Components Of Microbial Food Web Research Articles

Temporal variation of the small eukaryotic community in two freshwater lakes: emphasis on zoosporic fungi

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 67:91-105 (2012) - DOI: https://doi.org/10.3354/ame01592 FEATURE ARTICLE Temporal variation of the small eukaryotic community in two freshwater lakes: emphasis on zoosporic fungi Emilie Lefèvre1,2,*, Peter M. Letcher1, Martha J. Powell1 1Department of Biological Sciences, The University of Alabama, Tuscaloosa, Alabama 35487, USA 2Present address: Department of Biology, Duke University, Durham, North Carolina 27701, USA *Email: emilie.lefevre@duke.edu ABSTRACT: Applications of molecular approaches to the study of microbial eukaryotic communities in freshwater lakes are transforming our understanding of these ecosystems. One of the most unexpected discoveries is that zoosporic fungi significantly dominate the planktonic fungal diversity. Although zoosporic fungi are now recognized as an important component of aquatic microbial food webs, our knowledge of their community structure and temporal variability remains poor. The objectives of our study were (1) to compare and describe the contribution of zoosporic fungi to the eukaryotic diversity in 2 lakes differing in their trophic status during the mixing and the stratified seasons and (2) to phylogenetically identify the recovered zoosporic fungal sequences. The small eukaryotes (0.6 to 8 µm) of the euphotic zone of the oligotrophic Lake Tuscaloosa and meso-eutrophic Lake Lurleen (Alabama, USA) were collected over 1 yr. Analyses of the 28S rDNA clone libraries showed that zoosporic fungi dominated the small planktonic fungal community and were more diverse in the meso-eutrophic lake and during the thermal stratification. Although the overall structure of the eukaryotic community was similar between the 2 lakes, at lower taxonomic levels, community composition differed. Analyses of the retrieved fungal sequences revealed that zoosporic fungi mostly affiliated with Rhizophydiales and Chytridiales or formed environmental clades. Although the phytoplanktonic community was also monitored, zoosporic fungal parasites were rarely observed on algae. These results provide new insights into the diversity and seasonality of the zoosporic fungal community in lake ecosystems. KEY WORDS: Zoosporic fungi · Molecular diversity · Temporal variation · Freshwater lakes · Plankton Full text in pdf format Information about this Feature Article Supplementary material NextCite this article as: Lefèvre E, Letcher PM, Powell MJ (2012) Temporal variation of the small eukaryotic community in two freshwater lakes: emphasis on zoosporic fungi. Aquat Microb Ecol 67:91-105. https://doi.org/10.3354/ame01592 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 67, No. 2. Online publication date: October 02, 2012 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2012 Inter-Research.

Phylogenetic and Morphological Diversity of Novel Soil Cercomonad Species with a Description of Two New Genera (Nucleocercomonas and Metabolomonas)

Cercomonads are important components of microbial food webs in soils and aquatic sediments. Here, we investigated the general morphology, behaviour, life cycle and 18S rDNA phylogeny of cercomonad cultures from a German grassland soil habitat. We describe ten new species including two new genera from 23 strains. Three Cercomonas, two Eocercomonas and three Paracercomonas species are described. Based on large phylogenetic distance and distinct morphology, we erect two novel clade B genera near the root of the cercomonad tree. Nucleocercomonas nov. gen. bears a number of characters unusual for cercomonads: Its anterior flagellum is extremely long, it mostly does not glide, and in its most frequent life stage the cell body does not attach to the substratum, but produces unattached pseudopodia. Furthermore, it has a unique nucleus with a peripheral nucleolus that attaches to the nuclear envelope opposite the basal body connection. Metabolomonas nov. gen. is extremely metabolic. It is characterized by a very high beating frequency of the anterior flagellum, fast gliding, rapid changes in shape and strong cytoplasmic streams. A new genus Brevimastigomonas is erected for the previously described species Paracercomonas anaerobica. The general morphology of cercomonad species often does not correspond with their phylogenetic position: closely related species may have a very different morphology.

Abundance and Biomass Responses of Microbial Food Web Components to Hydrology and Environmental Gradients within a Floodplain of the River Danube

This study investigated the relationships of time-dependent hydrological variability and selected microbial food web components. Samples were collected monthly from the Kopački Rit floodplain in Croatia, over a period of 19 months, for analysis of bacterioplankton abundance, cell size and biomass; abundance of heterotrophic nanoflagellates and nanophytoplankton; and concentration of chlorophyll a. Similar hydrological variability at different times of the year enabled partition of seasonal effects from hydrological changes on microbial community properties. The results suggested that, unlike some other studies investigating sites with different connectivity, bacterioplankton abundance, and phytoplankton abundance and biomass increased during lentic conditions. At increasing water level, nanophytoplankton showed lower sensitivity to disturbance in comparison with total phytoplankton biomass: this could prolong autotrophic conditions within the floodplain. Bacterioplankton biomass, unlike phytoplankton, was not impacted by hydrology. The bacterial biomass less affected by hydrological changes can be an important additional food component for the floodplain food web. The results also suggested a mechanism controlling bacterial cell size independent of hydrology, as bacterial cell size was significantly decreased as nanoflagellate abundance increased. Hydrology, regardless of seasonal sucession, has the potential to structure microbial food webs, supporting microbial development during lentic conditions. Conversely, other components appear unaffected by hydrology or may be more strongly controlled by biotic interactions. This research, therefore, adds to understanding on microbial food web interactions in the context of flood and flow pulses in river-floodplain ecosystems.

Soil microbial food web responses to free-air ozone enrichment can depend on the ozone-tolerance of wheat cultivars

Soil processes are driven by soil organisms and their interactions with plants and soil abiotic conditions. Climate changes may directly or indirectly alter soil processes and the organisms mediating these processes. Although aboveground influences of ozone have been studied widely on agroecosystems, the effects on belowground processes are poorly understood. This study aimed to investigate the effects of elevated ozone concentration [O3] on the components of soil microbial food webs and compare their responses between ozone-sensitive and ozone-tolerant wheat cultivars. The responses of soil biota to elevated [O3] varied between the two wheat cultivars. Fungal PLFA and the fungi to bacteria ratio decreased following elevated [O3], especially in the rhizospheric soil of ozone-tolerant wheat. Nematode functional guilds were sensitive to elevated [O3] and cultivar effects. At wheat jointing stage, bacterivores belonging to K-strategies decreased following elevated [O3], while fungivores exhibited a reverse trend. The abundance of flagellates decreased in ozone-tolerant wheat, but increased in ozone-sensitive wheat following elevated [O3]. However, an opposite trend was found in the bacterivores belonging to r-strategies. In conclusion, wheat cultivars play an important role in determining the effects of elevated [O3] on soil food web. The responses of soil biota to elevated [O3] were greater in ozone-tolerant wheat than in ozone-sensitive wheat, which may in turn have influenced soil organic matter decomposition and nutrient turnover.

Preferential Feeding by the Ciliates Chilodonella and Tetrahymena spp. and Effects of These Protozoa on Bacterial Biofilm Structure and Composition

Protozoa are important components of microbial food webs, but protozoan feeding preferences and their effects in the context of bacterial biofilms are not well understood. The feeding interactions of two contrasting ciliates, the free-swimming filter feeder Tetrahymena sp. and the surface-associated predator Chilodonella sp., were investigated using biofilm-forming bacteria genetically modified to express fluorescent proteins. According to microscopy, both ciliates readily consumed cells from both Pseudomonas costantinii and Serratia plymuthica biofilms. When offered a choice between spatially separated biofilms, each ciliate showed a preference for P. costantinii biofilms. Experiments with bacterial cell extracts indicated that both ciliates used dissolved chemical cues to locate biofilms. Chilodonella sp. evidently used bacterial chemical cues as a basis for preferential feeding decisions, but it was unclear whether Tetrahymena sp. did also. Confocal microscopy of live biofilms revealed that Tetrahymena sp. had a major impact on biofilm morphology, forming holes and channels throughout S. plymuthica biofilms and reducing P. costantinii biofilms to isolated, grazing-resistant microcolonies. Grazing by Chilodonella sp. resulted in the development of less-defined trails through S. plymuthica biofilms and caused P. costantinii biofilms to become homogeneous scatterings of cells. It was not clear whether the observed feeding preferences for spatially separated P. costantinii biofilms over S. plymuthica biofilms resulted in selective targeting of P. costantinii cells in mixed biofilms. Grazing of mixed biofilms resulted in the depletion of both types of bacteria, with Tetrahymena sp. having a larger impact than Chilodonella sp., and effects similar to those seen in grazed single-species biofilms.

Dynamics of microbial planktonic food web components during a river flash flood in a Mediterranean coastal lagoon

Episodic river flash floods, characteristic of Mediterranean climates, are suspected to greatly affect the functioning of microbial food webs. For the first time, the abundance, biomass and diversities of microbial food web components were studied before and during 4 consecutive days after a flash flood that occurred in November 2008, in the surface waters of five stations along a salinity gradient from 20 to 36 in the Thau lagoon. Eukaryotic pico- and nanophytoplankton were discharged from the river into the lagoon and increased by 30- and 70-fold, respectively. Bacteria increased by only 2-fold in the lagoon, from around 4–8 × 106 cells ml−1, probably benefiting from river nutrient input. Chlorophyll a increased 4-fold, and pigment biomarkers showed that the dinophyceae, prasinophyceae and prymnesiophyceae were sensitive to the flood perturbation, whereas the bacillariophyceae, cryptophyceae and chlorophyceae were resistant and/or transported to the lagoon from the river. Predator responses were more complex as total heterotrophic flagellate abundance decreased slightly, whereas those of specific naked ciliates increased, particularly for Uronema sp. The flood also induced a specific change in diversity, from a community dominated by Strobilidium spiralis to a community dominated by Uronema sp. The tintinnid community was particularly sensitive to the flood event as the abundance of all species decreased greatly. The high increases in biomass, mainly brought by the river during the flood, could have eventually sedimented to the benthic layer and/or been transported further into the lagoon, supporting the pelagic food web, or have even been exported to the Mediterranean Sea.

Organic matter recycling in a beach environment influenced by sunscreen products and increased inorganic nutrient supply (Sturla, Ligurian Sea, NW Mediterranean)

The beaches are sites where the human influence may be strong and the beach ecosystems have often shown a high sensibility to environmental alterations. These zones may be affected by a large series of anthropogenic-derived pressures, such as unbalanced inorganic nutrient input, that may cause anomalous development of primary production, altering the structure of the trophic webs. Furthermore, the utilisation of cosmetic sunscreen products is reaching unexpected levels, thus assuming a potentially important as well as unknown role in the contamination of marine environments. The present study was planned to test the response of the beach ecosystem to increases in inorganic nutrients (nitrate and phosphate) and to the input of a widely used cosmetic sunscreenproduct. A short-term laboratory experiment was carried out on microsystems consisting of sediments and seawater from the swash zone of a Ligurian city beach (Sturla). The processes related to organic matter (OM) recycling and some microbial food web components (bacteria and micro-autotrophic organisms) were analysed. The multivariate statistical analysis of the results showed that the increase in inorganic nutrients and sunscreen caused only a transient alteration in the OM recycling processes in the seawater. The sedimentary processes, instead, were different in the different systems, although starting from the same condition. In the sediment, surprisingly, an increase in inorganic nutrients did not lead to an increase in the primary biomass nor to significantly higher bacterial abundance, while the sunscreen caused increased OM recycling, especially devoted to protein and lipid mobilisation, supporting a growing bacterial and autotrophic community by reducing the bottom-up pressure. Additional toxicity tests performed on protozoa highlighted that, while the inorganic nutrients seemed to show no effects, sunscreen decreased the protozoan viability, thus likely favouring microautotrophic and bacterial increases by reducing the top-down pressure.

Community heterogeneity and single‐cell digestive activity of estuarine heterotrophic nanoflagellates assessed using lysotracker and flow cytometry

Heterotrophic nanoflagellates (HNFs) are an essential component of all aquatic microbial food webs, and yet the exploration of the numerical and single-cell responses of these organisms in mixed assemblages still represents a major technical challenge. LysoTracker Green staining combined with flow cytometry was recently proposed for the enumeration of aquatic HNFs. Here we show that LysoTracker Green not only allows the enumeration of HNFs in estuarine samples with a wide range of HNF abundances, but also allows the discrimination of distinct HNF populations in mixed assemblages. In addition, the resulting cytometric parameters can be used to characterize cell size and the level of activity of the cells in the different populations that are detected. LysoTracker Green accumulates preferentially in lysosomes, and we demonstrate that the green fluorescence emission from HNF cells stained with LysoTracker strongly correlates with cell-specific beta-glucosaminidase (beta-Gam) activity, a key digestive enzyme of lysosomal origin in eukaryotic cells. Our results further show that different populations that develop in estuarine regrowth cultures are characterized by different intrinsic ranges of size and of feeding activity, and that there is a wide range of single-cell responses within these HNF populations. We found a large degree of uncoupling between cell size and feeding activity, both between and within HNF populations, and there appears to be no clear allometric scaling of feeding activity. We were able to reconstruct the succession of distinct HNF populations that developed during the regrowth experiments, and explore the complex interactions that occurred between numerical (change in abundance of the cytometric populations) and single-cell HNF responses.

Response of the plankton community to herbicide application (triazine herbicide, simetryn) in a eutrophicated system: short-term exposure experiment using microcosms

The methylthiotriazine herbicide, simetryn, is commonly used in Japan, and its concentration in surface water is often high enough to affect natural phytoplankton. To estimate how the plankton community in eutrophic systems respond to short-term exposure of realistic concentrations of simetryn, we collected plankton from a eutrophic lake and exposed them to low (20 μg l−1) and high (100 μg l−1) concentrations of simetryn for 12 days in microcosm tanks (50 l). High concentrations significantly lowered total phytoplankton biomass, particularly green algal density. Consequently, the species composition was severely modified by simetryn application. However, there was no apparent impact of simetryn on microbial food-web components, bacteria, heterotrophic nanoflagellates (HNF), and ciliates. Despite the decreased abundance of algal food, the zooplankton community showed subtle changes with simetryn application. The results indicate that the direct impact of simetryn on planktonic organisms other than phytoplankton, particularly on microbial food-web components, is weak. The indirect impact of simetryn on zooplankton through the change of food quality and quantity was also small. It has been suggested that the persistence of microorganisms, alternative food for zooplankton, probably dilutes the indirect impact of simetryn on zooplankton by compensating for the loss of food phytoplankton. Consequently, the plankton community in eutrophicated systems is resistant to the herbicide at a feasible concentration for a short period of time.

Impact of mesozooplankton grazing on the microbial community revealed by denaturing gradient gel electrophoresis (DGGE)

Although grazer inclusion incubation experiments are commonly used to evaluate mesozooplankton grazing mortality of phytoplankton, few studies have employed molecular methods to examine the changes in the composition of the microbial community across different grazing treatments. We used denaturing gradient gel electrophoresis (DGGE) to study the impact of mesozooplankton-mediated trophic cascades on microbial food web components at two subtropical coastal sites with contrasting trophic conditions. Reciprocal changes in microbial food web components resulting from trophic cascades were observed, with decreases in > 3 µm eukaryotes, increases in < 3 µm eukaryotes and decreases in bacterioplankton. At the estuarine station, diatoms and dinoflagellates accounted for the majority of the micro and nanoplankton, and they, particularly dinoflagellates, were selectively grazed by mesozooplankton. Although the total band numbers of picoeukaryotes did not vary significantly, the relative abundance of different phylogenetic groups changed with the appearance of mesozooplankton grazers. Both total band brightness and species richness and diversity of bacterioplankton declined sharply under mesozooplankton grazing at the coastal site, but not at the estuarine site, suggesting that mesozooplankton-mediated trophic cascade effect was less pronounced in eutrophic estuarine waters. We concluded that DGGE could be a useful tool to track the community shifts of microbial food web in experimental settings and we have demonstrated in this study such shifts in both eukaryotic and prokaryotic microbial communities induced by mesozooplankton grazing. The magnitude of mesozooplankton-mediated trophic cascade effect transferred to picoplankton appears to be stronger at the mesotrophic coastal water than at the eutrophic estuarine water.

Effect of zooplankton-mediated trophic cascades on marine microbial food web components (bacteria, nanoflagellates, ciliates)

To examine the grazing effects of copepod-dominated mesozooplankton on heterotrophic microbial communities, four mesocosm experiments using gradients of zooplankton abundance were carried out at a coastal marine site. The responses of different protist groups (nanoflagellates, ciliates) and bacterioplankton in terms of abundance and additionally, for bacteria, diversity, production, and exoenzymatic activity, were monitored during 1 week of incubation. Independent of the initial experimental abiotic conditions and the dominating copepod species, zooplankton caused order-of-magnitude changes in microbial functional groups in a clear community-wide four-link trophic cascade. The strongest predatory effects were observed for protist concentrations, thus generating inverse relationships between mesozooplankton and ciliates and between ciliates and nanoplankton. Copepod grazing effects propagated even further, not only reducing the abundance, production, and hydrolytic activity of bacterioplankton but also increasing bacterial diversity. The overall strength of this trophic cascade was dampened with respect to bacterial numbers, but more pronounced with respect to bacterial diversity and activity. High predation pressure by heterotrophic nanoflagellates, realized at the highest copepod abundance, was probably the underlying mechanism for these structural changes in the bacterial assemblages. Our results thus suggest a mechanism by which changes in higher trophic levels of marine plankton indirectly affect prokaryotic assemblages and microbially mediated ecosystem functions.

Comparison of the components of the planktonic food web in three large rivers (Rhine, Moselle and Saar)

AbstractThe dynamics of phyto‐, bacterio‐, proto‐ and metazooplankton were investigated for a period of 22 months fortnightly from March 1999 until December 2000 in the rivers Moselle, Saar and Rhine. Plankton abundances reached the lowest values in the Rhine and the highest in the Saar. Protozoa and not the much more studied metazoa clearly dominate the zooplankton not only by number but also by biomass by about 90%. We show that the plankton biomass in each of the rivers is the result of the interplay of the grazing by benthic predators and of the human induced factors such as river morphology, water residence time and nutrient load. Furthermore, we compare our study with the planktonic food web of three other large European rivers. The comparison between all studies revealed a general rise of the absolute mean biomass of all heterotrophic planktonic food web components (bacteria, protozoa, metazoa) parallel to an increase of the algal biomass of the river. For the rivers Rhine, Moselle and Saar, the average biomass of all investigated plankton groups was very low compared to the values obtained in the rivers Meuse, Loire and Elbe. The relative importance of organisms of the microbial food web increased with a decrease of the algal biomass or organic carbon content, respectively, in each of the rivers. We postulate that with a sinking nutrient level in the Rhine, the microbial food web components will become even more important within the plankton than they already are. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Microbial community development in the traps of aquatic Utricularia species

We examined trap fluid of three aquatic carnivorous species of Utricularia (Lentibulariaceae) to assess the role of microbial community within their traps in plant nutrient acquisition. In the context of increasing trap age, we characterized microbial community composition using phospholipid fatty acid (PLFA) analysis and microscopy. Nutrient content in various fractions of the trap fluid was analyzed and the abundance of free-suspended bacteria estimated. The activities of extracellular phosphatase in the trap fluid were determined using fluorometry and the contribution of the microbial community to phosphatase production assessed by epifluorescence microscopy. The trap microbial community seems to be largely derived from Utricularia associated periphyton. PLFA analysis revealed that trap fluid contained all components of a complex microbial food web with bacteria forming more than 58% of the viable microbial biomass in the trap. Trap age seems to be the key factor in determining the patterns of microbial community development as well as enzyme production. The amount of nutrients increases with increasing trap age, and the total amounts of C, N, and P accumulated within traps during their lifetime are relatively large—of the order of 100 mg L −1 for C and N, and between 0.2 and 0.6 mg L −1 for P. A significant part of the nutrient pool is present in the dissolved form. Trap fluid stoichiometry (molar N:P ratios about 100) as well as the presence of nutrient limited microbial cells (molar N:P ratios 25–61) indicates the importance of phosphorus rather than nitrogen for the nutrition of Utricularia. Our findings support the hypothesis that mutualism, apart from the predator–prey interaction, is an important association in aquatic Utricularia traps and that the trap-associated microbial community may be of benefit to the rootless aquatic Utricularia species facing problems with P acquisition due to the loss of roots in their evolution.

Freshwater and marine virioplankton: a brief overview of commonalities and differences

Summary1. Viruses are a pervasive component of microbial food webs in both marine and freshwater systems. The abundance of viruses in individual aquatic systems appears to be independent of salinity but related to the biomass of primary and secondary producers as well as seasonal effects. Burst size, virus production rate and the percentage of microbial cells carrying a viral burden also appear to be more closely correlated to trophic status than to salinity.2. In marine environments, the roles of planktonic viruses as regulators of carbon and nutrient cycling as well as microbial community structure have been a focus of numerous studies, yet the roles of freshwater virioplankton remain much less studied. Nevertheless, a survey of published freshwater studies demonstrates that virioplankton recycle important quantities of growth‐limiting nutrients from hosts via generation of dead particulate and dissolved organic matter during cell lysis, and suggests that both the chemical speciation and concentration of these organic compounds and nutrients may have important influences on the microbial community.3. Parallel observations on the spatial patterns and dynamics of microbial mortality due to viruses or grazing are more advanced in freshwaters than in marine environments. However, the constraints that determine whether virus‐ or grazer‐mediated mortality dominates are not yet understood in either environment.4. Application of molecular approaches has facilitated the examination of the diversity and ecological dynamics of specific viral populations and entire communities. The depth of detail achieved in marine environments towards characterizing these populations and communities is just beginning to be matched in freshwater systems. The few available data suggest that viruses targeting‐related hosts in freshwater and marine systems may be genetically distinct.5. Although the role of viruses in aquatic systems is complex and remains insufficiently studied, our survey of the literature indicates that, despite some differences, many of the controls on virioplankton activity and diversity are similar in marine and freshwater environments.

Protozooplankton characterization of two contrasting sites in a tropical coastal ecosystem (Guanabara Bay, RJ)

Much time and resources have been invested in understanding plankton dynamics in Guanabara Bay (Brazil), but no attention has been devoted to the protozooplankton. To fulfill this lacuna, abundance and composition of protozooplankton were investigated from January to December - 2000 in fortnightly surface water samplings at two distinct water quality sites (Urca - closer to the bay entrance, more saline and cleaner waters; Ramos - inner reaches, hypereutrophic waters). The density at Urca (10³ - 10(5) cell.l-1) was one to three orders of magnitude lower than at Ramos (10(4) - 10(5) cell.l-1). A seasonal trend for nanoplankton and protozooplankton was more evident at Urca, but both sites had lower densities during the colder period. Small heterotrophic dinoflagellates (20-30 mm) were dominant in over 50% of the samples. The protozooplankton abundance and composition reflected the distinct trophic conditions states found at the bay. During the wet-warm season, non-oligotrich ciliates were representative of Ramos site with Gymnodiniaceae dinoflagellates, while tintinnids and heterotrophic dinoflagellates were predominantly found at Urca mainly during the dry-cold season. This first descriptive study towards the understanding of the intricate relationships among the microbial food web components reveals that protozooplankton can be a good indicator of water quality conditions at the bay.

Relating planktonic microbial food web structure in lentic freshwater ecosystems to water quality and land use

Relationships among picoplankton, protozoa, phytoplankton, plant nutrients, lake type, drainage basin morphology and land cover were studied in 45 water bodies in South Island, New Zealand that ranged from large, deep, ultra-oligotrophic lakes to shallow, macrophyte-dominated ponds and swamps. The biomasses of most heterotrophic components of the pelagic microbial food webs were positively related to phytoplankton and features of the drainage basin that enhanced nutrient input, and imply strong resource-driven structuring of pelagic microbial food webs. Prokaryotic picophytoplankton biomass was negatively related to indices of eutrophication, and the picoautotroph contribution to total microbial food web biomass declined with increasing total phosphorus concentration from 16.5% in deep lakes to 70:30 in deep lakes, and indicate the potential importance of microbial food webs in carbon transfer to higher trophic levels in deep, less productive lakes. Strong relationships exist between land use in the catchment and pelagic microbial food web structure and biomass across a wide range in size and trophic state of water bodies in heterogeneous landscapes.

The role of ciliates within the microbial food web in the eutrophicated part of Kaštela Bay (Middle Adriatic Sea)

Interactions among phytoplankton, bacterioplankton, heterotrophic nanoflagellates (HNF), ciliated protozoa and copepod nauplii were studied in the eutrophicated part of Kas?tela Bay from May 1998 to November 1999. Special emphasis was placed on relationships between size categories of nonloricate ciliates (NLC) and other microbial food web components. Biomasses of phytoplankton and bacteria were primarily influenced by abiotic parameters. Temperature indirectly controlled variation in HNF biomass through the changes in biomass of bacteria and the smaller phytoplankton fraction. Besides HNF, bacterial biomass was affected by the NLC 3 (Cell Length 3 , CL 104 µm 3 (CL >40 µm) had a strong mutual correlation and they seemed to be controlling the microphytoplankton fraction. During the colder part of the year, HNF abundance was regulated by ciliate grazing. The high impact of ciliates in summer 1998 could have been influenced by the taxonomic composition of the phytoplankton community changing, as well as the higher eutrophication level in the study area. Predation by copepod nauplii on ciliates and carbon transfer to higher trophic levels appear to be of relative importance only in the period when they are most abundant. This paper outlines the dominant relationships within the microbial food web and suggests that a significant amount of bacterial production, phytoplankton and HNF biomass could be transferred to higher trophic levels through the microbial food web.

A cautionary note: Examples of possible microbial community dynamics in dilution grazing experiments

Dilution experiments are used commonly to provide estimates of grazing pressure exerted on phytoplankton and bacterioplankton as well as estimate their growth rates. However, very little attention has been given to the dynamics of grazers, especially heterotrophic nanoflagellates (HNF), in such experiments. We found temporal changes in concentrations of ciliates and HNF in a dilution experiment using water from the oligotrophic N.W. Mediterranean Sea. Ciliates decreased markedly over 24 h when held in seawater diluted with particle-free water (60% and 20% final conc whole seawater) while HNF increased in concentration in the same treatments. Using a time-course approach in a second experiment, we monitored changes in HNF and bacterioplankton concentrations in 20% whole seawater (80% particle-free seawater). Both HNF and heterotrophic bacteria displayed stable concentrations for the first 12 h and then grew rapidly, especially HNF, from 12 to 24 h. Examination of bacterial community composition using denaturing gel gradient electrophoresis (DGGE) showed a change in community composition over the 24 h incubation period. Dilution can have differential effects on the distinct components of the marine microbial food web.

Plankton Food Web Responses to Experimental Nutrient Additions in a Subtropical Lake

During two controlled enclosure experiments using water from a subtropical lake, the plankton food web displayed a highly variable response to combined addition of nitrogen and phosphorus. In July, the nutrients stimulated growth of Cylindrospermopsis raciborskii, and the biomass of macrozooplankton and microbial food web components did not increase. In October, the same addition of nutrients stimulated growth of small edible Lyngbya spp., and there were coincident increases in biomass of macrozooplankton and components of the microbial web. Past generalizations that cyanobacteria blooms inhibit growth of other food web components may not always hold true.

Phage community dynamics in hot springs.

In extreme thermal environments such as hot springs, phages are the only known microbial predators. Here we present the first study of prokaryotic and phage community dynamics in these environments. Phages were abundant in hot springs, reaching concentrations of a million viruses per milliliter. Hot spring phage particles were resistant to shifts to lower temperatures, possibly facilitating DNA transfer out of these extreme environments. The phages were actively produced, with a population turnover time of 1 to 2 days. Phage-mediated microbial mortality was significant, making phage lysis an important component of hot spring microbial food webs. Together, these results show that phages exert an important influence on microbial community structure and energy flow in extreme thermal environments.