Loss processes influencing growth of planktonic bacterial populations in Lake Constance

The fate of bacterial production in a deep mesotrophic lakehas been investigated on the basis of a multiple approach combiningfield and experimental studies. Bacterial production gains asestimated by measurements of methyl-[³H]thymidine incorporationinto macromolecules appeared to be more or less balanced bycorresponding grazing losses. This was suggested by consideringabundance and grazing potential of presumed grazers as wellas by following in time the binding of labelled bacteria bylarger sized organisms. Moreover, strong increases in bacterialnet growth were always observed after removal of grazers fromthe water samples. As was indicated by independent approaches,phagotrophic microflagellates were mainly responsible for theobserved grazing losses. Support for the importance of alternativeloss processes such as sinking or autolysis was given neitherby field observations nor by long-term expenments with exclusionof grazers. It is concluded that phagotrophic microflagellatesmay be regarded as important functional components of planktoniccommunities in freshwater lakes.     

Ciliates are the Dominant Grazers on Pico- and Nanoplankton in a Shallow,Naturally Highly Eutrophic Lake

Abundance and biomass of the microbial loop members [bacteria, heterotrophic nanoflagellates (HNF), and ciliates] were seasonally measured in the naturally eutrophic and shallow (2.8 mean depth) Lake Võrtsjärv, which has a large open surface area (average 270 km²) and highly turbid water (Secchi depth <1 m). Grazing rates (filter feeding rates) on 0.5-, 3-, and 6-μm-diameter particles were measured to estimate pico- and nanoplankton grazing (filter feeding) by micro- and metazooplankton. Among grazers, HNF had a low abundance (<50 cells mL^?1) and, due to their low specific filtering rates, they only grazed a minor fraction of the bacterioplankton (≤4.2% of total grazing). Ciliates were relatively abundant (≤158 cells mL^?1) and, considering their high specific feeding rates, were able to graze more than 100% of the bacterial biomass production in the open part of the lake, whereas the average daily grazing accounted for 9.3% of the bacterial standing stock. Ciliates were potentially important grazers of nanoplanktonic organisms (on average, approximately 20% of the standing stock of 3-μm-size particles was grazed daily). Metazooplankton grazed a minor part of the bacterioplankton, accounting for only 0.1% of standing stock of bacteria. Grazing on nanoplankton (3–6 μm) by metazooplankton was higher (0.4% of standing stock). The hypothesis is proposed that ciliates dominate due to a lack of top–down regulation by predators, and HNF have a low abundance due to strong grazing pressure by ciliates.     

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.     

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.     

Direct and indirect influences of crustacean zooplankton on bacterioplankton of Lake Constance

Herbivorous crustacean zooplankton may influence bacterial populations of lakes directly by grazing on them or indirectly by grazing on algae. In Lake Constance a regularly observed decrease of bacterial density during periods of high abundance of cladocerans (clearwater phase) indicated bacterial grazing losses. However, cladoceran grazing on bacteria appeared to be less efficient than on algae. Moreover, cladocera reduced grazing pressure on bacteria by grazing on bacterivorous flagellates. Additionally, a shift of bacterial composition from an originally higher percentage of filamentous and aggregate growth forms towards a population of homogenously distributed small single celled bacteria was observed regularly at the beginning of the clearwater phase. Transient increases of bacterial abundance and productivity coinciding with the increase of cladocera at the end of the algal spring bloom were interpreted as field indications of indirect bacteria-zooplankton interactions due to crustacean grazing on phytoplankton. The release of organic carbon during grazing of crustacea on algae was considered as explanation for the observed stimulation of bacterial populations. Thereby, additional, otherwise inaccessible algal carbon would be made available to bacteria by zooplankton. Experimental support for this hypothesis was given by showing that bacteria were able to respond to crustacean grazing on algae by enhanced growth and activities. The possible impact of these direct and indirect crustacea-bacteria interactions on the abundance, activity and composition of bacterioplankton as well as on the structure and function of the total planktonic community is discussed.     

Predation-mediated shifts in size distribution of microbial biomass and activity during detritus decomposition

Many experimental studies on detritus decomposition revealed a comparable microbial succession after the addition of a substrate pulse: from small, freely suspended single bacteria at the beginning, to more complex and larger growth forms during a later stage, accompanied by the appearance of bacterivorous protists. We examined in three model experiments with different organic carbon sources whether this shift in bacterial size structure is linked to the grazing impact of bacterivores. In short‐term (8–10 d) microcosm experiments we added natural dissolved and particulate detritus (macrophyte leaves and leachate, dead phytoplankton cells) as an organic substrate source. By the use of size‐fractionated inocula and eucaryotic inhibitors we obtained treatments without protists, in which bacteria developed without predation. These were compared, by measurements of bacterial activity and microscopical analysis of bacterial size structure, to incubations in which either cultured heterotrophic nanoflagellates or a natural protist assemblage was included in the inoculum. The presence of bacterial grazers resulted in a 50–90% reduction of bacterial biomass compared to grazer‐free trials. The selective removal of freely suspended bacteria produced a very different relative composition of bacterial biomass: it became dominated by large, grazing‐resistant forms such as filaments and cells attached to particles or clustered in small aggregates. In grazer‐free treatments, bacterial biomass was always dominated (>80%) by free‐living, single bacterial cells. The time course of the bacterial development suggested different underlying mechanisms for the appearance of predation resistant filamentous and of aggregated or attached bacteria. As bacterial aggregates developed in approximately similar amounts with and without grazers no specific growth stimulation by protists could be detected. In contrast, concentrations of filamentous bacteria were 2–10 times higher in treatments with protists, thus indicating a stimulation of this growth form during enhanced grazing pressure. Measurements of ectoenzymatic activity and H‐leucine uptake indicated that microbial activity was also shifted to larger size fractions. In most cases more than 50% of bacterial activity in treatments with protists was associated with the size fraction>10 μm whereas this value was <2% without grazers. Grazing by protists also enhanced the specific activity of the bacterial assemblage which is in contrast to an assumed lower competitive ability of complex bacterial growth forms. The results imply that the selective force of bacterivory in nutrient‐rich environments changes the structure and possibly the function of aquatic bacteria and their position in the food web, making protist‐resistant bacteria more vulnerable to metazoan filter feeders and detritivores, and possibly also subject to sedimentation.     

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.     

Nitrogen Mineralization by Acanthamoeba polyphaga in Grazed Pseudomonas paucimobilis Populations

Nitrogen mineralization was studied in a simple grazing system in which the protozoan Acanthamoeba polyphaga was grown with the bacterium Pseudomonas paucimobilis (two soil organisms isolated from the shortgrass prairie in northern Colorado). In different experiments, either carbon or nitrogen was adjusted to be in limiting amounts. When carbon was limiting, grazers were almost entirely responsible for nitrogen mineralization, with bacteria themselves contributing little. When nitrogen was limiting, nitrogen mineralization by grazers permitted continued growth by the grazed bacteria and a greater bacterial biomass production. The increased growth of the grazed bacteria did not result in an increased total amount of carbon used, but the grazed bacteria used carbon more efficiently than the ungrazed bacteria.     

Size-selective feeding by Cyclidium sp. on bacterioplankton and various sizes of cultured bacteria

Abstract Size-selective grazing by Cyclidium sp., isolated as a dominant ciliate bacterivore from the Římov Reservoir (South Bohemia), was examined using fluorescent labelled bacteria (FLB) produced from natural bacterioplankton or pure bacterial cultures. Sizes of ingested bacteria in food vacuoles were measured directly. Three experimental arrangements were used: (1) Ciliates were grown on the pure culture of Alcaligenes xylosoxidans and fed with various proportions of ‘large’ and ‘small’ FLB (mean biovolume, 0.377 and 0.202 μm³, respectively) prepared from the same bacterial species. Results clearly showed significant selection of larger bacteria. (2) Ciliates were grown on natural bacterioplankton from the reservoir and subsequently fed on FLB prepared from the reservoir bacterioplankton (mean biovolume, 0.065 μm³). Independent of either prey or predator abundance, larger FLB (> 0.100 μ m³, and especially those > 0.200 μ m³) were ingested with much higher frequency than their occurrence i the natural assemblage. (3) Ciliates were grown on the reservoir baterioplankton and fed by FLB prepared from the culture of Pseudomonas sp. In contrast with previous results, no size selection of the ciliate was found when FLB were different from the bacterial food used to grow the ciliate. Ecological impacts of size-selective bacterivory are suggested.     

Experimental manipulations of microbial food web interactions in a humic lake: shifting biological drivers of bacterial community structure

A previous multiyear study observed correlations between bacterioplankton community composition (BCC) and abundance and the dynamics of phytoplankton populations and bacterivorous grazers in a humic lake. These observations generated hypotheses about the importance of trophic interactions (both top-down and bottom-up) for structuring bacterial communities in this lake, which were tested using two multifactorial food web manipulation experiments that separately manipulated the intensity of grazing and the composition of the phytoplankton community. Our results, combined with field observations, suggest that a hierarchy of drivers structures bacterial communities in this lake. While other studies have noted links between aggregate measures of phytoplankton and bacterioplankton communities, we demonstrate here correlations between succession of phytoplankton assemblages and BCC as assessed by automated ribosomal intergenic spacer analysis (ARISA). We used a novel approach linking community ARISA data to phylogenetic assignments from sequence analysis of 16S rRNA gene clone libraries to examine the responses of specific bacterial phylotypes to the experimental manipulations. The synchronous dynamics of these populations suggests that primary producers may mediate BCC and diversity through labile organic matter production, which evolves in quality and quantity during phytoplankton succession. Superimposed on this resource-mediated control of BCC are brief periods of intense bacterivory that impact bacterial abundance and composition.     

Protozoan Grazing Increases Mineralization of Naphthalene in Marine Sediment

Bacterial decomposition of organic matter is frequently enhanced when protozoa are present. Various mechanisms have been proposed to account for this phenomenon, including effects associated with grazing by protozoa (such as increased recycling of limiting nutrients, removal of senescent cells, or reduction of competition among bacteria) and indirect effects of grazers (such as excretion of bacterial growth factors). Few studies have examined the role of protozoa in bacterial degradation of xenobiotic compounds in sediment containing a natural community of microbes. The effect of protozoa on mineralization of naphthalene was investigated in this study. Laboratory experiments were conducted using field-contaminated estuarine sediment, with the indigenous microbial populations. Mineralization of naphthalene was up to four times greater in treatments with actively grazing protozoa than in treatments containing the grazing inhibitor cytochalasin B. Control experiments confirmed that the grazing inhibitor was not toxic to ciliates but did prevent them from grazing. The grazing inhibitor did not affect growth rates of a mixed culture of sediment bacteria or a pure polycyclic-aromatic-hydrocarbon-degrading strain. Once grazing had been inhibited, supplementing treatments with inorganic N and P, glucose, or additional protozoa failed to stimulate naphthalene mineralization. Naphthalene-degrading bacteria were four to nine times less abundant when protozoan grazing was suppressed. We suggest that protozoa enhance naphthalene mineralization by selectively grazing on those sediment bacteria that ordinarily would outcompete naphthalene-degrading bacteria.     

Effects of Nutrients on Specific Growth Rate of Bacterioplankton in Oligotrophic Lake Water Cultures

The effects of organic and inorganic nutrient additions on the specific growth rates of bacterioplankton in oligotrophic lake water cultures were investigated. Lake water was first passed through 0.8-μm-pore-size filters (prescreening) to remove bacterivores and to minimize confounding effects of algae. Specific growth rates were calculated from changes in both bacterial cell numbers and biovolumes over 36 h. Gross specific growth rates in unmanipulated control samples were estimated through separate measurements of grazing losses by use of penicillin. The addition of mixed organic substrates alone to prescreened water did not significantly increase bacterioplankton specific growth rates. The addition of inorganic phosphorus alone significantly increased one or both specific growth rates in three of four experiments, and one experiment showed a secondary stimulation by organic substrates. The stimulatory effects of phosphorus addition were greatest concurrently with the highest alkaline phosphatase activity in the lake water. Because bacteria have been shown to dominate inorganic phosphorus uptake in other P-deficient systems, the demonstration that phosphorus, rather than organic carbon, can limit bacterioplankton growth suggests direct competition between phytoplankton and bacterioplankton for inorganic phosphorus.     

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.     

Aquatic Microbiology for Ecosystem Scientists: New and Recycled Paradigms in Ecological Microbiology

In all ecosystems, bacteria are the most numerous organisms and through them flows a large fraction of annual primary production. In the past decade we have learned a great deal about some of the factors that regulate bacteria and their activities, and how these activities, in turn, alter ecosystem-level processes. Here I review three areas in which recent progress has been made with particular reference to pelagic ecosystems: the problem of bacterial cell dormancy; the effect of solar radiation on organic matter lability; and, the maintenance of net heterotrophy. In a system in which grazing is the major source of mortality for bacteria, bacterial cell dormancy is something of a paradox. I argue that the degree to which bacteria are grazed by flagellates (highly selective grazers) versus other grazers (cladocerans, bivalves) may explain the degree and variation in the proportion of active cells which recent evidence shows to be large. Another factor affecting bacterial activity that has come to the fore in recent years is solar radiation. Irradiation, especially in the ultra-violet range has long been thought of as simply deleterious to some bacteria. A wealth of newer evidence shows that refractory dissolved organic compounds may be converted into microbially labile compounds by solar radiation in several wavebands. This interaction between irradiation and organic matter (photolysis) may explain, in part, how dissolved organic carbon (C) may be refractory in the dark environment of the soil but become labile in the illumunated surface waters of lakes or rivers. The newer evidence shows that aquatic ecosystems, at least oligotrophic ones, are significantly subsidized by terrestrially-produced organic matter. I review here multiple lines of evidence that suggest that freshwater ecosystems are predominantly systems which respire more organic C than they produce by photosynthesis, and are therefore net heterotrophic. While net heterotrophy is an interesting exception for terrestrial ecosystems, it appears to be commonplace for aquatic systems and represents an important linkage between terrestrial and aquatic ecosystems.     

Bacterioplankton growth, grazing mortality and quantitative relationship to primary production in a humic and a clearwater lake

Bacterial growth and grazing mortality were estimated from Mayto October in two south Swedish oligotrophic lakes, one beinga clearwater lake (water colour 5–10 mg Pt l^–1 DOC2.9–3.4 mg l^–1, Secchi disk depth 5.0–9.4m) and the other a humic, brownwater lake (water colour 105–165mg Pt l^–1, DOC 13.7–22.7mg l^–1, Secchi diskdepth 1.3–2.1 m). Specific rates of growth and grazingmortality were generally similar for both lakes. However, theabundance of bacteria was consistently 2–3 times higherin the water of the humic lake, suggesting that the total productionand consumption of bacterial cells were also higher than inthe dearwater lake. The ratio of bacterial secondary productionto primary production was higher in the humic lake than in theclearwater lake, indicating that the bacterioplankton of thehumic lake utilize allochthonous substrates, in addition tosubstrates originating from autochthonous primary production.Most of the bacterial loss in both lakes could be attributedto small protozoan grazers. This implies that allochthonousand autochthonous organic carbon fixed by bacterioplankton isless important in terms of carbon flow to higher trophic levelsthan would be expected if macrozooplankton were the dominantbacterivores, providing a more direct and efficient transferof carbon to larger organisms.     

Effects of streptomycin,cycloheximide, Fungizone,captan, carbofuran,cygon, and PCNB on soil microorganisms

Eight biocides were chosen to determine whether they had any effects on nontarget organisms in soil and to what extent they would reduce their target populations under laboratory experimental conditions. A simplified microcosm system was utilized in which reduced species arrays that included field populations of either only bacteria and fungi, or bacteria, fungi, and protozoa (no nematodes, arthropods, or plants) were inoculated into sterilized soil. In a second set of experiments, plants were grown in sterilized soil. A bactericide-streptomycin-four fungicides-cycloheximide, Fungizone (amphotericin B), captan, and PCNB (quintozene)-an acaricide-cygon-an insecticide-nematicide-carbofuran-and an insecticide-diazinon-were used. Each biocide had effects on nontarget organisms although the increases or decreases, with respect to the control, were of only limited duration. Reductions in target groups were typically of longer duration. Streptomycin, applied at 1 mg·g^–1 soil, did not decrease bacterial populations during the experimental incubation. At 3 mg·g^–1 soil, streptomycin decreased the numbers of bacteria that grew on tryptone agar, but also reduced active hyphae. Fungizone was the most effective of the 4 fungicides tested in reducing active hyphae. Increased bacterial populations were usually observed following fungal reductions. Carbofuran had the fewest effects on the test organisms (bacteria, fungi, and protozoa). Only an initial stimulation of bacterial and fungal populations was observed with cygon although it also increased NH₄ ⁺-N concentrations in soil during most of the incubation, as did streptomycin and cycloheximide. A transitory increase in fungal populations following a decrease in ciliate numbers was observed in the cygon with grazers treatments. Diazinon reduced all microbial populations and inorganic nitrogen concentrations measured. Cygon and PCNB decreased growth of blue grama plants, while streptomycin reduced shoot weights of blue grama. These results should be useful in assessing the effects of these biocides when applied to more complex systems.     

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.     

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%.     

Bacterioplankton interactions with Daphnia and algae in experimental enclosures

Development of bacterioplankton was studied by manipulation of planktivorous fish and/or nutrients in experimental enclosures in a fish pond. Grazing pressure exerted by large zooplankton (Daphnia galeata and Daphnia pulicaria) strongly influenced the counts and size distribution of bacterial populations. Morphometric analyses by scanning electron microscope revealed a shift in size distribution from larger mainly rod-type bacteria under low grazing pressure towards smaller mainly coccus-type under strong grazing pressure. The metabolic activity of bacteria measured as glucose uptake was higher under strong grazing pressure. After removal of large daphnids, the increase in bacterial density was probably the result of two additive factors: low grazing pressure and high level of dissolved organic matter (DOM) due to photosynthetic activity of more abundant algae. Composition of bacterial populations shifted toward larger, rod-type bacteria, and their metabolic efficiency measured by uptake, was lowered. The basic dimensionality of the system and interactions between variables was describe by R-mode factor analysis. The manipulated enclosures were relate with factor score.     

Predation on marine picoplankton populations examined with an 'add-in' approach

NW Mediterranean surface water was spiked with picoplanktonprey (heterotrophic bacteria or cyanobacteria) or predators(bacterivorous microflagellates or ciliates) to investigatedifferential grazing pressure on picoplankton populations. Addinga particular prey type did not yield different growth patternsfor heterotrophic bacteria and cyanobacteria. but gave eithersimilar, positive, effects on both picoplankton types or similarnegative effects. Natural populations of both predator typesincreased with additions of cyanobacteria, but not heterotrophicbacteria. Ciliate additions gave marked decreases in cyanobacteria.While individual groups of grazers may preferentially consumecyanobacteria, selective grazing is probably not responsiblefor the maintenance of apparently stable populations of differentgroups of picoplankters during the summer.