacteria in eutrophic reservoirs of the Middle Volga: abundance, production, viral infection].

During the summer season of 2010, abundance and productivity of picocyanobacteria in Gorky and Cheboksary Reservoirs have being examined. It is found out that in the eutrophic reservoirs of the Middle Volga abundance and biomass of picocyanobacteria, averaged over the water column, varied within the range of (34-322) x 10(3) cells/ml and 38-455 mg/m3 respectively. In more productive Cheboksary Reservoir, the contribution of picocyanobacteria in total biomass and production of phytoplankton (4.7 ± 0.7 and 8.3 ± 1.3% respectively) was lower than in less productive Gorky Reservoir (10.6 ± 2.1 and 19.2 ± 3.0% respectively). In both reservoirs, high level of picocyanobacteria infection by viruses was detected. The frequency of visible infected cells and virus-induced mortality of picocyanobacteria in Cheboksary Reservoir were substantially higher (3.2 ± 0.4% of total abundance and 21.8 ± 2.9% of daily production) than in Gorky Reservoir (1.7 ± 0.2% of total abundance and 11.0 ± 1.7% of daily production). The results obtained indicate that in eutrophic reservoirs during summer bloom of large cyanobacteria their abundance is regulated to a great extent by viruses.

[Heterotrophic organisms and viruses in the Oka River and Cheboksary Reservoir during the abnormally hot summer of 2010].

In July 2010, abnormally high water temperature (25-29 degrees C), as well as increased biomass and phytoplankton production caused intensive development of heterotrophic bacteria and heterotrophic nanoflagellates. It was found that the abundance, biomass, and production of heterotrophic bacterioplankton, as well as the abundance and biomass of heterotrophic nanoflagellates, and the number of planktonic viruses, which were calculated on average for the reservoir under study, turned out to be higher in the years with lower water temperature (20-23 degrees C). The virus-induced mortality of bacterioplankton in the Oka River and the Cheboksary Reservoir averaged 25.4 +/- 3.4 and 22.4 +/- 2.7% of the daily bacterioplankton production.

[Distribution of picocyanobacteria and virioplankton in mesotrophic and eutrophic reservoirs: the role of viruses in mortality of picocyanobacteria].

The quantitative distribution and relationships of picocyanobacteria and viruses were studied in the mesotrophic Sheksna and mesoeutrophic Rybinsk reservoirs belonging to the system of the Volga-Baltic Waterway. The share of picocyanobacteria in the sum biomass and production of phytoplankton decreased along with increase in the productivity in the waterbody. However, the abundance of virioplankton increased in such conditions. The extent of infection by phages and virus-induced mortality of picocyanobacteria were two times higher on average in the mesoeutrophic reservoir compared to the mesotrophic one: 14 and 7% of the number of picocyanobacteria, 21 and 11% of daily production of picocyanobacteria, respectively. Mortality in picocyanobacteria resulted from lysis by viruses increased (up to 52% of daily production of picocyanobacteria) at the local parts where water was blooming with phytoplankton and at the parts affected by communal pollution.

[The microbial loop in the planktonic communities in lakes with various trophic status].

The structure of planktic trophic chains was studied in eight lakes of European Russia and five lakes in Central Asia. The lakes differed in the level of productivity, morphometric parameters, and the type of agitation and mineralization. It is found that the microbial loop of picophototrophic organisms, bacteria, heterotrophic flagellates, infusoria, and viruses constitutes 12.3-64.7% of the total plankton biomass. Positive correlation between the biomass of microbial community and the primary production of phytoplankton is observed, whereas no relation is revealed between the share of microorganisms in the plankton biomass and the trophic status of the water body. The presence of a great number of cladocerans decreased the role of the microbial loop in the structural organization of the planktic community. Heterotrophic flagellates consuming 3-81% of daily bacterial production were the principal cause of bacteria elimination only in some of the studied water bodies.

[Viruses in the plankton of the Rybinsk Reservoir].

The role of autochthonous viruses in the regulation of bacterioplankton abundance and production was studied in the Rybinsk Reservoir. During the ice-free period, the number of virus-like particles varied within the range of (11.0-57.4) x 10(6) particles/ml. The virus to bacterioplankton abundance ratio ranged within 3.0-9.4. From 4 to 25% of bacterioplankton was infected by phages. A single infected cell contained up to 80 mature virus particles. The phage-induced bacterioplankton mortality in different parts of the reservoir constituted 3.7-41.8% (22.5% on average) of bacterioplankton daily production. Heterotrophic flagellates grazed from 7.6 to 68.8% (27.5% on average) of the daily bacterial production. Thus, along with flagellates, viruses are an important factor controlling bacterioplankton development in the reservoir.

[Enumeration of active cells in bacterial plankton of the Rybinsk reservoir with 5-cyano-2,3-ditolyltetrasolium: comparison with other methods]. / Opredelenie soderzhaniia aktivnykh kletok v bakterioplanktone Rybinskogo vodokhranilishcha s pomoshch'iu 5-tsiano-2,3-ditoliltetrazoliia: sravnenie s drugimi metodami.

The enumeration of actively respiring bacterial cells in different biotopes of the littoral zone of the Rybinsk Reservoir during the spring period of ice thaw using the fluorescent dye 5-cyano-2,3-ditolyl tetrazolium chloride showed that bacterial communities growing on the bottom surface of the ice cover and in water overgrown by higher aquatic plants were most active. The number of active cells among individual bacterial cells averaged 20% and reached about 40% among aggregated and filamentous bacterial cells. The results of the count of active bacteria by this method were compared with those obtained by other methods.