Feeding and grazing impact by small marine heterotrophic dinoflagellates on heterotrophic bacteria

ABSTRACT. We investigated the feeding of the small heterotrophic dinoflagellates (HTDs) Oxyrrhis marina , Gyrodinium cf. guttula , Gyrodinium sp., Pfiesteria piscicida , and Protoperidinium bipes on marine heterotrophic bacteria. To investigate whether they are able to feed on bacteria, we observed the protoplasm of target heterotrophic dinoflagellate cells under an epifluorescence microscope and transmission electron microscope. In addition, we measured ingestion rates of the dominant heterotrophic dinoflagellate, Gyrodinium spp., on natural populations of marine bacteria (mostly heterotrophic bacteria) in Masan Bay, Korea in 2006–2007. Furthermore, we measured the ingestion rates of O. marina , G . cf. guttula , and P. piscicida on bacteria as a function of bacterial concentration under laboratory conditions. All HTDs tested were able to feed on a single bacterium. Oxyrrhis marina and Gyrodinium spp. intercepted and then ingested a single bacterial cell in feeding currents that were generated by the flagella of the predators. During the field experiments, the ingestion rates and grazing coefficients of Gyrodinium spp. on natural populations of bacteria were 14–61 bacteria/dinoflagellate/h and 0.003–0.972 day⁻¹, respectively. With increasing prey concentration, the ingestion rates of O. marina , G . cf. guttula , and P. piscicida on bacteria increased rapidly at prey concentrations of ca 0.7–2.2 × 10⁶ cells/ml, but increased only slowly or became saturated at higher prey concentrations. The maximum ingestion rate of O. marina on bacteria was much higher than those of G . cf. guttula and P. piscicida . Bacteria alone supported the growth of O. marina . The results of the present study suggest that some HTDs may sometimes have a considerable grazing impact on populations of marine bacteria, and that bacteria may be important prey.     

Distribution of viruses in the water column of the ice-covered Rybinsk Reservoir and their contribution to heterotrophic bacteria mortality

Virioplankton and bacterioplankton abundance has been determined in the pelagic and littoral zones of the Rybinsk Reservoir during the ice-covered period. The role of viruses in heterotrophic bacterioplankton infection and mortality is assessed. At water temperatures between 0.3 and 0.9°C, the number of planktonic virus particles and planktonic bacteria varies from 37.1 × 10⁶ to 84.1 × 10⁶ particles/mL, (57.3 ± 2.1) × 10⁶ particles/mL on average and from 2.50 × 10⁶ to 6.11 × 10⁶ cells/mL, (3.66 ± 0.16) × 10⁶ cells/mL on average, respectively. The ratio of the virus number to the bacteria number varies from 8.8 to 27.9, being 16.5 ± 0.7 on average. Visually infected cells comprise 0.3–0.5% (1.5 ± 0.2% on average) of the total number of bacterioplankton. Infected bacterial cells contain from 5 to 107 (17 ± 4 on average) mature virus particles. The average virus-induced mortality of bacteria accounts for 13.0 ± 1.9% (variations range from 2 to 55%) of the daily bacterial production, indicating that viruses play an important role in the regulation of bacterioplankton production and abundance in the Rybinsk Reservoir during the ice-covered period.     

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.     

A study on biological activity measurements and heterotrophic bacteria in a small freshwater lake

A 6-m-deep lake has been sampled to measure the temporal and depth-wise distribution of heterotrophic bacteria and biological activity in the water. Surface, mid-depth and bottom waters were analysed at monthly intervals for a period of one year. The coefficient of heterotrophic activity, alkaline phosphatase activity and biological oxygen demand are used as an index of biological activity. The bacterial community was at maximum during spring, coinciding with high values of biological activity. Highest biological activity was observed in the bottom waters. Dissolved organic carbon showed a significant positive correlation with most of the biological activity parameters. This suggests that biological activity, as measured by the coefficient of heterotrophic activity, was more closely related to the concentration of substrates than to population density of heterotrophic bacteria.     

Aquatic heterotrophic bacteria have highly flexible phosphorus content and biomass stoichiometry

Bacteria are central to the cycling of carbon (C), nitrogen (N) and phosphorus (P) in every ecosystem, yet our understanding of how tightly these cycles are coupled to bacterial biomass composition is based upon data from only a few species. Bacteria are commonly assumed to have high P content, low biomass C:P and N:P ratios, and inflexible stoichiometry. Here, we show that bacterial assemblages from lakes exhibit unprecedented flexibility in their P content (3% to less than 0.01% of dry mass) and stoichiometry (C:N:P of 28: 7: 1 to more than 8500: 1200: 1). The flexibility in C:P and N:P stoichiometry was greater than any species or assemblage, including terrestrial and aquatic autotrophs, and suggests a highly dynamic role for bacteria in coupling multiple element cycles.Terrestrial ecosystems are an important source of nutrients and organic carbon (C) to freshwater rivers and lakes as well as the coastal ocean. Past work has shown that heterotrophic bacteria, a group of organisms that process terrestrial inputs of organic carbon, nitrogen (N) and phosphorus (P) (Biddanda et al., 2001), are C-poor and P-rich (Makino et al., 2003) relative to terrestrial inputs characterized by high C:P ratios. As a result, bacterial assemblages in freshwater ecosystems should experience elemental imbalance and act as efficient exporters of organic carbon to downstream ecosystems. However, freshwater ecosystems metabolize most of the organic C they receive from terrestrial ecosystems (Cole et al., 2007) and it has been shown recently that strains (Scott et al., 2012) and assemblages (Godwin and Cotner, 2014) of bacteria from lakes can be P-poor and stoichiometrically flexible. Here, we demonstrate that bacterial assemblages from lakes exhibit unprecedented plasticity in their stoichiometry and discuss the implications of flexible composition to ecosystem processes.To determine the extent of stoichiometric flexibility within assemblages, we performed two experiments in which we cultured the bacteria-sized fraction of plankton from a northern temperate lake under varying C:P supply ratios and measured their biomass composition (Supplementary Methods). We created C:P_supply ratios from 31.6:1 to more than 2 20 000:1 by manipulating the supply of phosphate in a defined medium, with all other nutrients in excess of C and P. At each level of C:P_supply, we enriched the lake assemblages in batch cultures and used these enrichments to inoculate chemostats at the same C:P_supply. The chemostats were maintained at a dilution rate (0.33 d⁻¹) that is low relative to assemblage growth rates measured in lakes (Cotner et al., 2001).The bacterial P content decreased from a mean of 3.55% of dry mass when the assemblage was C-limited to 0.006–0.05% when the assemblage was most P-limited (Figure 1). The range of P content measured in the assemblage cultures was nearly equal to the range of existing data in the literature, particularly for P relative to dry mass (Supplementary Table 6). Single-cell measurements from plankton environments indicated the potential for even lower phosphorus quotas (Norland et al., 1995; Cotner et al., 2010), although many of those cells may not be actively growing, potentially decreasing their demand for P-rich RNA, where much of the P resides in bacterial cells (Makino et al., 2003). The P relative to dry mass values measured here were lower than those reported for a bacterium grown in the absence of added phosphate and high concentrations of arsenate (0.012% of dry mass as P, Wolfe-Simon et al., 2010). The results presented here clearly demonstrate that bacteria can have P content less than 0.01% of dry mass when growing at low levels of P.Open in a separate windowFigure 1Effect of C:P_supply ratio on biomass P/cell (a), P/dry mass (b), C:P_biomass (c) and N:P_biomass (d) ratios in chemostats diluted at 0.33 d⁻¹. Data from Experiment 1 are displayed as solid circles, and open circles denote data from Experiment 2. At each level of C:P_supply, the data from replicate chemostat are staggered to improve clarity. The error bars represent the s.e. of the ratio for each chemostat, following propagation of errors from the numerator and denominator. In Experiment 1, C:P_biomass and N:P_biomass (analysis of variance, P<1 × 10^-5) increased and P/dry mass and P/cell decreased (P<0.005) significantly with increasing C:P_supply. On the basis of changes in C:P_biomass, the assemblage was defined as P-sufficient at C:P_supply of 31.6:1 and P-limited at C:P_supply of 10 000:1 and greater. At C:P_supply of 1 00 000:1, only one chemostat had P content above the analytical detection limit and only two chemostats without added P had N above the detection limit.The C:P_biomass and N:P_biomass of the bacterial assemblages increased from 28:1 and 6:1, respectively, when C-limited to a maximum of >8500:1 and >1200:1 when P-limited (Figure 1). The ranges of C:P_biomass and N:P_biomass observed in this study cover nearly the entire range of measurements recorded in previous studies for bacterial cultures and assemblages (Figure 2; Supplementary Table 6) and nearly match the ranges of C:P_biomass and N:P_biomass observed in vascular plant tissues (Elser et al., 2000; Sterner and Elser, 2002; Reich and Oleksyn, 2004). Furthermore, the bacterial assemblage (of multiple strains) exhibited greater stoichiometric plasticity than has been documented in any other species or assemblage, including terrestrial and aquatic primary producers (Sterner and Elser, 2002; Persson et al., 2010). These experiments demonstrate that previous assumptions of low and invariant C:P_biomass (Tanaka et al., 2009; Fanin et al., 2013) and high relative P content for bacteria (Wolfe-Simon et al., 2010) do not represent the physiological flexibility of bacteria in natural assemblages. Although mean cellular P content decreased under P limitation, much of the flexibility in C:P_biomass was due to a substantial increase in cellular C content (Supplementary Figure 1), likely owing to the accumulation of C-rich storage molecules (Thingstad et al., 2005).Open in a separate windowFigure 2Ranges of C:P_biomass and N:P_biomass for bacterial cultures and other organisms, with separate panels for C:P_biomass (panel a) and N:P_biomass (panel b). Data for heterotrophic bacteria are separated by sources: literature data (Supplementary Table 7), assemblage chemostat cultures (Godwin and Cotner, 2014) and this study. Ranges for other organisms were from ^a (Cross et al., 2005) and ^b (Elser et al., 2000). ^cRanges for E. coli were compiled from multiple studies (Supplementary Table 7). Seston refers to suspended particulate matter (phytoplankton, heterotrophs and detritus). The boxplots display data for individual replicate cultures where data are available, with the centerline representing the median, the edges of the box representing the 25% and 75% quantiles, and the whiskers representing the maximum and minimum values. Dashed lines indicate the Redfield ratio (C:N:P=106:16:1).The range of stoichiometric flexibility present in natural assemblages is critical to understanding homeostasis within ecosystems. Strict homeostasis of assemblage C:N:P_biomass leads to the prediction that the ratio of regenerated C:P increases dramatically with increasing resource C:P (Sterner, 1990), but flexible biomass stoichiometry allows tight coupling and negative feedback between bacterial biomass stoichiometry and resource stoichiometry, facilitating the inherent resilience of ecosystems to nutrient perturbations (Scheffer et al., 2001). It is increasingly recognized that much of the organic matter metabolized in rivers and lakes originates in terrestrial ecosystems where C:P and N:P ratios can be much higher than for organic matter originating in aquatic ecosystems (Lennon and Pfaff, 2005). The observations in this study of extreme flexibility in bacterial biomass stoichiometry are consistent with observations of higher and more variable biomass C:P and N:P in the seston (suspended particulate matter) in freshwaters than in pelagic (offshore) marine systems where terrestrial influences and nutrient gradients are less profound (Cotner et al., 2010).The bacteria in inland waters and the coastal ocean experience stoichiometric imbalance when they process terrestrial inputs of dissolved and particulate organic matter with high C:P ratios. Compared with bacteria with low and invariant C:P_biomass, assemblages that increase their C:P_biomass in response to this imbalance will remineralize less ‘excess'' C through respiration and could decrease the export of organic matter to downstream ecosystems. In ecosystems where internal nutrient cycling processes are dominant and bacteria regenerate a large fraction of available nutrients (for example, offshore marine systems), flexible bacterial stoichiometry likely stabilizes dissolved inorganic nutrient concentrations and inhibits fluctuations.The capacity of heterotrophic bacteria to continue to buffer C and nutrient feedbacks in ecosystems is likely challenged by the use of inorganic fertilizers that decrease the exported C:N and C:P ratios to aquatic systems (Arbuckle and Downing, 2001) and anthropogenic warming that increases both the export of organic carbon and the C:N:P stoichiometry of that material (Freeman et al., 2001; Urban et al., 2011). Additionally, because stoichiometric flexibility decreases with increasing relative growth rates (Makino and Cotner., 2004; Hillebrand et al., 2013), bacterial assemblages in low-temperature environments could become less flexible as the result of anthropogenic warming. By examining the capacity of aquatic bacterial assemblages to respond to C:N:P imbalance, we can evaluate the influence of stoichiometric flexibility on aquatic ecosystem productivity and the extent and periodicity of nutrient fluctuations.     

Sunlight effects on the DMSP-sulfur and leucine assimilation activities of polar heterotrophic bacterioplankton

The influence of solar ultraviolet radiation and photosynthetically active radiation (PAR) on summertime marine bacterial uptake and assimilation of sulfur from radiolabeled dimethlysulfoniopropionate (³⁵S-DMSP) was studied at four Arctic and two Antarctic stations. Incubations with ³H-leucine were also conducted for comparative purposes as a measurement of bacterial activity. Arctic waters were characterized by large numbers of colonial Phaeocystis pouchetii and higher DMSP concentrations than in the two diatom-dominated Antarctic samples. Exposure to full sunlight radiation (280–700?nm), and to a lesser extent to PAR?+?UVA (320–700?nm), generally decreased the bacterial assimilation of ³H-leucine with respect to darkness, and caused variable effects on ³⁵S-DMSP assimilation. By using a single-cell approach involving microautoradiography we found high percentages of sulfur assimilating cells within the bacterial groups Gammaproteobacteria, Bacteroidetes, SAR11 and Roseobacter despite the varying DMSP concentrations between Arctic and Antarctic samples. The dominant SAR11 clade contributed 50–70% of the cells assimilating both substrates in the Arctic stations, whereas either Gammaproteobacteria or SAR11 were the largest contributors to ³H-leucine uptake in samples from the two Antarctic stations. Only one station was analyzed for single-cell ³⁵S-DMSP assimilation in Antarctica, and Gammaproteobacteria were major contributors to its uptake, providing the first evidence for Antarctic bacteria actively taking up ³⁵S-DMSP. PAR?+?UVA repeatedly increased the number of SAR11 cells assimilating ³H-leucine. This pattern also occurred with other ³⁵S-DMSP assimilating groups, though not so consistently. Our results support a widespread capability of polar bacteria to assimilate DMSP-sulfur during the season of maximum DMSP concentrations, and show for the first time that all major polar taxa can be highly active at this assimilation under the appropriate circumstances. Our findings further confirm the role of sunlight as a modulator of heterotrophic carbon and sulfur fluxes in the surface ocean.     

Activity profiles of bacterioplankton in a eutrophic river

1. The significance of microbial diversity in processing dissolved organic matter (DOM) is largely unknown. We investigated the range of functional diversity in the bacterioplankton from a eutrophic river by profiling extracellular enzyme activities (EEA) and substrate-induced respiration (SIR) patterns.
2. The EEA profiles consisted of assays for 21 hydrolases, measured using fluorogenic substrates arrayed on 96-well microplates. Commercially available BiOLOG^® GN and ECO plates, which contained 120 different substrates, were used for the SIR profiles.
3. The EEA data were more dynamic than the SIR. Five enzymes, leucine aminopeptidase, alkaline phosphatase, alanine aminopeptidase, arginine aminopeptidase and β-glucosidase, showed consistently high activity; ten others were ubiquitous at lower activity levels; the remainder were detected intermittently. The SIR data showed less temporal variability. With one exception (citrate), the 20 substrates that generated the largest responses were all saccharides or their derivatives.
4. The EEA and SIR data did not generally correlate. Both methods were effective for ordinating bacterioplankton although, unlike the SIR, the EEA ordination followed a clear temporal trajectory.
5. Because the SIR profiles are based on a culture response, whilst the EEA profiles measure activity of the extant community, the latter appear to be more directly linked to the mechanics of DOM processing.     

The dynamics of heterotrophic nanoflagellates and bacterioplankton in a large ultra-oligotrophic Antarctic lake

The abundance of both heterotrophic nanoflagellates (HNAN) andbacterioplankton in a large (9km²) ultraoligotrophic Antarcticlake (Crooked Lake) were investigated from December 1992 untilNovember 1993. HNAN abundance peaked in spring, summer and autumn,falling to lowest numbers during the winter. Numbers rangedbetween 0 and 50.9x10⁴ l^–1. Bacterioplankton abundancewas highest during the late summer and then fell progressivelytowards winter and autumn (range 1.19–4.46x10⁶ l^–1)In contrast to numbers, mean cell volumes (MCV) of the bacteriareached their highest in spring, and consequently highest bacterialbiomass occurred at this time. MCV ranged between 0.052 and0.224µm³. Bacterial production measurements followingthe incorporation of [³H] thymidine into DNA and [¹⁴C] leucineinto protein using a doubling-labelling procedure were undertakenin January, June, August, October and November. Rates variedbetween 2.8 and 52 ng C l¹ h¹. On occasions, a significant differencein production rates based on the uptake of leucine and thymidinewas observed, suggesting unbalanced growth. Highest rates ofproduction coincided with times of high dissolved organic carbonlevels in the water column and lowest production with low levelsof DOC. HNAN grazing rates were measured by following the uptakeof fluorescently labelled bacteria and averaged 4.8 bacterialcells individual¹ day¹ at 2 and 4°C. Specific growth rates(h¹) ranged around 0.00070–0.00077 in both the field andlaboratory, giving doubling times of 37.3 and 41.0 days, respectively.These low rates of grazing and growth indicate that there isno adaptation to low temperatures in these freshwater protists.Based on these data, the gross production efficiency is 24%.HNAN removed between 0.1 and 9.7% of bacterial production perday.     

Predation rates of flagellate and ciliated protozoa on bacterioplankton in a river

Abstract Predation rates of flagellate and ciliate protozoa on the bacterioplankton of Butrón River (Spain) were determined from FLB (fluorescently labelled bacteria) uptake rates. Bacterial and ciliate protozoa counts were higher when higher water temperature was recorded. Flagellate counts did not show this pattern, which suggested predation of flagellates by other organisms, or some other different nutritional mode besides phagotrophy. Average individual ciliate predation rates were up to 40-times higher than those of flagellates. These results were compared with similar data obtained from other authors in several aquatic systems. However, the population predation rates of flagellate protozoa were on average 6-times higher than that of ciliate protozoa, due to the low population numbers of the latter. Thus, flagellate protozoa can be considered as more important bacterial consumers than ciliates in this aquatic system.     

嗜酸异养菌对自养菌Acidithiobacillus ferrooxidans金属离子抗性和生物浸出的影响

【目的】了解嗜酸异养菌在诸如酸性矿坑水(AMD)和生物浸出体系等极端酸性环境中对浸矿微生物产生的影响。【方法】研究由嗜酸异养菌Acidiphilium acidophilum和自养菌Acidithiobacillus ferrooxidans经长期驯化后形成的共培养体系分别在Cd2+、Cu2+、Ni2+和Mg2+胁迫下的稳定性;并将此共培养体系应用于黄铁矿和低品位黄铜矿的生物浸出实验。【结果】在上述4种金属离子分别存在的条件下,异养菌Aph.acidophilum均能促进At.ferrooxidans对亚铁的氧化,提高其对能源利用的效率。共培养体系中的异养菌Aph.acidophilum使At.ferrooxidans对Cu2+的最大耐受浓度(MTC)由2.0 g/L提高到5.0 g/L,而且共培养的细胞数量与2.0 g/L Cu2+条件下生长的At.ferrooxidans纯培养相似。另外,共培养中的At.ferrooxidans对Mg2+的MTC也由12.0 g/L提高到17.0 g/L。生物浸出实验中嗜酸异养菌Aph.acidophilum促进了At.ferrooxidans对黄铁矿样品的浸出,浸出率较其纯培养提高了22.7%;但在含铁量较低的低品位黄铜矿浸出体系中共培养和At.ferrooxidans纯培养的浸出率均低于33%。在加入2.0 g/L Fe2+的低品位黄铜矿浸出体系中,共培养和At.ferrooxidans纯培养的浸出率均得到提高,分别达到52.22%和41.27%。【结论】以上结果表明,Aph.acidophilum与At.ferrooxidans共培养在一定的环境胁迫下仍能保持其稳定性并完成各自的生态功能,并且嗜酸异养菌Aph.acidophilum适合在含铁量较高的浸出体系中与铁氧化细菌共同作用来提高生物浸出的效率。     

Influence of small river conditions on the abundance of Tubificidae

Two small rivers, the Yerik and Nezhegolyok, were studied seasonally in 1979–1980. Increase in concentration of nitrogen and phosphorus compounds in the river water as well as proliferation of enterococci and enterobacilli favoured the abundance of Tubificidae while, at the same time, their numbers were reduced by chironomids and leeches.     

Recombination between RNA viruses and plasmids might have played a central role in the origin and evolution of small DNA viruses

The finding that viruses with RNA and DNA genomes can recombine to produce chimeric entities provides valuable insights into the origin and evolution of viruses. It also substantiates the hypothesis that certain groups of DNA viruses could have emerged from plasmids via acquisition of capsid protein-coding genes from RNA viruses.     

Metabolic diversity of heterotrophic bacterioplankton over winter and spring in the coastal Arctic Ocean

Metabolic diversity of heterotrophic bacterioplankton was tracked from early winter through spring with Biolog Ecoplates under the seasonally ice covered arctic shelf in the Canadian Arctic (Franklin Bay, Beaufort Sea). Samples were taken every 6 days from December 2003 to May 2004 at the surface, the halocline where a temperature inversion occurs, and at 200 m, close to the bottom. Despite the low nutrient levels and low chlorophyll a , suggesting oligotrophy in the winter surface waters, the number of substrates used (NSU) was greater than in spring, when chlorophyll a concentrations increased. Denaturing gradient gel electrophorisis analysis also indicated that the winter and spring bacterial communities were phylogenetically distinct, with several new bands appearing in spring. In spring, the bacterial community would have access to the freshly produced organic carbon from the early phytoplankton bloom and the growth of rapidly growing specialist phenotypes would be favoured. In contrast, in winter bacterioplankton consumed more complex organic matter originated during the previous year's phytoplankton production. At the other depths we tested the NSU was similar to that for the winter surface, with no seasonal pattern. Instead, bacterioplankton metabolism seemed to be influenced by resuspension, advection, and sedimentation events that contributed organic matter that enhanced bacterial metabolism.     

Isolation and gene quantification of heterotrophic N2-fixing bacterioplankton in the Baltic Sea

Cyanobacteria are regarded as the main N(2)-fixing organisms in marine waters. However, recent clone libraries from various oceans show a wide distribution of the dinitrogenase reductase gene (nifH) originating from heterotrophic bacterioplankton. We isolated heterotrophic N(2)-fixing bacteria from Baltic Sea bacterioplankton using low-nitrogen plates and semi-solid diazotroph medium (SSDM) tubes. Isolates were analysed for the nitrogenase (nifH) gene and active N(2) fixation by nested polymerase chain reaction (PCR) and acetylene reduction respectively. A primer-probe set targeting the nifH gene from a gamma-proteobacterial isolate, 97% 16S rDNA similarity to Pseudomonas stutzeri, was designed for measuring in situ dynamics using quantitative real-time PCR. This nifH gene sequence was detected at two of 11 stations in a Baltic Proper transect at abundances of 3 x 10(4) and 0.8 x 10(3) copies per litre seawater respectively. Oxygen requirements of isolates were examined by cultivation in SSDM tubes where oxygen gradients were determined with microelectrodes. Growth, and thereby N(2) fixation, was observed as horizontal bands formed at oxygen levels of 0-6% air saturation. The apparent microaerophilic or facultative anaerobic nature of the isolates explains why the SSDM approach is the most appropriate isolation method. Our study illustrates how combined isolation, functional analyses and in situ quantification yielded insights into the oxygen requirements of heterotrophic N(2)-fixing bacterioplankton isolates, which were confirmed to be present in situ.     

Halobacteriovorax,an underestimated predator on bacteria: potential impact relative to viruses on bacterial mortality

Predation on bacteria and accompanying mortality are important mechanisms in controlling bacterial populations and recycling of nutrients through the microbial loop. The agents most investigated and seen as responsible for bacterial mortality are viruses and protists. However, a body of evidence suggests that predatory bacteria such as the Halobacteriovorax (formerly Bacteriovorax), a Bdellovibrio-like organism, contribute substantially to bacterial death. Until now, conclusive evidence has been lacking. The goal of this study was to better understand the contributors to bacterial mortality by addressing the poorly understood role of Halobacteriovorax and how their role compares with that of viruses. The results revealed that when a concentrated suspension of Vibrio parahaemolyticus was added into microcosms of estuarine waters, the native Halobacteriovorax were the predators that responded first and most rapidly. Their numbers increased by four orders of magnitude, whereas V. parahaemolyticus prey numbers decreased by three orders of magnitude. In contrast, the extant virus population showed little increase and produced little change in the prey density. An independent experiment with stable isotope probing confirmed that Halobacteriovorax were the predators primarily responsible for the mortality of the V. parahaemolyticus. The results show that Halobacteriovorax have the potential to be significant contributors to bacterial mortality, and in such cases, predation by Halobacteriovorax may be an important mechanism of nutrient recycling. These conclusions add another dimension to bacterial mortality and the recycling of nutrients.     

Water discharge-regulated bacteria/heterotrophic nanoflagellate (HNF) interactions in the water column of the river Rhine

Heterotrophic nanoflagellates (HNF) make up a large fraction of the zooplankton biomass of rivers. Their abundance can be strongly affected by water discharge, but the consequences of this highly dynamic factor for their main prey, the bacteria, is still unknown. The focus of this study was on bacterial/HNF interactions in the Lower River Rhine (Germany) with respect to the discharge-dependent dynamics. The bacterial and HNF abundances and biomasses were determined over the course of 17 months. The potential consumption of bacteria by HNF was calculated based on the biomass data and on data on the HNF production. The mean bacterial abundance in the Rhine at Cologne ranged from 0.3 x 10(6) to 3.5 x 10(6) cells mL(-1), with lowest abundances in winter and highest in late spring. No significant changes in abundance during the downstream passage were found. Neither could a significant correlation be found between bacterial and HNF abundance. The ratio of bacterial to HNF abundance showed high variations which lay between 166 and 19,055 and was negatively dependent on water discharge. Monthly routine calculations on the potential bacterial consumption by HNF revealed a clearance of between 2 and 82% of the bacterial standing stock d(-1). The values increased greatly with water discharge and could exceed 100% d(-1) at times of high water flow. The presented data suggests a change in the top-down control of the planktonic bacteria due to the water discharge: The importance of benthic predation at low water flow (high contact probability to benthic predators) gives way to an increased importance in predation by planktonic HNF at high water flow.     

Response of riverine and transition zone bacterioplankton communities to a pulsed river inflow

Hydrobiologia - We investigated the impact of such a summer storm pulse on the riverine zone bacterioplankton and subsequently the transition zone bacterioplankton. Discharge varied from 0.08 m3...     

Distribution of viruses and their impact on bacterioplankton in mesotrophic and eutrophic reservoirs

Spatial distribution of planktonic viral particles (virioplankton) and mortality of heterotrophic bacteria caused by viral lysis were studied in the eutrophic Ivan’kovskoe and mesotrophic Uglichskoe reservoirs (the Upper Volga). During the summer peak of phytoplankton, the number of viral particles was higher in the Ivan’kovskoe Reservoir ((55.1 ± 9.5) × 10⁶ ml⁻¹ on average) than in the Uglichskoe Reservoir ((42.9 ± 5.1) × 10⁶ ml⁻¹ on average). The ratio of viral to bacterial abundances ranged from 2.5 to 7.0. The average number of mature phages in infected heterotrophic bacteria varied from 17 to 109 particles/cell. Most of the infected bacterial cells in the Ivan’kovskoe Reservoir were rod-shaped, and, in the Uglichskoe Reservoir, they were mainly vibrio-shaped. In the Ivan’kovskoe Reservoir, from 8.3 to 22.4% of planktonic bacteria were infected by phages, suggesting phage-induced mortality of bacterioplankton equal to 10.5–34.8% (19.1% on average) of the daily bacterial production. In the Uglichskoe Reservoir, from 9.4 to 33.5% of bacteria were phage-infected, suggesting phage-induced bacterial mortality of 13.7–40.2% (23.5% on average) of the daily bacterial production. The obtained results testify to an important role of autochthonous viruses in the regulation of bacterioplankton abundance and production in the reservoirs.