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°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°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.     

Pico- and Nanoplankton in Aquatic Ecosystems in the Valley of the Lakes and Great Lakes Depression (Mongolia)

The abundance and biomass of bacterioplankton, phototrophic picoplankton, and heterotrophic nanoflagellates has been determined in lakes, rivers, and reservoirs located in the Valley of the Lakes and Great Lakes Depression (Mongolia). The species richness of the heterotrophic flagellates and their consumption of bacteria are estimated. Pico- and nanoplankton are the most abundant in shallow mineral lakes Orog and Tatsyn and in the freshwater Durgun Reservoir. Heterotrophic nanoflagellates consume 26–92% (on average 66%) of the daily bacterioplankton production. Thus, flagellates are important in the transfer of bacterial carbon to the higher levels of planktonic trophic webs. A total of 30 species and their forms of heterotrophic flagellates from 14 large taxa are identified. The highest species diversity of these protists are found in the Durgun and Taishyr reservoirs.     

Structure and functioning of the microbial loop in a boreal reservoir

The abundance and biomass of the main components of the microbial plankton food web (“microbial loop”)—heterotrophic bacteria, phototrophic picoplankton and nanoplankton, heterotrophic nanoflagellates, ciliates and viruses, production of phytoplankton and bacterioplankton, bacterivory of nanoflagellates, bacterial lysis by viruses, and the species composition of protists—have been determined in summer time in the Sheksna Reservoir (the Upper Volga basin). A total of 34 species of heterotrophic nanoflagellates from 15 taxa and 15 species of ciliates from 4 classes are identified. In different parts of the reservoir, the biomass of the microbial community varies from 26.2 to 64.3% (on average 45.5%) of the total plankton biomass. Heterotrophic bacteria are the main component of the microbial community, averaging 63.9% of the total microbial biomass. They are the second (after the phytoplankton) component of the plankton and contribute on average 28.6% to the plankton biomass. The high ratio of the production of heterotrophic bacteria to the production of phytoplankton indicates the important role of bacteria, which transfer carbon of allochthonous dissolved organic substances to a food web of the reservoir.     

Planktonic viruses,heterotrophic bacteria,and nanoflagellates in fresh and coastal marine waters of the Kara Sea Basin (the Arctic)

In August–September 2009, the concentration of dissolved organic matter and quantitative distribution of virioplankton, bacterioplankton, and heterotrophic nanoflagellates were studied in the coastal waters of the Kara Sea, the fresh waters of the islands and the coasts of the sea, and the estuaries of the Ob’ and Yenisei rivers. A high positive correlation was observed between the abundances of viruses and bacteria. The frequency of visibly infected bacteria in marine waters ranged from 0.6 to 4.3% (an average of 1.6%); in the fresh waters of islands and coastline and in estuaries, it ranged from 0.3 to 3.9% (an average of 1.5%) and from 0.5 to 1.6% (an average of 1.1%) respectively. In most surveyed water bodies, the role of viruses in bacterioplankton mortality was considerably higher than that of heterotrophic flagellates.     

Abundance and Biomass Responses of Microbial Food Web Components to Hydrology and Environmental Gradients within a Floodplain of the River Danube

This study investigated the relationships of time-dependent hydrological variability and selected microbial food web components. Samples were collected monthly from the Kopa?ki Rit floodplain in Croatia, over a period of 19 months, for analysis of bacterioplankton abundance, cell size and biomass; abundance of heterotrophic nanoflagellates and nanophytoplankton; and concentration of chlorophyll a. Similar hydrological variability at different times of the year enabled partition of seasonal effects from hydrological changes on microbial community properties. The results suggested that, unlike some other studies investigating sites with different connectivity, bacterioplankton abundance, and phytoplankton abundance and biomass increased during lentic conditions. At increasing water level, nanophytoplankton showed lower sensitivity to disturbance in comparison with total phytoplankton biomass: this could prolong autotrophic conditions within the floodplain. Bacterioplankton biomass, unlike phytoplankton, was not impacted by hydrology. The bacterial biomass less affected by hydrological changes can be an important additional food component for the floodplain food web. The results also suggested a mechanism controlling bacterial cell size independent of hydrology, as bacterial cell size was significantly decreased as nanoflagellate abundance increased. Hydrology, regardless of seasonal sucession, has the potential to structure microbial food webs, supporting microbial development during lentic conditions. Conversely, other components appear unaffected by hydrology or may be more strongly controlled by biotic interactions. This research, therefore, adds to understanding on microbial food web interactions in the context of flood and flow pulses in river-floodplain ecosystems.     

Trophic interactions within the microbial food web in a tropical floodplain lake (Laguna Bufeos, Bolivia)

Whether the primary role of bacterioplankton is to act as "remineralizers" of nutrients or as direct nutritional source for higher trophic levels will depend on factors controlling their production and abundance. In tropical lakes, low nutrient concentration is probably the main factor limiting bacterial growth, while grazing by microzooplankton is generally assumed to be the main loss factor for bacteria. Bottom-up and top-down regulation of microbial abundance was studied in six nutrient limitation and dilution gradient-size fractionation in situ experiments. Bacteria, heterotrophic nanoflagellates (HNF), ciliates and rotifers showed relatively low densities. Predation losses of HNF and ciliates accounted for a major part of their daily production, suggesting a top-down regulation of protistan populations by rotifers. Phosphorus was found to be strongly limiting for bacterial growth, whereas no response to enrichment with Nitrogen or DOC was detected. HNF were the major grazers on bacteria (g-0.43 d(-1)), the grazing coefficient increased when ciliates were added (g- 0.80 d(-1)) but decreased when rotifers were added (g- 0.23 d(-1)) probably due to nutrient recycling or top-down control of HNF and ciliates by rotifers.     

The densities of bacteria, picophytoplankton, heterotrophic nanoflagellates and ciliates in Lake Kasumigaura (Japan) monitored monthly since 1996

This data paper describes the densities of the bacterioplankton, picocyanobacteria, eukaryotic picoplankton, heterotrophic nanoflagellates and ciliates in the water of Lake Kasumigaura??a shallow, eutrophic lake that is the second largest in Japan. All of these planktonic organisms are components of a microbial loop and are countable using an epifluorescence microscope. These data represent the results of monthly collections from April 1996 through March 2010 at two sites on the lake, and this data set is unique among the available published data papers concerning lakes or plankton and continues to be freely available. The monitoring was performed as a component of the Lake Kasumigaura Long-term Environmental Monitoring program conducted by National Institute for Environmental Studies since 1977 and includes water quality, plankton, and benthos. The data have been used for ecological studies and for studies on the management of water quality.     

Direct and indirect evidence of size-selective grazing on pelagic bacteria by freshwater nanoflagellates

Size-selective grazing of three heterotrophic nanoflagellates (with cell sizes of 21, 44, and 66 mum) isolated from Lake Arlington, Texas was examined by using a natural mixture of fluorescence labelled lake bacteria. Sizes of ingested bacteria in food vacuoles were directly measured. Larger bacterial cells were ingested at a frequency much higher than that at which they occurred in the assemblage, indicating preferential flagellate grazing on the larger size classes within the lake bacterioplankton. Water samples were collected biweekly from June through September, 1989, fractionated by filtration, and incubated for 40 h at in situ temperatures. The average bacterial size was always larger in water which was passed through 1-mum-pore-size filters (1-mum-filtered water) (which was predator free) than in 5-mum-filtered water (which contained flagellates only) or in unfiltered water (in which all bacterivores were present). The increase of bacterial-cell size in 1-mum-filtered water was caused by a shift in the size structure of the bacterioplankton population. Larger cells became more abundant in the absence of flagellate grazing.     

Prey selectivity of bacterivorous protists in different size fractions of reservoir water amended with nutrients

An experiment designed to examine food preferences of heterotrophic nanoflagellates (HNF) grazing on bacterioplankton was performed in the freshwater Rímov reservoir (Czech Republic). Water samples were size-fractionated to obtain < 5 microm filtrate containing bacteria and HNF. To manipulate resource availability, < 5 microm treatments were incubated in dialysis bags submerged in the barrels filled with the unfiltered reservoir water amended with either orthophosphate or glucose or combination of both. We employed rRNA-targeted probes to assess HNF prey preferences by analysing bacterial prey in HNF food vacuoles compared with available bacteria. Actinobacteria (the HGC69a probe) were avoided by HNF in all treatments. Cytophaga-Flavobacterium-Bacteroidetes bacteria (the CF319a probe) were positively selected mainly in treatments in which bacteria were heavily grazed, the < 5 microm treatments, but this trend was less pronounced towards the end of the study. The members of a small subcluster of Betaproteobacteria (the R-BT065 probe) were mostly positively selected. The nutrient amendments differentially affected bacterioplankton dynamics in almost all treatments, and together with the size fractionation, altered HNF overall bacterivory as well as prey selection. Analyses of bacterivores in unfiltered treatments allowed to detect the effect of different protists on shifts in HNF selectivity observed in < 5 microm compared with unfiltered treatments.     

Effect of mineral suspended matter on the productive characteristics of bacterioplankton and the degradation of labile organic materials

The effect of mineral particulate matter on the population of bacterioplankton, its aggregation, and productive characteristics was studied in model experiments with different concentrations of particulate kaolin and the same concentration of organic substance (sodium humate). It was found that the presence of mineral particulate matter stimulated the aggregation of bacterioplankton, improved bacterial production, and extended the productive period of bacterioplankton. The integral specific production of aggregated bacterioplankton was higher than that of free-swimming bacterioplankton. The energy metabolic coefficient K2 of bacterioplankton in the presence of mineral particulate matter was higher than in its absence.     

Flow cytometric detection and quantification of heterotrophic nanoflagellates in enriched seawater and cultures

A flow cytometric protocol to detect and enumerate heterotrophic nanoflagellates (HNF) in enriched waters is reported. At present, the cytometric protocols that allow accurate quantification of bacterioplankton cannot be used to quantify protozoa for the following reasons: i) the background produced by the bacterial acquisitions does not allow the discrimination of protozoa at low abundance, ii) since the final protozoan fluorescence is much higher than the bacterioplankton fluorescence (more than 35 fold) the protozoa acquisitions lie outside the range. With an increase in the fluorescence threshold and a reduction of the fluorescence detector voltage, low fluorescence particles (bacteria) are beneath the detection limits and only higher fluorescence particles (most of them heterotrophic nanoflagellates) are detected. The main limitation for the application of the cytometric protocol developed is that a ratio of bacteria/HNF below 1000 is needed. At higher ratios, the background of larger cells of bacterioplankton makes it difficult to discriminate protozoa. The proposed protocol has been validated by epifluorescence microscopy analyzing both a mixed community and two single species of HFN: Rhynchomonas nasuta and Jakoba libera. Taking into account the required bacteria/HNF ratio cited above, the results provide evidence that the flow cytometric protocol reported here is valid for counting mixed communities of HNF in enriched seawater and in experimental micro or mesocosms. In the case of single species of HNF previous knowledge of the biological characteristics of the protist and how they can affect the effectiveness of the flow cytometric count is necessary.     

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.     

Heterotrophic microorganisms and viruses in the water of the Gorky reservoir during a period of anomalously high water temperature

During the anomalously hot summer of 2010, the water temperature in the Gorky reservoir reached 27–33°C. Pronounced cyanobacterial blooms occurred in the limnetic part of the reservoir. The average values for bacterioplankton abundance (11.58 ± 1.25 × 10⁶ cell/mL), biomass (886 ± 96 mg/m³), and production [169 ± 32 mg C/(m³ day)] were twice as high as in the year with temperatures comparable to long-term average values. These parameters were higher in the limnetic part than in the river one. The abundance (4.86 ± 0.75 × 10³ cell/mL) and biomass (138 ± 9 mg/m³) of heterotrophic nanoflagellates were 2.3 and 1.7 times higher, respectively, than in years with regular temperature regimes. The average number of plank-tonic viral particles (N _v) in 2010 was 48.89 ± 9.54 × 10⁶ particles/mL, while virus-induced bacterial mortality (VMB) accounted for 26.9 ± 4.6% of the bacterial production. The N _v and VMB values in the limnetic part of the reservoir were, respectively, 1.5 and 1.8 times higher than in the river one.     

Seasonality of top-down control of bacterioplankton at two central Red Sea sites with different trophic status

The role of bottom-up (nutrient availability) and top-down (grazers and viruses mortality) controls on tropical bacterioplankton have been rarely investigated simultaneously from a seasonal perspective. We have assessed them through monthly samplings over 2 years in inshore and offshore waters of the central Red Sea differing in trophic status. Flow cytometric analysis allowed us to distinguish five groups of heterotrophic bacteria based on physiological properties (nucleic acid content, membrane integrity and active respiration), three groups of cyanobacteria (two populations of Synechococcus and Prochlorococcus), heterotrophic nanoflagellates (HNFs) and three groups of viruses based on nucleic acid content. The dynamics of bacterioplankton and their top-down controls varied with season and location, being more pronounced in inshore waters. HNFs abundances showed a strong preference for larger prey inshore (r = −0.62 to −0.59, p = 0.001–0.002). Positive relationships between viruses and heterotrophic bacterioplankton abundances were more marked inshore (r = 0.67, p < 0.001) than offshore (r = 0.44, p = 0.03). The negative correlation between HNFs and viruses abundances (r = −0.47, p = 0.02) in shallow waters indicates a persistent seasonal switch between protistan grazing and viral lysis that maintains the low bacterioplankton stocks in the central Red Sea area.     

The Effect of Mineral Particulate Matter on the Productive Characteristics of Bacterioplankton and the Degradation of Labile Organic Material

The effect of mineral particulate matter on the population of bacterioplankton, its aggregation, and productive characteristics was studied in model experiments with different concentrations of particulate kaolin and the same concentration of organic substance (sodium humate). It was found that the presence of mineral particulate matter stimulated the aggregation of bacterioplankton, improved bacterial production, and extended the productive period of bacterioplankton. The integral specific production of aggregated bacterioplankton was higher than that of free-swimming bacterioplankton. The energy metabolic coefficient K ₂ of bacterioplankton in the presence of mineral particulate matter was higher than in its absence.     

Trophic relationships between planktonic bacteria,heterotrophic nanoflagellates and viruses in a mesoeutrophic reservoir

The abundance, biomass, and production (Р _В) of bacrerioplankton; the taxonomic composition, abundance, biomass of heterotrophic nanoflagellates (HNF) and the rate of consumption of bacteria by HNFs; and the abundance of virioplankton, frequency of visibly infected bacterial cells, virus-induced mortality of bacterioplankton, and viral production were estimated in the mesoeutrophic Rybinsk Reservoir. The rate of bacterial mortality due to viral lysis (7.8–34.1%, on average 17.2 ± 2.0% of daily Р _В) was lower than the consumption of bacteria by the HNF community (15.4–61.3%, on average 32.0 ± 4.2% of daily Р _В). While consuming bacteria, HNFs simultaneously absorbed a significant number of viruses residing on the surface and inside the bacterial cells.     

Patterns of marine bacterioplankton biodiversity in the surface waters of the Scotia Arc, Southern Ocean

Spatial patchiness in marine surface bacterioplankton populations was investigated in the Southern Ocean, where the Antarctic Circumpolar Current meets the islands of the Scotia Arc and is subjected to terrestrial input, upwelling of nutrients and seasonal phytoplankton blooms. Total bacterioplankton population density, group-specific taxonomic distribution and six of eight dominant members of the bacterioplankton community were found to be consistent across 18 nearshore sites at eight locations around the Scotia Arc. Results from seven independent 16S rRNA gene clone libraries (1223 sequences in total) and fluorescent in situ hybridization suggested that microbial assemblages were predominantly homogeneous between Scotia Arc sites, where the Alphaproteobacteria, Gammaproteobacteria and the Cytophaga-Flavobacterium-Bacteroidetes cluster were the dominant bacterial groups. Of the 1223 useable sequences generated, 1087 (89%) shared ≥?97% similarity with marine microorganisms and 331 (27%) matched published sequences previously detected in permanently cold Arctic and Antarctic marine environments. Taken together, results suggest that the dominant bacterioplankton groups are consistent between locations, but significant differences may be detected across the rare biodiversity.     

Heterotrophic nanoflagellates in water column and bottom sediments of the Rybinsk Reservoir: Species composition,abundance, biomass and their grazing impact on bacteria

Abundance, biomass, and taxonomic composition of heterotrophic nanoflagellates (HNFs) have been determined in the water column and bottom sediments of the large lowland meso-eutrophic reservoir (Rybinsk Reservoir, Upper Volga) in summer. The role of HNFs in the consumption of the bacterial production is estimated. In the reservoir, 55 species from 15 large taxa, including 35 species from the plankton, are identified and 45 species are from benthos samples. The orders Kinetoplastida, Choanomonada, and Chrysomonadida are distinguished by the highest species diversity. Abundance and biomass of HNFs in the water column average 991 ± 326 cells/mL and 41.4 ± 14.1 mg/m³, while in the bottom sediments they are (236 ± 61) × 10³ cells/mL and 10.7 ± 4.0 μg/mL, respectively. The biomass of HNFs average 11.2% of the bacterial biomass in the water column and only 0.8% of that in the sediments. Flagellates are found to be a major factor which control the development of bacterioplankton grazing, on average, 32.3% of its daily production, whereas their impact on bacteriobenthos is insignificant, as they consume, on average, only 2.0% of its production.     

Relation entre habitat et peuplements de poissons: Essai de definition d'une methode numerique pour des rivieres nordiques

Estimates of primary production in relation to bacterioplankton growth kinetics have been made in three fish ponds with mono and polyculture in the Fish Farm Nivka, Kiev, USSR. Since the basic process of primary production is dependent upon such prevailing conditions as temperature, light and nutrient rather than upon the introduction of fish species, almost similar production rates were obtained in these ponds, with the variance ratio found not significantly different from zero (P > 0.05). So was the case with bacterioplankton growth kinetics (P > 0.05). A maximal gross production coupled with a bacterioplankton peak was demonstrated in the summer month of July with the rise of temperature of the water, while minimal production was observed in September. The effect of season was highly significant (P < 0.001) upon both primary production and bacterioplankton. Extreme deficiency of the net photosynthetic production of O₂ was observed in summer due to less prevalence of phytoplanktonic algae as a consequence of heavy grazing by the herbivorous fishes at the time of their active growth. A plot of biochemical oxygen demand as function of the respiratory loss of O₂ due t0 bacterial population showed a direct linear relationship (P < 0.001). Lack of definite correlation (P > 0.05) was indicated between the BOD and the total bacterioplankton population. There existed no definite relationship (P > 0.05) between primary production and bacterioplankton, perhaps because of the very low percentage of autotrophic photosynthetic bacteria and abundance of heterotrophic bacteria in the composition of total bacterioplankton.     

Viruses in the plankton of the Rybinsk Reservoir

The role of autochthonous viruses in the regulation of bacterioplantkon 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) × 10⁶ 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.