Direct and Indirect Effects of Protist Predation on Population Size Structure of a Bacterial Strain with High Phenotypic Plasticity

We studied the impact of grazing and substrate supply on the size structure of a freshwater bacterial strain (Flectobacillus sp.) which showed pronounced morphological plasticity. The cell length varied from 2 to >40 μm and encompassed rods, curved cells, and long filaments. Without grazers and with a sufficient substrate supply, bacteria grew mainly in the form of medium-sized rods (4 to 7 μm), with a smaller proportion (<10%) of filamentous forms. Grazing experiments with the bacterivorous flagellate Ochromonas sp. showed that freely suspended cells of <7 μm were highly vulnerable to grazers, whereas filamentous cells were resistant to grazing and became enriched during predation. A comparison of long-term growth in carbon-limited chemostats with and without grazers revealed that strikingly different bacterial populations developed: treatments with flagellates were composed of >80% filamentous cells. These attained a biomass comparable to that of populations in chemostats without grazers, which were composed of medium-sized rods and c-shaped cells. Carbon starvation resulted in a fast decrease in cell length and a shift towards small rods, which were highly vulnerable to grazing. Dialysis bag experiments in combination with continuous cultivation revealed that filament formation was significantly enhanced even without direct contact of bacteria with bacterivores and was thus probably stimulated by grazer excretory products.     

Direct and indirect effects of protist predation on population size structure of a bacterial strain with high phenotypic plasticity

We studied the impact of grazing and substrate supply on the size structure of a freshwater bacterial strain (Flectobacillus sp.) which showed pronounced morphological plasticity. The cell length varied from 2 to >40 microm and encompassed rods, curved cells, and long filaments. Without grazers and with a sufficient substrate supply, bacteria grew mainly in the form of medium-sized rods (4 to 7 microm), with a smaller proportion (<10%) of filamentous forms. Grazing experiments with the bacterivorous flagellate Ochromonas sp. showed that freely suspended cells of <7 microm were highly vulnerable to grazers, whereas filamentous cells were resistant to grazing and became enriched during predation. A comparison of long-term growth in carbon-limited chemostats with and without grazers revealed that strikingly different bacterial populations developed: treatments with flagellates were composed of >80% filamentous cells. These attained a biomass comparable to that of populations in chemostats without grazers, which were composed of medium-sized rods and c-shaped cells. Carbon starvation resulted in a fast decrease in cell length and a shift towards small rods, which were highly vulnerable to grazing. Dialysis bag experiments in combination with continuous cultivation revealed that filament formation was significantly enhanced even without direct contact of bacteria with bacterivores and was thus probably stimulated by grazer excretory products.     

Predation as a shaping force for the phenotypic and genotypic composition of planktonic bacteria

Predation is a major mortality factor of planktonic bacteria and an important shaping force for the phenotypic and taxonomic structure of bacterial communities. In this paper we: (1) summarise current knowledge on bacterial phenotypic properties which affect their vulnerability towards grazers, and (2) review experimental evidence demonstrating that this phenotypic heterogeneity results in shifts of bacterial community composition during enhanced protist grazing pressure. Size-structured interactions are especially important in planktonic systems and bacterial cell size influences the mortality rate and the type of grazer to which bacteria are most susceptible. When protists are the major bacterivores, both very small and large bacterial cells gain some size refuge. Recent studies have revealed that also various non-morphological traits such as motility, physicochemical surface characters and toxicity affect bacterial vulnerability and protist feeding success. These properties are effective at different stages during the feeding process of interception feeding flagellates (encounter, capture, ingestion, digestion). Grazing-resistant bacteria in natural communities can account for a substantial portion of the total bacterial biomass at least in more productive aquatic systems. In field and laboratory experiments it has been demonstrated that increased protozoan grazing results in shifts in the phenotypic and genotypic composition of the bacterial assemblage. The importance of this shaping force for the bacterial community structure depends, however, on the overall food web structure, especially on the composition of the metazooplankton. Whereas the structuring impact of bacterial grazers is well documented, relatively little is known about how grazing-mediated changes in bacterial communities influence microbially mediated processes and biogeochemically important transformations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Variability in protist grazing and growth on different marine Synechococcus isolates

Grazing mortality of the marine phytoplankton Synechococcus is dominated by planktonic protists, yet rates of consumption and factors regulating grazer-Synechococcus interactions are poorly understood. One aspect of predator-prey interactions for which little is known are the mechanisms by which Synechococcus avoids or resists predation and, in turn, how this relates to the ability of Synechococcus to support growth of protist grazer populations. Grazing experiments conducted with the raptorial dinoflagellate Oxyrrhis marina and phylogenetically diverse Synechococcus isolates (strains WH8102, CC9605, CC9311, and CC9902) revealed marked differences in grazing rates-specifically that WH8102 was grazed at significantly lower rates than all other isolates. Additional experiments using the heterotrophic nanoflagellate Goniomonas pacifica and the filter-feeding tintinnid ciliate Eutintinnis sp. revealed that this pattern in grazing susceptibility among the isolates transcended feeding guilds and grazer taxon. Synechococcus cell size, elemental ratios, and motility were not able to explain differences in grazing rates, indicating that other features play a primary role in grazing resistance. Growth of heterotrophic protists was poorly coupled to prey ingestion and was influenced by the strain of Synechococcus being consumed. Although Synechococcus was generally a poor-quality food source, it tended to support higher growth and survival of G. pacifica and O. marina relative to Eutintinnis sp., indicating that suitability of Synechococcus varies among grazer taxa and may be a more suitable food source for the smaller protist grazers. This work has developed tractable model systems for further studies of grazer-Synechococcus interactions in marine microbial food webs.     

Consequences of protist-stimulated bacterial production for estimating protist growth efficiencies

The trophic link between bacteria and bacterivorous protists is a complex interaction that involves feedback of inorganic nutrients and growth substrates that are immeadiately available for prey growth. These interactions were examined in the laboratory and in incubations of concentrated natural assemblages of bacterioplankton. Growth dynamics of estuarine and marine bacterivorous protists were determined in laboratory culture using Vibrio natriegens as prey and were compared to growth of protists on bacterioplankton assemblages concentrated by tangential flow filtration from four northwest Florida Estuaries. Biomass transfers from bacteria to protists were monitored by tracing elemental carbon and nitrogen in particulate fractions of protist added and grazer free controls. Gross growth efficiencies of the protists on naturally occurring bacteria were within the range determined in lab estimates of growth efficiency on cultured bacteria (50%). However, bacterial response to protist excretion products was different in the lab and field incubations, and bacterial growth contributed to the biomass available to protists in the field incubations. As determined by radioisotope-labeled substrate incorporation, a time lag in bacterial reponse to protist excretion products was observed for laboratory batch cultures, allowing accurate estimation of growth efficiency. In incubations with concentrated natural bacterial assemblages, bacterial growth response coincided with protist growth and excretion. The additional bacterial production on protist excretion products reached a maximum of 2–3-fold higher than protist-free controls. In addition, ammonium concentrations increased with protist grazing and growth in lab cultures, but ammonium excreted by protists in concentrates did not accumulate. The C:N values for the bacterial concentrates suggests that these bacteria were nitrogen limited. It is speculated that dissolved organic carbon, concentrated by tangential flow filtration (> 100,000 MW membrane) with the bacterioplankton, was utilized by bacteria when nitrogen was supplied as ammonium and amino acids from protist excretion. Thus, estimates of protist growth efficiency on naturally occurring bacterioplankton, corrected for protist-stimulated bacterial production, were in the range of 13–21%.     

When do grazers accelerate or decelerate soil carbon and nitrogen cycling in tundra? A test of theory on grazing effects in fertile and infertile habitats

It is generally predicted that grazers enhance soil microbial activity and nutrient availability and promote soil bacteria in fertile ecosystems, but retard microbial activity and nutrient availability and promote soil fungi in infertile ecosystems. We tested these predictions in tundra by comparing grazing effects between fertile and infertile habitats and with/without nutrient manipulation by fertilization. Grazing decreased soil N content in fertile and in fertilized plots in infertile habitats while increased it in infertile tundra habitats, which directly opposed our prediction. We conclude that this unpredicted outcome probably resulted from nutrient transport between habitats. Also contrasting with our hypothesis, grazing increased fungal rather than bacterial abundance in fertilized plots at both habitats. In support with predictions, grazing increased microbial activity for soil C decomposition in fertile but decreased it in infertile habitats. The effect of grazing on soil C decomposition followed same patterns as grazer‐induced changes in the activity of β‐glucosidase, which is an extracellular enzyme synthesized by soil microorganisms for degrading soil cellulose. We suggest that the theoretical framework on grazer–soil interactions should incorporate microbial potential for extracellular enzyme production (‘microscale’ grazer effects) and nutrient translocation by grazers among habitats (‘macroscale’ grazer effects) as important mechanisms by which grazers influence soil processes and nutrient availability for plants at contrasting levels of habitat fertility.     

The effect of grazing and nutrient supply on periphyton associated bacteria

The effects of nutrient additions and grazing by macro-invertebrates on periphyton-associated algae and bacteria were studied by performing an enclosure experiment on three occasions from early spring to summer at mesotrophic Lake Erken and V?dd?, at the Swedish Baltic coast. There were significant interactions between nutrient additions and grazing on bacterial biomass and specific activity in Lake Erken. Thus, the importance of either bottom-up or top-down effects could not be singled out. Bacterial biomass increased with enrichment only in the absence of grazers. Grazer presence tended to increase bacterial biomass in ambient nutrient conditions, but to decrease bacterial biomass under enrichment. For specific activity the positive response to enrichment was restricted to grazer presence. Hence, grazing by macro-invertebrates may have an indirect positive effect on bacterial activity by enhancing nutrient conditions through their feeding activities and/or fecal pellets production. In addition, we found a significant relationship between bacterial production and chlorophyll a at both sites. This relationship weakened in the presence of macro-invertebrates. Thus, the importance of internal nutrient regeneration by bacteria and algae decreased, possibly due to increased nutrient availability, in the presence of macro-invertebrate grazers.     

The influence of moss on grazers in high‐altitude streams: food,refuge or both?

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Secondary metabolites help biocontrol strain Pseudomonas fluorescens CHA0 to escape protozoan grazing

In soil ecosystems, bacteria must cope with predation activity, which is attributed mainly to protists. The development of antipredation strategies may help bacteria maintain higher populations and persist longer in the soil. We analyzed the interaction between the root-colonizing and biocontrol strain Pseudomonas fluorescens CHA0 and three different protist isolates (an amoeba, a flagellate, and a ciliate). CHA0 produces a set of antibiotics, HCN, and an exoprotease. We observed that protists cannot grow on CHA0 but can multiply on isogenic regulatory mutants that do not produce the extracellular metabolites. The in vitro responses to CHA0 cells and its exoproducts included growth inhibition, encystation, paralysis, and cell lysis. By analyzing the responses of protists to bacterial supernatants obtained from different isogenic mutants whose production of one or more exometabolites was affected and also to culture extracts with antibiotic enrichment, we observed different contributions of the phenolic antifungal compound 2,4-diacetylphloroglucinol (DAPG) and the extracellular protease AprA to CHA0 toxicity for protists and to the encystation-reactivation cycle. The grazing pressure artificially produced by a mixture of the three protists in a microcosm system resulted in reduced colonization of cucumber roots by a regulatory isogenic CHA0 mutant unable to produce toxins. These results suggest that exometabolite production in biocontrol strain CHA0 may contribute to avoidance of protist grazing and help sustain higher populations in the rhizosphere, which may be a desirable and advantageous trait for competition with other bacteria for available resources.     

Secondary Metabolites Help Biocontrol Strain Pseudomonas fluorescens CHA0 To Escape Protozoan Grazing

In soil ecosystems, bacteria must cope with predation activity, which is attributed mainly to protists. The development of antipredation strategies may help bacteria maintain higher populations and persist longer in the soil. We analyzed the interaction between the root-colonizing and biocontrol strain Pseudomonas fluorescens CHA0 and three different protist isolates (an amoeba, a flagellate, and a ciliate). CHA0 produces a set of antibiotics, HCN, and an exoprotease. We observed that protists cannot grow on CHA0 but can multiply on isogenic regulatory mutants that do not produce the extracellular metabolites. The in vitro responses to CHA0 cells and its exoproducts included growth inhibition, encystation, paralysis, and cell lysis. By analyzing the responses of protists to bacterial supernatants obtained from different isogenic mutants whose production of one or more exometabolites was affected and also to culture extracts with antibiotic enrichment, we observed different contributions of the phenolic antifungal compound 2,4-diacetylphloroglucinol (DAPG) and the extracellular protease AprA to CHA0 toxicity for protists and to the encystation-reactivation cycle. The grazing pressure artificially produced by a mixture of the three protists in a microcosm system resulted in reduced colonization of cucumber roots by a regulatory isogenic CHA0 mutant unable to produce toxins. These results suggest that exometabolite production in biocontrol strain CHA0 may contribute to avoidance of protist grazing and help sustain higher populations in the rhizosphere, which may be a desirable and advantageous trait for competition with other bacteria for available resources.     

Effects of nutrient availability and Ochromonas sp. predation on size and composition of a simplified aquatic bacterial community

Predation and competition are two main factors that determine the size and composition of aquatic bacterial populations. Using a simplified bacterial community, composed of three strains characterized by different responses to predation, a short-term laboratory experiment was performed to evaluate adaptations and relative success in communities with experimentally controlled levels of predation and nutrient availability. A strain with a short generation time (Pseudomonas putida), one with high plasticity in cell morphology (Flectobacillus sp. GC5), and one that develops microcolonies (Pseudomonas sp. CM10), were selected. The voracious flagellate Ochromonas sp. was chosen as a predator. To describe adaptations against grazing and starvation, abundance, biomass and relative heterogeneity of bacteria were measured. On the whole, the strains in the predation-free cultures exhibited unicellular growth, and P. putida represented the largest group. The presence of Ochromonas strongly reduced bacterial abundance, but not always the total biomass. The activity of grazers changed the morphological composition of the bacterial communities. Under grazing pressure the relative composition of the community depended on the substrate availability. In the presence of predators, P. putida abundance declined in both high and low nutrient treatments, and Pseudomonas CM10 developed colonies. Flectobacillus was only numerically codominant in the nutrient-rich environments.     

The importance of bacterivorous protists in the decomposition of stream leaf litter

1. Allochthonous organic matter, in the form of senesced leaves, is a major source of carbon supporting detrital food webs. While studies have documented the role of bacteria and fungi in the decomposition of leaf litter, little information is available regarding the role of protists in the decomposition process. 2. We tested the hypothesis that the presence of stream‐dwelling bacterivorous protists leads to an increased rate of leaf decomposition through grazing pressure on bacteria. We isolated live protists from decomposing leaves collected in a stream in Northern Virginia, U.S.A. (Goose Creek) and established laboratory cultures of common bacterivorous protists. 3. Recently senesced leaves from the field were used in laboratory microcosm experiments to determine if the rate of litter decomposition differed between four treatments: bacteria only, bacteria + flagellates, bacteria + flagellates + ciliates, autoclaved stream water (control). We determined the dry weight of leaf remaining, bacterial abundance, flagellate abundance and ciliate abundance for each replicate on days 0, 7, 14, 30, 60 and 120. 4. The rate of leaf decomposition was significantly higher in treatments with protists than without and bacterial abundance declined in protist treatments compared with bacteria only treatment. Weight loss in the presence of flagellates was three to four times higher when protists were present compared with treatments with bacteria alone. These results provide experimental evidence that protists could play a significant role in the detrital processes of streams.     

The effect of grazer size manipulation on periphyton communities

Summary We examined the effect of grazer size on periphyton biomass, size structure, and species compostion by removing the largest invertebrate grazers on artificial macrophytes planted in the littoral of Lake Memphremagog (Que-Vt). A series of exclosures with increasingly fine mesh prevented colonization by large invertebrates but allowed in smaller grazers. Oligochaetes, chironomids, and cladocerans effectively replaced snails so that total grazer biomass in the various treatments was not significantly different from the controls. With one exception, algal biomass, measured as chlorophyll a, did not differ significantly among the various treatments. However algal size and taxonomy were affected because the dominance of large blue-green colonies was apperantly related to the presence of large grazers. The results of the size manipulations were qualitatively similar to those induced in phytoplankton communities by size selective zooplankton grazing and are consistent with models based on general allometric equations.Contribution 181 of the Lake Memphremagog Project, Limnology Research Centre     

Body size and the division of niche space: food and predation differentially shape the distribution of Serengeti grazers

1. Theory predicts that small grazers are regulated by the digestive quality of grass, while large grazers extract sufficient nutrients from low-quality forage and are regulated by its abundance instead. In addition, predation potentially affects populations of small grazers more than large grazers, because predators have difficulty capturing and handling large prey. 2. We analyse the spatial distribution of five grazer species of different body size in relation to gradients of food availability and predation risk. Specifically, we investigate how the quality of grass, the abundance of grass biomass and the associated risks of predation affect the habitat use of small, intermediate and large savanna grazers at a landscape level. 3. Resource selection functions of five mammalian grazer species surveyed over a 21-year period in Serengeti are calculated using logistic regressions. Variables included in the analyses are grass nitrogen, rainfall, topographic wetness index, woody cover, drainage lines, landscape curvature, water and human habitation. Structural equation modelling (SEM) is used to aggregate predictor variables into 'composites' representing food quality, food abundance and predation risk. Subsequently, SEM is used to investigate species' habitat use, defined as their recurrence in 5 × 5 km cells across repeated censuses. 4. The distribution of small grazers is constrained by predation and food quality, whereas the distribution of large grazers is relatively unconstrained. The distribution of the largest grazer (African buffalo) is primarily associated with forage abundance but not predation risk, while the distributions of the smallest grazers (Thomson's gazelle and Grant's gazelle) are associated with high grass quality and negatively with the risk of predation. The distributions of intermediate sized grazers (Coke's hartebeest and topi) suggest they optimize access to grass biomass of sufficient quality in relatively predator-safe areas. 5. The results illustrate how top-down (vegetation-mediated predation risk) and bottom-up factors (biomass and nutrient content of vegetation) predictably contribute to the division of niche space for herbivores that vary in body size. Furthermore, diverse grazing assemblages are composed of herbivores of many body sizes (rather than similar body sizes), because these herbivores best exploit the resources of different habitat types.     

Assimilation of toluene carbon along a bacteria-protist food chain determined by 13C-enrichment of biomarker fatty acids

A food chain consisting of toluene, toluene-degrading Pseudomonas sp. PS+ and a bacterivorous flagellated amoebae Vahlkampfia sp. was established in a batch culture. This culture was amended with [U-13C]toluene and served as a model system to elucidate the flux of carbon in the food chain by quantifying bacterial biovolumes and 13C enrichment of phospholipid fatty acid (PLFA) biomarkers of the bacteria and the heterotrophic protists. Major PLFA detected in the batch co-culture included those derived from Pseudomonas sp. PS+ (16:1omega7c and 18:1omega7c) and Vahlkampfia sp. (20:4omega6c and 20:3omega6c). A numerical model including consumption of toluene by the bacteria and predation of the bacteria by the heterotrophic protists was adjusted to the measured toluene carbon, bacterial carbon and delta13C values of bacterial and protist biomass. Using this model, we estimated that 28+/-7% of the consumed toluene carbon was transformed into bacterial biomass, and 12+/-4% of the predated bacterial carbon was incorporated into heterotrophic protist biomass. Our study showed that the 13C enrichment of PLFA biomarkers coupled to biomass determination via biovolume calculations is a suitable method to trace carbon fluxes in protist-inclusive microbial food chains because it does not require the separation of protist cells from bacterial cells and soil particles.     

Microbial community dynamics in Mediterranean nutrient-enriched seawater mesocosms: changes in abundances, activity and composition

Quantitative and qualitative changes in bacterial communities from the Mediterranean Sea were compared in duplicate batch mesocosms with or without addition of inorganic nutrients. Methods including traditional microbial ecology techniques, molecular biology and flow cytometry were combined to determine abundances, production, cell size, activity, culturability and taxonomic diversity of bacterial cells. Addition of nutrients and confinement resulted in an increase of bacterial densities which were rapidly controlled by protozoan grazing. Changes in bacterial activity and morphology were observed during the growth phase of bacteria and under grazing pressure. The proportion of medium-size and culturable cells increased during the growth phase. These cells were preferentially consumed by grazers resulting in a strong limitation of bacterial production. As a consequence of the grazing pressure, large cells were produced and contributed to the remaining bacterial productivity after grazing. Grazing had an effect on the taxonomic composition of bacterial communities by preferentially eliminating gamma-Proteobacteria, alpha-Proteobacteria were preserved. It seems that some species from the genera Ruegeria and Cytophaga may have developed defence strategies to escape predation.     

Diverse protist grazers select for virulence-related traits in Legionella

It is generally accepted that selection for resistance to grazing by protists has contributed to the evolution of Legionella pneumophila as a pathogen. Grazing resistance is becoming more generally recognized as having an important role in the ecology and evolution of bacterial pathogenesis. However, selection for grazing resistance presupposes the existence of protist grazers that provide the selective pressure. To determine whether there are protists that graze on pathogenic Legionella species, we investigated the existence of such organisms in a variety of environmental samples. We isolated and characterized diverse protists that graze on L. pneumophila and determined the effects of adding L. pneumophila on the protist community structures in microcosms made from these environmental samples. Several unrelated organisms were able to graze efficiently on L. pneumophila. The community structures of all samples were markedly altered by the addition of L. pneumophila. Surprisingly, some of the Legionella grazers were closely related to species that are known hosts for L. pneumophila, indicating the presence of unknown specificity determinants for this interaction. These results provide the first direct support for the hypothesis that protist grazers exert selective pressure on Legionella to acquire and retain adaptations that contribute to survival, and that these properties are relevant to the ability of the bacteria to cause disease in people. We also report a novel mechanism of killing of amoebae by one Legionella species that requires an intact Type IV secretion system but does not involve intracellular replication. We refer to this phenomenon as ‘food poisoning''.     

Ecology of grazing lawns in Africa

Grazing lawns are a distinct grassland community type, characterised by short‐stature and with their persistence and spread promoted by grazing. In Africa, they reveal a long co‐evolutionary history of grasses and large mammal grazers. The attractiveness to grazers of a low‐biomass sward lies in the relatively high quality of forage, largely due to the low proportion of stem material in the sward; this encourages repeat grazing that concomitantly suppresses tall‐grass growth forms that would otherwise outcompete lawn species for light. Regular grazing that prevents shading and maintains sward quality is thus the cornerstone of grazing lawn dynamics. The strong interplay between abiotic conditions and disturbance factors, which are central to grazing lawn existence, can also cause these systems to be highly dynamic. Here we identify differences in growth form among grazing lawn grass species, and assess how compositional differences among lawn types, as well as environmental variables, influence their maintenance requirements (i.e. grazing frequency) and vulnerability to degradation. We also make a clear distinction between the processes of lawn establishment and lawn maintenance. Rainfall, soil nutrient status, grazer community composition and fire regime have strong and interactive influences on both processes. However, factors that concentrate grazing pressure (e.g. nutrient hotspots and sodic sites) have more bearing on where lawns establish. Similarly, we discuss the relevance of enhanced rates of nitrogen cycling and of sodium levels to lawn maintenance. Grazer community composition and density has considerable significance to grazing lawn dynamics; not all grazers are adapted to foraging on short‐grass swards, and differences in body size and relative mouth dimensions determine which species are able to convert tall‐grass swards into grazing lawns under different conditions. Hence, we evaluate the roles of different grazers in lawn dynamics, as well as the benefits that grazer populations derive from having access to grazing lawns. The effects of grazing lawns can extend well beyond their borders, due to their influence on grazer densities, behaviour and movements as well as fire spread, intensity and frequency. Variation in the area and proportion of a landscape that is grazing lawn can thus have a profound impact on system dynamics. We provide a conceptual model that summarises grazing lawn dynamics, and identify a rainfall range where we predict grazing lawns to be most prevalent. We also examine the biodiversity associated with grazing lawn systems, and consider their functional contribution to the conservation of this biodiversity. Finally, we assess the utility of grazing lawns as a resource in a rangeland context.     

Cascading predation effects of Daphnia and copepods on microbial food web components

1. We performed a mesocosm experiment to investigate the structuring and cascading effects of two predominant crustacean mesozooplankton groups on microbial food web components. The natural summer plankton community of a mesotrophic lake was exposed to density gradients of Daphnia and copepods. Regression analysis was used to reveal top–down impacts of mesozooplankton on protists and bacteria after days 9 and 15. 2. Selective grazing by copepods caused a clear trophic cascade via ciliates to nanoplankton. Medium‐sized (20–40 μm) ciliates (mainly Oligotrichida) were particularly negatively affected by copepods whereas nanociliates (mainly Prostomatida) became more abundant. Phototrophic and heterotrophic nanoflagellates increased significantly with increasing copepod biomass, which we interpret as an indirect response to reduced grazing pressure from the medium‐sized ciliates. 3. In Daphnia‐treatments, ciliates of all size classes as well as nanoflagellates were reduced directly but the overall predation effect became most strongly visible after 15 days at higher Daphnia biomass. 4. The response of bacterioplankton involved only modest changes in bacterial biomass and cell‐size distribution along the zooplankton gradients. Increasing zooplankton biomass resulted either in a reduction (with Daphnia) or in an increase (with copepods) of bacterial biovolume, activity and production. Patterns of bacterial diversity, as measured by polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE), showed no distinct grouping after 9 days, whereas a clear treatment‐coupled similarity clustering occurred after 15 days. 5. The experiment demonstrated that zooplankton‐mediated predatory interactions cascade down to the bacterial level, but also revealed that changes occurred rather slowly in this summer plankton community and were most pronounced with respect to bacterial activity and composition.