Abstract
The distribution of virioplankton, abundance and production, frequency of visibly infected cells of heterotrophic bacteria and autotrophic picocyanobacteria and their virus-induced mortality have been studied in mesotrophic and eutrophic reservoirs of the Upper and Middle Volga (Ivankovo, Uglich, Rybinsk, Gorky, Cheboksary, and Sheksna reservoirs). The abundance of planktonic viruses (VA) is on average by 4.6 ± 1.2 times greater than the abundance of bacterioplankton (BA). The distribution of VA in the Volga reservoirs was largely determined by the distribution of BA and heterotrophic bacterioplankton production (PB). There was a positive correlation between VA and BA and between VA and PB. In addition, BA and VA were both positively correlated with primary production of phytoplankton. Viral particles of 60 to 100 µm in size dominated in the phytoplankton composition. A large number of bacteria and picocyanobacteria with viruses attached to the surface of their cells were found in the reservoirs. Viruses as the most numerous component of plankton make a significant contribution to the formation of the planktonic microbial community biomass. The number of phages inside infected cells of bacteria and picocyanobacteria reached 74‒109 phages/cell. Easily digestible organic matter, which entered the aquatic environment as a result of viral lysis of bacteria and picocyanobacteria, could be an additional source of carbon for living bacteria. The results of long-term studies indicate a significant role of viruses in functioning of planktonic microbial communities in the Volga reservoirs.
Highlights
Research over the past three decades has firmly established that viruses are the most abundant and diverse biological entities (Bratbak & Heldal, 2000; Wommack & Colwell, 2000; Weinbauer, 2004), thereby forming an integral component of microbial food web in a great variety of aquatic environments (Bratbak et al, 1994; Clasen et al, 2008; Wilhelm & Matteson, 2008)
We analyze the results of long-term studies of the abundance and production of free viruses, the abundance of viruses attached to bacteria, the size of viral particles, frequencies of visibly infected cells, and virus-induced mortality of heterotrophic bacteria and picocyanobacteria in six reservoirs of the Upper and Middle Volga basin
The frequency of visibly infected bacterial cells (FVIC) by viruses, i.e. the proportion of bacteria containing mature phage particles inside cells in NB (Fig. 2) varied from 0.5% of NB in the Sheksna Reservoir to 5.0% of NB in the Gorky and Cheboksary reservoirs averaging 2.3 ± 0.5% of NB (Table 3)
Summary
Research over the past three decades has firmly established that viruses are the most abundant and diverse biological entities (Bratbak & Heldal, 2000; Wommack & Colwell, 2000; Weinbauer, 2004), thereby forming an integral component of microbial food web (viruses, bacteria, picophytoplankton, protozoa) in a great variety of aquatic environments (Bratbak et al, 1994; Clasen et al, 2008; Wilhelm & Matteson, 2008). In pelagic food webs carbon content of bacteria and picocyanobacteria can be either transferred to the upper trophic levels when consumed by protozoa and multicellular filter feeders or, alternatively, contribute to the dissolved organic matter stocks as a result of viralinduced cell lysis (Ram et al, 2015; Vaqué et al, 2017). We analyze the results of long-term studies of the abundance and production of free viruses, the abundance of viruses attached to bacteria, the size of viral particles, frequencies of visibly infected cells, and virus-induced mortality of heterotrophic bacteria and picocyanobacteria in six reservoirs of the Upper and Middle Volga basin. A quantitative assessment of the role of viruses in carbon fluxes in planktonic food webs of the reservoirs has been performed
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