Altering the balance between bacterial production and protistan bacterivory triggers shifts in freshwater bacterial community composition

Bacterivorous protists are known to induce changes in bacterial community composition (BCC). We hypothesized that changes in BCC could be related quantitatively to a measure of grazing: the ratio of bacterial mortality to growth rate. To test this hypothesis, we analyzed time-course changes in BCC, protistan grazing rate, and bacterial production from 3 in situ studies conducted in a freshwater reservoir and three laboratory studies. In the field experiments, samples were manipulated to yield different levels of protistan bacterivory and incubated in dialysis bags. Laboratory investigations were continuous cultivation studies in which different bacterivorous protists were added to bacterial communities. BCC was assessed using 4–6 different rRNA-targeted oligonucleotide probes for community analysis. Change in BCC (Δ BCC) was estimated as the sum of changes in the proportions of the two phylogenetic groups that showed the largest shifts. Analysis of a set of 22 estimates of shifts in the ratio of grazing to production rate over periods of 48–72 h and Δ BCC showed that Δ BCC was positively and tightly correlated (r ² = 0.784) with shifts in the ratio of grazing mortality to cell production. While the nature of a shift in BCC is unpredictable, the magnitude of the change can be related to changes in the balance between bacterial production and protistan grazing. This revised version was published online in August 2006 with corrections to the Cover Date.     

Food selection by bacterivorous protists: insight from the analysis of the food vacuole content by means of fluorescence in situ hybridization

A modified fluorescence in situ hybridization (FISH) method was used to analyze bacterial prey composition in protistan food vacuoles in both laboratory and natural populations. Under laboratory conditions, we exposed two bacterial strains (affiliated with beta- and gamma-Proteobacteria -- Aeromonas hydrophila and Pseudomonas fluorescens, respectively) to grazing by three protists: the flagellates Bodo saltans and Goniomonas sp., and the ciliate Cyclidium glaucoma. Both flagellate species preferably ingested A. hydrophila over P. fluorescens, while C. glaucoma showed no clear preferences. Differences were found in the digestion of bacterial prey with B. saltans digesting significantly faster P. fluorescens compared to two other protists. The field study was conducted in a reservoir as part of a larger experiment. We monitored changes in the bacterial prey composition available compared to the bacteria ingested in flagellate food vacuoles. Bacteria detected by probe HGC69a (Actinobacteria) and R-BT065 were negatively selected by flagellates. Bacteria detected by probe CF319a were initially positively selected but along with a temporal shift in bacterial cell size, this trend changed to negative selection during the experiment. Overall, our analysis of protistan food vacuole content indicated marked effects of flagellate prey selectivity on bacterioplankton community composition.     

STIMULATING EFFECT OF ANABAENA SP. (CYANOBACTERIA) EXUDATE ON PRYMNESIUM PARVUM (HAPTOPHYTA)1

Prymnesium parvum blooms have become more frequent in the south‐central United States, leading to significant ecological and economic impacts. Allelopathic effects from cyanobacteria were suggested as a mechanism that might limit the development of P. parvum blooms. This research focused on the effects of cultured cyanobacteria, Anabaena sp., on P. parvum. Over a 6‐d period, daily additions of filtrate from the senescent Anabaena culture were made to P. parvum cultures growing in log phase. All treatments, including several types of controls, showed reductions in P. parvum biomass over the course of the experiment, but the treatments receiving Anabaena filtrate were reduced to a lesser degree, suggesting that filtrate from the senescent cyanobacteria culture was beneficial to P. parvum in some way. This unexpected outcome may have resulted from stimulation of heterotrophic bacteria by the addition of Anabaena filtrate, which likely contained exudates rich in dissolved organic carbon compounds. P. parvum was then able to supplement its nutritional requirements for growth by feeding on the elevated bacteria population. These findings coupled to previous observations suggest that interactions between cyanobacteria and P. parvum in natural environments are complex, where both allelopathic and growth‐stimulating interactions are possible.     

Temperature‐dependent phagotrophy and phototrophy in a mixotrophic chrysophyte

The roles of temperature and light on grazing and photosynthesis were examined for Dinobryon sociale, a common freshwater mixotrophic alga. Photosynthetic rate was determined for D. sociale adapted to temperatures of 8, 12, 16, and 20°C under photosynthetically active radiation light irradiances of 25, 66, and 130 μmol photons · m^?2 · s^?1, with concurrent measurement of bacterial ingestion at all temperatures under medium and high light (66 and 130 μmol photons · m^?2 · s^?1). Rates of ingestion and photosynthesis increased with temperature to a maximum at 16°C under the two higher light regimes, and declined at 20°C. Although both light and temperature had a marked effect on photosynthesis, there was no significant difference in bacterivory at medium and high irradiances at any given temperature. At the lowest light condition (25 μmol photons · m^?2 · s^?1), photosynthesis remained low and relatively stable at all temperatures. D. sociale acquired the majority of carbon from photosynthesis, although the low photosynthetic rate without a concurrent decline in feeding rate at 8°C suggested 20%–30% of the carbon budget could be attributed to bacterivory at low temperatures. Grazing experiments in nutrient‐modified media revealed that this mixotroph had increased ingestion rates when either dissolved nitrogen or phosphorus was decreased. This work increases our understanding of environmental effects on mixotrophic nutrition. Although the influence of abiotic factors on phagotrophy and phototrophy in pure heterotrophs and phototrophs has been well studied, much less is known for mixotrophic organisms.     

Ecological Traits of the Algae‐Bearing Tetrahymena utriculariae (Ciliophora) from Traps of the Aquatic Carnivorous Plant Utricularia reflexa

Trap fluid of aquatic carnivorous plants of the genus Utricularia hosts specific microbiomes consisting of commensal pro‐ and eukaryotes of largely unknown ecology. We examined the characteristics and dynamics of bacteria and the three dominant eukaryotes, i.e. the algae‐bearing ciliate Tetrahymena utriculariae (Ciliophora), a green flagellate Euglena agilis (Euglenophyta), and the alga Scenedesmus alternans (Chlorophyta), associated with the traps of Utricularia reflexa. Our study focused on ecological traits and life strategies of the highly abundant ciliate whose biomass by far exceeds that of other eukaryotes and bacteria independent of the trap age. The ciliate was the only bacterivore in the traps, driving rapid turnover of bacterial standing stock. However, given the large size of the ciliate and the cell‐specific uptake rates of bacteria we estimated that bacterivory alone would likely be insufficient to support its apparent rapid growth in traps. We suggest that mixotrophy based on algal symbionts contributes significantly to the diet and survival strategy of the ciliate in the extreme (anaerobic, low pH) trap‐fluid environment. We propose a revised concept of major microbial interactions in the trap fluid where ciliate bacterivory plays a central role in regeneration of nutrients bound in rapidly growing bacterial biomass.     

Pelagic Ciliates in a Large Mesotrophic Lake: Seasonal Succession and Taxon‐Specific Bacterivory in Lake Constance

The taxonomic composition of the ciliate assemblage and their taxon‐specific bacterial grazing rates in Lake Constance were investigated over the course of one year. Bacterial grazing rates were measured using natural fluorescently labelled bacteria (FLB) and compared to bacterial production. Small species such as Balanion planctonicum/Urotricha furcata and Rimostrombidium spp./Halteria sp. were the most numerous ciliates on the annual average. Larger ciliates such as Rimostrombidium lacustris and Limnostrombidium spp. contributed significantly to total ciliate biomass, but were relatively unimportant as bacterial grazers. Per capita ingestion rates ranged from 0–194 bacteria ciliate⁻¹ h⁻¹ and changed seasonally up to a hundredfold within a given taxon. Approximately 1% of the bacterial production were removed by the ciliate community on the annual average. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mixotrophy and the toxicity of Ochromonas in pelagic food webs

1. Toxic compounds produced by many phytoplankton taxa are known to have negative effects on competitors (allelopathy), anti‐predatory effects on grazers (mortality or impaired reproduction) or both. Although mixotrophs of the genus Ochromonas are known to be toxic to zooplankton, it has often been assumed in studies of plankton community processes that all flagellates in the size range of this taxon are edible to typical zooplankton grazers (i.e. cells ≤30 μm for Daphnia, ≤6 μm for rotifers). 2. We explored the toxicity of a species of Ochromonas to other planktonic taxa, including its competitors (two species of phytoplankton and protists) and consumers (two species of zooplankton). To test if mode of nutrition by this mixotroph influences its toxicity to other taxa, we exposed each test species to Ochromonas cultured in chemostats under four different nutritional regimes: osmotrophy (labile dissolved organic carbon) and phagotrophy (bacterial prey) in both light and dark conditions (i.e. with or without photosynthesis). 3. Filtrate from osmotrophically fed Ochromonas had a significant negative effect on the population growth rate of two obligate phototrophic phytoplankton, Cryptomonasozolini and Chlamydomonas reinhardtii. The protists Tetrahymena tetrahymena and Paramecium aurelia were also negatively affected by Ochromonas filtrate. Ochromonas cells were toxic to both the rotifer Brachionus calicyflorus and the cladoceran Daphnia pulicaria, with the toxic effects significantly more severe when fed at high cell densities (75 000 cells mL⁻¹) than at low densities (7500 cells mL⁻¹). Ochromonas cultured osmotrophically in the light was more toxic to the Daphnia than cells cultured under other conditions. In contrast, Ochromonas from all nutritional conditions was equally highly toxic to Brachionus. 4. Our findings support the view that Ochromonas can be toxic to other components of the food web with which it interacts. It is especially toxic to zooplankton that directly consume it, although the effect depends upon Ochromonas cell density and whether or not a good food source is simultaneously present. Our results call into question the common practice of pooling flagellates into a single ‘functional group’ included in an ‘edible phytoplankton’ category of cells <30 μm in diameter.     

Phytoplankton, bacterial production and protozoan bacterivory in stratified, brackish-water Lake Shira (Khakasia, Siberia)

Rates of oxygenic and anoxygenic photosynthesis, chemoautotrophic and heterotrophic bacterial production and protozoan bacterivory were measured in the pelagic zone of the stratified brackish-water lake with the purpose to determine the vertical distribution of these processes and to estimate their significance in the functioning of planktonic community of the lake. In midsummer, total daily primary productivity was about 1.3 g C m^–2, of which 72% was produced by the phytoplankton, 24% by the chemoautotrophic bacteria, and only 4% by the phototrophic sulphur bacteria. Thus anoxygenic photosynthesis is a negligible source of organic matter in the lake. The production of heterotrophic bacteria averaged 1.5 g C m^–2 d^–1 and exceeded the total photosynthesis of phytoplankton and photosynthetic bacteria by a factor of 1.5. The estimated total primary production was too low to sustain the bacterial production. Probably the carbon cycle in the lake is dependent on the input of allochthonous organic matter. As a rule, the maximal rates of primary production and heterotrophic bacterial production were found in the chemocline or at the upper boundary of the chemocline. Heterotrophic flagellates dominated among the protozoan populations and were the major consumers of the bacterioplankton production in the lake. They showed maximal ingestion rates from 2.3 to 2.9 mg C m^–3 h^–1 at the upper boundary of the chemocline, where they consumed from 50 to 54% of the production of heterotrophic bacteria. Data obtained indicate that in Lake Shira the oxic-anoxic interface is the site of the most intensive production and mineralization of organic matter.     

The bacterivory of interstitial ciliates in association with bacterial biomass and production in the hyporheic zone of a lowland stream

Rates of bacteria ingestion by interstitial ciliates were estimated and compared to bacterial biomass and production. Investigation was carried out in the hyporheic zone of a lowland stream. FISH was applied to quantitatively determine bacteria within the ciliate's food vacuoles. To estimate bacteria ingestion rates using FISH, we had to strike a new path. When numbers of bacteria in the food vacuoles remains constant with time (bacterial digestion and ingestion are at equilibrium), ingestion rate can be estimated based on the digestion time and the average number of bacteria per cell. Ciliate community was predominantly composed of bacterivorous ciliates. FISH-signals deriving from ingested bacteria were detected in Cinetochilum margaritaceum, 'other small scuticociliates', Pleuronema spp., and Vorticella spp. Ingestion rates for these taxa were 78, 150, 86, and 38 bacteria ind(-1) h(-1), respectively. The grazing impacts on bacterial biomass and carbon production were calculated based on these ingestion rates. Ciliate grazing caused a decrease in bacterial biomass of 0.024% day(-1) and in bacterial carbon production of 1.60%. These findings suggest that interstitial ciliate grazing impact on bacteria biomass and production was too low to represent an important link in the carbon flow of the hyporheic zone under study.     

Disentangling the role of light and nutrient limitation on bacterivory by mixotrophic nanoflagellates

Many phytoplankton taxa function on multiple trophic levels by combining photosynthesis and ingestion of bacteria, termed mixotrophy. Despite the recognition of mixotrophy as a universal functional trait, we have yet to fully resolve how environmental conditions influence community grazing rates in situ. A microcosm study was used to assess bacterivory by mixotrophic nanoflagellates following nutrient enrichment and light attenuation in a temperate lake. We found contrasting results based on assessment of mixotroph abundance or bacterivory. Despite an interactive effect of nutrient enrichment and light attenuation on mixotroph abundance, significant differences within light treatments were observed only after enrichment with P or N + P. The greatest abundance of mixotrophs across treatments occurred under co-nutrient enrichment with full exposure to irradiance. However, bacterivory by mixotrophic nanoflagellates was greatest under shaded conditions after either N or P enrichment. We suggest that PAR availability dampened the stimulatory effect of nutrient limitation, and bacterivory supplemented a suboptimal photosynthetic environment. In a saturating light regime, the mixotrophic community was less driven to ingest bacteria because photosynthesis was able to satisfy energetic demands. These findings quantify community bacterivory in response to environmental drivers that may characterize future ecosystem conditions and highlight the importance of considering grazing rates in conjunction with abundance of mixotrophic protists.