Components Of Microbial Food Web Research Articles

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

Summary1. 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.

Rotifer fecundity in relation to components of microbial food web in a eutrophic reservoir

The relationship among rotifers and their potential food sources such as bacteria, heterotrophic nanoflagelates and algae were examined in the eutrophic Řimov reservoir. The egg ratio of five rotifer species (Polyarthra spp., Keratella cochlearis, K. quadrata, Kellicottia longispina and Synchaeta spp.) was estimated and related to the concentration of food particles. Generally, the highest egg ratio was found during the spring time, before the clear water phase, although the densities of some species reached their highest numbers in summer. All investigated food sources were found to have a significant relationship to rotifer egg ratio. The fecundity of all species, except Synchaeta, depend on chlorophyll-a. The Monod curve of food-fecundity relationship reflects that K. cochlearis and P. dolichoptera have high affinities to lower concentrations of chl-a. This is in contrast to K. longispina and K. quadrata, which maximized their egg ratio under higher food concentrations. The two components of the microbial food web (bacterial carbon and HNF) were correlated with fecundity but only for K. cochlearis. The seasonal differences in food-fecundity relationships for K. cochlearis andK. longispina indicated that there might be differential food preferences of these two species during the season.

The Effects of Freshwater Flushing on Marine Heterotrophic Protists – Implications for Ballast Water Management

Survivorship of ballast-entrained marine heterotrophic protists was examined following freshwater flushing. The recovered taxa, including typical marine rhizopods such as Platyamoeba murchelanoi, Labyrinthula spp, Pontifex maximus, Thecamoeba orbis, and the ciliate Condylostoma arenarium, were reared in waters of various salinities. After 2 months, the original salinity subsample retained five protist taxa, the freshwater six, including the amoeba Cochliopodium bilimbosum, the brackish water 22 taxa, and the seawater 19 taxa. Since protists form a major component of marine microbial food webs, their survival may be instrumental in supporting complex ballast-entrained food webs. Our study raises questions as to the reliability of open-ocean exchange (OOE) or freshwater flushing as effective control measures.

Effects of resuspension and mixing on population dynamics and trophic interactions in a model benthic microbial food web

The effects of resuspension and mixing on the population dynamics and trophic inter- actions in a simple benthic microbial food web were studied during 3 plankton wheel experiments of 9 to 15 d duration. The food web consisted of a mixed bacterial assemblage, the heterotrophic flagel- late Bodo designis, the ciliate Euplotes balteatus and the rhizopod Vannella platypodia. The diatom Amphora coffeaeformis was included to maintain oxygen concentrations. B. designis, E. balteatus, V. platypodia and A. coffeaeformis are hereafter referred to by genus name alone. Population dynamics in 500 ml microcosms on rotating and non-rotating plankton wheels were compared. Final abun- dances of Amphora increased in suspension in all experiments. Resuspension increased initial growth rates and final abundances of Euplotes and Vannella during a whole-community experiment and a community subset experiment during which resuspension effects with and without the top predator Euplotes were compared. During the community subset experiment, suspended Bodo grew faster and reached higher final abundances than non-resuspended Bodo when Euplotes was absent, but experienced higher loss to Euplotes when the ciliate was present. Individual consumption rates of Bodo by Euplotes were estimated to be higher in suspension (5.5 vs 3.6 Bodo Euplotes -1 h -1 ), which could not be explained by higher abundance of Bodo alone. Similar but non-significant trends were found for bacteria. During a third experiment, Euplotes did not benefit from resuspension at low food concentrations. Our results show that resuspension and mixing can enhance population growth of autotrophic and heterotrophic protists, and that the trophic coupling between flagellates and ciliates, and possibly between other microbial food web components, can become closer in suspension when the food supply is sufficient. This suggests that subsidiary energy input by resuspension and mixing may increase the biomass turnover in benthic microbial food webs.

Trophic interactions within the microbial food web in Piburger See (Austria)

The structure, seasonal dynamics and trophic interactions of the microbial food web were investigated in the oligo-mesotrophic Piburger See. Abundance and biomass of the microbial food web components as well as bacterial production and predation by protists were quantified at weekly intervals in the euphotic zone. Phytoplankton biomass showed a maximum under ice and in summer. Abundance of plastidic flagellates was correlated with chlorophyll-a (r = 0.48; p < 0.02). High abundance of nanociliates at the surface (ca. 1 x 10 5 ind. l −1 ), mainly Balanion planctonicum, were portant for the start of the clear-water phase. Mixotrophic ciliates were present under ice and during summer. Among them Pelagohalteria viridis reached abundances of up to 3 x 10 3 ind. l −1 . Bacterivory exceeded bacterial production on several occasions although, on average, they were balanced. Aplastidic flagellates were the main controllers of bacterial production, ciliates had a very low impact (< 3.1 % of water column cleared per day). Autotrophic picoplankton represented a significant alternative C pool for protists making up to 43 % of total picoplankton biomass. During most time of the year the microbial food web (MFW) seems to act as a sink for carbon, except during the clear-water phase when cladocerans have a strong grazing impact on the whole MFW. Using a multiple regression model we found that 35 % of the variability of bacterial production was explained by the abundance of bacteria and temperature. This value increased to 50 % when the number of active (INT) bacteria was used instead of total bacteria. Bacterial abundances showed a low predictive value on aplastidic flagellate abundances, suggesting that trophic pathways in this lake are strongly fluctuating and that a coupling between both populations does not always exist

A framework for the assessment of top-down vs. bottom-up control of heterotrophic nanoflagellate abundance

A model is presented that allows inferences about the main mode of control of planktonic heterotrophic nanoflagellate (HNF) abundance, control by resources or by predation, from the abundances of bacterial and HNF. The model is built upon the observation that field measurements of HNF abundance at most reach concentrations close to those expected for a system in steady-state (with no variation of bacterial abundance). Field and enclosure data from freshwater and marine systems are used to test its utility by regressing D, the distance between the maximal possible concentration of HNF and the actual concentration, to the abundance of micro- or macrozooplankton. It is concluded that the model may help ascertain the impact of microzooplankton on the components of microbial food webs when data on their abundances are not available, and that it can also be used to generate hypotheses on the degree of influence of sources of carbon other than bacteria on flagellates, and the effect of HNF grazing on bacterial specific growth rates.

Distribution and abundance of choanoflagellates (Acanthoecidae) across the ice-edge zone in the Weddell Sea, Antarctica

Choanoflagellates are thought to be an important component of oceanic microbial food webs, but little quantitative data exists on their abundance,, distribution, or relationship to potential food sources. In an Antarctic ice edge zone (northern Weddell Sea, March 1986), choanoflagellate abundance varied over two orders of magnitude in the upper 100 m. The lowest abundances were recorded at the bottom of the water column under ice cover and the highest abundances occurred in the upper 30 m of open water. Species that were predominantly in colonies dominated the open-water samples. Abundances of total choanoflagellates and some individual species were correlated with primary and secondary biomass and production, indicating a response to gradients in potential food sources. This suggests that choanoflagellates are tightly coupled with their food sources and supports the contention that they may an important link between bacteria-sized particles and metazoan grazers.