Ciliate Taxa Research Articles

Structure and dynamics of the protoplankton community in an environmentally protected urban stream.

Samples of water for analysis of biological communities (testate amoebae, ciliates and bacteria) and limnological variables were taken in the Mandacaru stream located in the Conservation Unit of Parque do Cinquentenário, in the city of Maringá, Paraná State, Brazil, in two hydrological periods (dry and rainy). We calculated the Water Quality Index (WQI) composed by nine parameters of water quality (dissolved oxygen, thermotolerant coliforms, pH, BOD, water temperature, total nitrogen, total phosphorous, turbidity, and total solids), Trophic State Index (TSI) for phosphorous and we used a Redundancy Analysis (RDA) to verify the influence of environmental variables in the protozoan community. The WQI showed that water quality was considered good in some points and considered bad in other points. The TSI for phosphorus classified the stream as mesotrophic in the majority of sampled sites points (mean between 53.09 and 58.35). We identified 19 taxa of testate amoebae, belonging to six families, being Difflugiidae, Centropyxidae, and Arcellidae those with more species and 71 infrageneric taxa of ciliates, distributed in 12 orders being Peniculida the most representative order, followed by Euplotida. According to RDA analysis, samples of the dry period were characterized by higher values of bacteria density and concentrations of chlorophyll-a, total phosphorous, and total nitrogen. Ciliates and testate amoebae presented higher abundance values in some of the months characterized by higher precipitation and in conditions of higher system productivity. we may conclude that the Mandacaru stream, although strongly influenced by anthropic action, still presents an acceptable water quality. Lastly, we emphasize that protists abundance was strongly influenced by system productivity. This was evidenced by elevated protozoan densities where there was higher primary and bacterial productivity. Thus, these organisms must be considered in studies that aim at the identification of organisms that may indicate anthropic impacts and environmental quality.

Ciliates as model organisms for the ecotoxicological risk assessment of heavy metals: A meta-analysis.

Ciliates are key components of aquatic ecosystems, significantly contributing to the decomposition of organic matter and energy transfer to higher trophic levels. They are considered good biological indicators of chemical pollution and relatively sensitive to heavy metal contamination. In this study, we performed a meta-analysis of the available toxicity data of heavy metals and ciliates to assess: (1) the sensitivity of freshwater ciliates to different heavy metals, (2) the relative sensitivity of ciliates in comparison to the standard test species used in ecotoxicological risk assessment, and (3) the difference in sensitivity across ciliate taxa. Our study shows that the tolerance of ciliates to heavy metals varies notably, which is partly influenced by differences in methodological conditions across studies. Ciliates are, in general, sensitive to Mercury>Cadmium>Copper>Zinc>Lead>Chromium. Also, this study shows that most ciliates are more tolerant to heavy metal pollution than the standard test species used in ecotoxicological risk assessments, i.e., Raphidocelis subcapitata, Daphnia magna, and Onchornyncus mykiss. Threshold concentrations derived from toxicity data for these species is expected to confer sufficient protection for the vast majority of ciliate species. Our data analysis also shows that the most commonly tested ciliate species, Paramecium caudatum and Tetrahymena thermophila, are not necessarily the most sensitive ones to heavy metal pollution. Finally, this study stresses the importance of developing standard toxicity test protocols for ciliates, which could lead to a better comprehension of the toxicological impact of heavy metals and other contaminants to ciliate species.

Delimitation of five astome ciliate species isolated from the digestive tube of three ecologically different groups of lumbricid earthworms, using the internal transcribed spacer region and the hypervariable D1/D2 region of the 28S rRNA gene

BackgroundVarious ecological groups of earthworms very likely constitute sharply isolated niches that might permit speciation of their symbiotic ciliates, even though no distinct morphological features appear to be recognizable among ciliates originating from different host groups. The nuclear highly variable ITS1–5.8S-ITS2 region and the hypervariable D1/D2 region of the 28S rRNA gene have proven to be useful tools for the delimitation of species boundaries in closely related free-living ciliate taxa. In the present study, the power of these molecular markers as well as of the secondary structure of the ITS2 molecule were tested for the first time in order to discriminate the species of endosymbiotic ciliates that were isolated from the gastrointestinal tract of three ecologically different groups of lumbricid earthworms.ResultsNineteen new ITS1–5.8S-ITS2 region and D1/D2-28S rRNA gene sequences were obtained from five astome species (Anoplophrya lumbrici, A. vulgaris, Metaradiophrya lumbrici, M. varians, and Subanoplophrya nodulata comb. n.), which were living in the digestive tube of three ecological groups of earthworms. Phylogenetic analyses of the rRNA locus and secondary structure analyses of the ITS2 molecule robustly resolved their phylogenetic relationships and supported the distinctness of all five species, although previous multivariate morphometric analyses were not able to separate congeners in the genera Anoplophrya and Metaradiophrya. The occurrence of all five taxa, as delimited by molecular analyses, was perfectly correlated with the ecological groups of their host earthworms.ConclusionsThe present study indicates that morphology-based taxonomy of astome ciliates needs to be tested in the light of molecular and ecological data as well. The use of morphological identification alone is likely to miss species that are well delimited based on molecular markers and ecological traits and can lead to the underestimation of diversity and overestimation of host range. An integrative approach along with distinctly increased taxon sampling would be helpful to assess the consistency of the eco-evolutionary trend in astome ciliates.

Ciliate species from tank-less bromeliads in a dry tropical forest and their geographical distribution in the Neotropics.

The study of ciliate diversity in tropical environments remains scarce. In Neotropical forests, bromeliads are a common component of the vascular flora; bromeliads with tank morphology intercept rain water and detritus, resulting in the formation of a phytotelm, where heterotrophic protist communities like ciliates can establish. However, it is not known if ciliates inhabit tank-less bromeliads. For this reason, the goal of the present study was to investigate if ciliates can establish between the leaf axils of five terrestrial and epiphytic tank-less bromeliad species in a dry tropical forest in west Mexico. We collected samples of rain water and detritus from the leaf axils during the humid season of years 2015 and 2016. For ciliate taxonomical identification, we used optical microscopy, in vivo observation, and silver impregnation techniques. To summarize information about geographical distribution of ciliates identified at species level in the Neotropics, we provided their records from previous works, at country level with locality and georeferenciation. We recorded 27 taxa of ciliates, where the class Oligohymenophorea contained the largest richness of taxa. Drepanomonas revoluta, Leptopharynx bromeliophilus, and Tetrahymena sp. were recorded from all the species of bromeliads. Bromelia karatas was the species that hosted the largest number of ciliate taxa (22). Our results indicated that Glaucomides bromelicola, Gonostomum bromelicola, Leptopharynx bromelicola and L. bromeliophilus, species which are considered endemic to tank bromeliads, can also inhabit tank-less bromeliads. We provided previous records of 19 ciliate species from eleven countries within the Neotropical region, and Bromeliothrix metopoides was the species most frequently recorded in Neotropical countries (9). Therefore, tank-less bromeliads can constitute a temporal habitat for ciliates, and function as cysts reservoirs in environments with a pronounced seasonality like dry tropical forests.

Molecular phylogeny and comparative morphology indicate that odontostomatids (Alveolata, Ciliophora) form a distinct class-level taxon related to Armophorea

The odontostomatids are among the least studied ciliates, possibly due to their small sizes, restriction to anaerobic environments and difficulty in culturing. Consequently, their phylogenetic affinities to other ciliate taxa are still poorly understood. In the present study, we analyzed newly obtained ribosomal gene sequences of the odontostomatids Discomorphella pedroeneasi and Saprodinium dentatum, together with sequences from the literature, including Epalxella antiquorum and a large assemblage of ciliate sequences representing the major recognized classes. The results show that D. pedroeneasi and S. dentatum form a deep-diverging branch related to metopid and clevelandellid armophoreans, corroborating the old literature. However E. antiquorum clustered with the morphologically discrepant plagiopylids, indicating that either the complex odontostomatid body architecture evolved convergently, or the positioning of E. antiquorum as a plagiopylid is artifactual. A new ciliate class, Odontostomatea n. cl., is proposed based on molecular analyses and comparative morphology of odontostomatids with related taxa.

Ciliate diversity and distribution patterns in the sediments of a seamount and adjacent abyssal plains in the tropical Western Pacific Ocean

BackgroundBenthic ciliates and the environmental factors shaping their distribution are far from being completely understood. Likewise, deep-sea systems are amongst the least understood ecosystems on Earth. In this study, using high-throughput DNA sequencing, we investigated the diversity and community composition of benthic ciliates in different sediment layers of a seamount and an adjacent abyssal plain in the tropical Western Pacific Ocean with water depths ranging between 813 m and 4566 m. Statistical analyses were used to assess shifts in ciliate communities across vertical sediment gradients and water depth.ResultsNine out of 12 ciliate classes were detected in the different sediment samples, with Litostomatea accounting for the most diverse group, followed by Plagiopylea and Oligohymenophorea. The novelty of ciliate genetic diversity was extremely high, with a mean similarity of 93.25% to previously described sequences. On a sediment depth gradient, ciliate community structure was more similar within the upper sediment layers (0-1 and 9-10 cm) compared to the lower sediment layers (19-20 and 29-30 cm) at each site. Some unknown ciliate taxa which were absent from the surface sediments were found in deeper sediments layers. On a water depth gradient, the proportion of unique OTUs was between 42.2% and 54.3%, and that of OTUs shared by all sites around 14%. However, alpha diversity of the different ciliate communities was relatively stable in the surface layers along the water depth gradient, and about 78% of the ciliate OTUs retrieved from the surface layer of the shallowest site were shared with the surface layers of sites deeper than 3800 m. Correlation analyses did not reveal any significant effects of measured environmental factors on ciliate community composition and structure.ConclusionsWe revealed an obvious variation in ciliate community along a sediment depth gradient in the seamount and the adjacent abyssal plain and showed that water depth is a less important factor shaping ciliate distribution in deep-sea sediments unlike observed for benthic ciliates in shallow seafloors. Additionally, an extremely high genetic novelty of ciliate diversity was found in these habitats, which points to a hot spot for the discovery of new ciliate species.

Further consideration on the phylogeny of the Ciliophora: Analyses using both mitochondrial and nuclear data with focus on the extremely confused class Phyllopharyngea

Most ciliate phylogenetic analyses have largely relied on the nuclear small subunit ribosome DNA (nSSU-rDNA) locus. However, single locus or multi-loci from the same genome or chromosome may not be sufficient enough to elucidate phylogenetic relationships among ciliate taxa. Therefore, in addition to nSSU-rDNA, the mitochondrial small subunit ribosome DNA (mtSSU-rDNA) was applied in this study. We expanded the taxon sampling especially within the class Phyllopharyngea. Phylogenetic analyses based on nSSU-rDNA and mtSSU-rDNA, independently, as well as concatenated were performed and revealed the following: (1) mtSSU-rDNA is more variable than nSSU-rDNA, and is better at elucidating relationships at lower levels, e.g. intra-/inter-specific or generic relationships; (2) the validity of the two genera Mirodysteria and Spirodysteria is challenged based on their similar morphology with Dysteria and the analyses from both mtSSU-rDNA and nSSU-rDNA; (3) Brooklynella is confirmed to be an intermediate taxon between Dysteriidae and Hartmannulidae, and may represent a distinct family; (4) Trithigmostoma should remain in Chilodonellidae; (5) the separation of Paraspathidium from Litostomatea is supported and it groups with prostomateans and plagiopyleans. In summary, results from mtSSU-rDNA corroborated those of nSSU-rDNA for highly supported clades, and the mtSSU-rDNA tree with its secondary structure gave topologies that could be explained by the morphology; therefore it can be useful in some cases towards better resolution of robust phylogenies.

Space-time variation of ciliates related to environmental factors in 15 nearshore stations of the Gulf of Gabes

Diversity and structure of ciliate communities in the Gulf of Gabes (Tunisia) were investigated based on a survey of 15 nearshore stations along 237 Km, by monthly sampling over a 1-year. Ciliated protozoa were identified to genus and/or species level and enumerated. Statistic tools were used to explain the ciliates assemblage. High ciliates species richness from 133 taxa was recorded, including new records of 76 species. This study showed a longitudinal distribution of ciliate communities, which are organized in northern stations (from Tabia to Harbor of Gabes) and southern stations (from Zarrat to Jabiat Haj Ali). The number of taxa increased significantly in northern stations but decreased in the southern. This distribution was mainly influenced by the salinity and phytoplankton abundance. Ciliate taxa were grouped into fives size-classes: 15-30 µm, 30-50 µm, 50-100 µm, 100-200 µm and >200 µm. In terms of abundance, most abundant size groups were small ciliates (15-30 μm) accounted from 15 to 79 %, while the greatest biomass contribution came from the 50-100 μm size classes. We thus conclude high diversity of ciliates communities that showed a geographical distribution influenced by abiotic and biotic factors along the coast of Gulf of Gabes.

Ciliates along oxyclines of permanently stratified marine water columns.

Studies of microbial communities in areas of the world where permanent marine water column oxyclines exist suggest they are "hotspots" of microbial activity, and that these water features and the anoxic waters below them are inhabited by diverse protist taxa, including ciliates. These communities have minimal taxonomic overlap with those in overlying oxic water columns. Some ciliate taxa have been detected in multiple locations where these stable water column oxyclines exist; however, differences in such factors as hydrochemistry in the habitats that have been studied suggest local selection for distinct communities. We compare published data on ciliate communities from studies of deep marine water column oxyclines in Caricao Basin, Venezuela, and the Black Sea, with data from coastal, shallower oxycline waters in Framvaren and Mariager fjords, and from several deep-sea hypersaline anoxic basins in the Eastern Mediterranean Sea. Putative symbioses between Bacteria, Archaea, and ciliates observed along these oxyclines suggests a strategy of cooperative metabolism for survival that includes chemosynthetic autotrophy and exchanges of metabolic intermediates or end products between hosts and their prokaryotic partners.

Focusing on Genera to Improve Species Identification: Revised Systematics of the Ciliate Spirostomum

Although many papers dealing with the description of new ciliate taxa are published each year, species taxonomy and identification in most groups of the phylum Ciliophora remain confused. This is largely due to a scarcity of surveys on the systematics of immediately higher levels (genera and families) providing data for old and new species together. Spirostomum is a common and distinctive inhabitant of fresh- and brackish water environments, including artificial and eutrophic ones, and is a good model for applied ecology and symbiosis research. Despite this, only 3 of the numerous species are commonly cited, and no studies have yet confirmed their monophyly, with the consequence that reproducibility of the results may be flawed. In this paper we present morphological and molecular data for 30 Spirostomum populations representing 6 different morphospecies, some of which were collected in previously unreported countries. We performed a detailed revision of Spirostomum systematics combining literature surveys, new data on hundreds of organisms and statistical and phylogenetic analyses; our results provide insights on the evolution, ecology and distribution of known morphospecies and a novel one: Spirostomum subtilis sp. n. We also offer tools for quick species identification.

Temporal structure of the peritrich ciliate assemblage in a large Neotropical lake

Abstract Background Periphytic communities are usually composed by prokaryotic and eukaryotic microbes and small metazoans and could be found in any submerged surface in aquatic environments. Ciliates generally are the dominant organisms in periphytic communities where they can form assemblages of complex taxonomic composition. Among these ciliate taxa, peritrichs are very common organisms found in periphyton; also, they are easy to collect, be easily recognized, and have been widely used to evaluate and monitor ecological and ecotoxicological investigations. Several studies have been focused on periphytic communities in freshwaters from the Northern Hemisphere, with very little data on similar environments in the South. In the present study, we analyzed the structure and temporal dynamics of the ciliate peritrich community in a Neotropical shallow lake, comparing the fluctuation of the peritrich community with environmental factors. Results Peritrichia comprised a total of 22 morphospecies throughout the year with genera Epistylis and Vorticella the most diverse and abundant genera. Peritrich density was considerably higher during fall and winter, demonstrating a clear seasonal cycle. Small, solitary species showed no pattern of dominance during any particular stage of succession, reaching abundance peak any time during the sampling period. On the other hand, large colonial species were abundant only in the last half of each successional cycle. Species abundance was correlated to temporal and environmental drivers. Conclusion Our results support the hypothesis of a temporal pattern of succession in the community of peritrich ciliates that composes the periphyton of the studied lake with different responses by individual species to successional time, year, season, and environmental factors.

The distribution of ciliates on Ecology Glacier (King George Island, Antarctica): relationships between species assemblages and environmental parameters

Ciliates are important consumers of pico- and nano-sized producers, are nutrient regenerators, and are an important food source for metazoans. To date, ecological research on ciliates has focused on marine ecosystems rather than on glacier habitats. This paper presents the first major study on ciliates from the Ecology Glacier (South Shetland Islands, Antarctica). The objective of the study was to investigate the structure and spatial distribution of ciliate communities and to identify the environmental factors determining the structure of the assemblages. Microbial communities were collected from three habitats: surface snow, cryoconite holes, and glacier streams. Sam- pling was carried out every 3-4 days from January 17 to February 24, 2012. A total of 18 ciliate taxa were identi- fied. The species richness, abundance, and biomass of protozoa differed significantly between the stations studied with the lowest numbers in streams on the glacier surface and the highest numbers in cryoconite holes. The RDA performed to specify the direct relationships between the abundance of ciliate taxa and environmental variables showed obvious differences between studied habitats. The analysis showed that all variables together explained 62.4 % of total variance. However, variables that significantly explained the variance in ciliate communities in cryoconite holes, snow, and surface streams were tem- perature, conductivity, and total nitrogen. Further research is required to explain the impact of biotic factors influ- encing the presence of ciliates, including the abundance of bacteria, microalgae, and small Metazoa.

Diversity and seasonality in structure of ciliate communities in the Neva Estuary (Baltic Sea)

Diversity and structure of ciliate communities in the Neva Estuary (Baltic Sea) were studied at two near-shore stations from October 2007 to January 2009. Ciliates from 111 taxa were detected, including new records of 24 species for the Baltic Sea. The ciliate taxa were grouped into size-categories and trophic guilds and their species composition, abundance and biomass were monitored in different seasons. Two distinct associations replaced each other during seasonal succession at water temperatures 5–128C. During the warm season (late April–October) ciliate communities were composed essentially of picoand nano-filterers (mostly algivorous) that became less important in the cold season (October–early April). The predator, Monodinium balbiani, and bactivorous peritrichs were characteristic of the warm season, while in the cold season, other predatory (Lacrymaria coronata group) and bactivorous (Cyclidium spp., Aspidisca sp., Chilodonella sp.) ciliates occurred. The most abundant size groups were small ciliates (20–30 mm) and nanociliates (,20 mm). A proportion of large ciliates (.60 mm) increased in the cold season due to the appearance of benthic species in the plankton. Total ciliate abundance and biomass ranged 0.12–10.3 10 ind L and 0.3– 53.3 mg C L, respectively. An unusual winter peak of the ciliate Trithigmostoma sp. was observed (32.8 mg C L) although generally the overall ciliate numbers decreased in the cold season.

Ciliate plankton dynamics and survey of ciliate diversity in the Salton Sea, California, 1997–1999

Planktonic ciliates and other protozoa were monitored at mid-lake stations in the saline, polymictic Salton Sea during the period 1997–1999, at approximately two-week intervals. Additionally, in 1999, a survey of ciliate diversity in a variety of microhabitats was undertaken. Ciliates generally comprised < 20 percent of the total zooplankton biovolume, with copepods, rotifers and larvae of a barnacle and polychaete worm making up the rest. However, in early 1999 tintinnids constituted ~40 percent of total zooplankton biovolume, and in September 1998 when metazooplankters were very scarce, ciliates represented nearly 100 percent. An anaerobic ciliate, Sonderia sp., invaded the mid-water column during periods of anoxia and high sulfide levels in 1998 and 1999. Large ciliates, such as Condylostoma spp. and Favella sp. increased in abundance over the three-year period while the smaller forms, mostly scuticociliates, did not. This pattern may be due to a decrease during our study in abundance of the filter-feeding hybrid tilapia (Oreochromis mossambicus x O. urolepis honorum), which may have selectively grazed upon the larger forms. An inverse relationship between copepod abundance and large ciliate abundance suggests copepods also prey on the larger forms. A total of 143 ciliate taxa were found as well as protozoans in other groups such as heliozoans and choanoflagellates.

Size spectra and abundance of planktonic ciliates within various habitats in a macrophyte-dominated lake (Eastern Poland)

Body size, community structure, abundance and biomass of ciliates were compared in various stands of macrophytes in a macrophyte-abundant shallow lake in Eastern Poland. Samples were collected in belts of Phragmites, Typha, Ceratophyllum, Elodea, Stratiotes and Chara. Additionally, protozooplankton was collected from the open water zone surrounding the vegetation belts. Differences in numbers of ciliate taxa between micro-sites were statistically significant. The highest numbers were found in Chara and Ceratophyllum stands, lower numbers in Stratiotes and Elodea stands and the lowest in the open water, Phragmites and Typha areas. Ciliate biomass was, like density, significantly higher in submerged macrophytes than in emergent macrophytes and open water zones. Based on differences in macrophyte structure, two groups of habitats with similar patterns of size-related ciliate distribution were distinguished. The first group consisted of two vegetated zones of sparse stem structure (Phragmites and Typha) and the open water zone, the second group comprised submerged macrophyte species, which were more dense and complex. Generally, the abundance of ciliates correlated positively with total suspension solid (TSS) and total organic carbon (TOC) concentrations. In the Chara and Ceratophyllum stands, relations between ciliate numbers, TSS and TOC were stronger.

Evolution and implications of genome rearrangements in ciliates

We are exploring genome evolution in diverse ciliates to assess whether patterns of protein evolution correspond to the presence of genome duality, and to levels of genome rearrangements among ciliates. Ciliate genomes contain an estimated 25,000–35,000 genes, a number higher than that of other characterized microbial eukaryotes and on a similar scale to that of humans and Arabidopsis thaliana. We have previously hypothesized that the divergence among ciliate proteins, and the accumulation of paralogs within ciliate taxa, is due to differential selection on the functional macronucleus and the transcriptionally inactive micronucleus. To assess this hypothesis, we are expanding our analyses of protein‐coding gene sequences to compare the fate of proteins in ciliates to that of other eukaryotes, and to test whether the most divergent ciliate proteins are found in ciliates with extensively processed genomes. These analyses, combined with a phylogenetic perspective that reveals multiple origins of extensive fragmentation in ciliates, provide a possible explanation for the large genome size and diversity of proteins in ciliates.

Seasonal changes in the abundance of autotrophic picoplankton and some environmental factors in hypereutrophic Furuike Pond

Seasonal change in the abundance of autotrophic picoplankton (APP) was investigated once or twice a week in relation to some environmental variables in a hypereutrophic pond, from July 1999 to June 2000. Cell density of APP ranged between 0.3×105 and 10.1×105 cells ml−1, overlapping the lower range of APP abundances given in the literature for hypereutrophic systems. The pattern of seasonal change in concentration of dissolved inorganic phosphorus (0.3–20.3 μ mol P l−1) was similar to that of cell density of APP, suggesting that phosphorus limitation on APP abundance. By contrast, nitrogen limitation seemed unlikely since the pattern of seasonal change in concentration of dissolved inorganic nitrogen was different from that of APP cell density. We could not find any coupled oscillations between APP abundance and heterotrophic nanoflagellates, or between that of APP and ciliates. The dominant ciliate taxa, based on their cell densities, were Cinetochilum margaritaceum, Cyclidium glaucoma, Halteria grandinella, Strobilidium sp. and Urotricha spp. The relative contribution of the < 2 μm fraction to total chlorophyll concentration was seasonally high (up to 16.2%), indicating seasonal importance of APP abundance as food for heterotrophs.

Proliferation of cryptic protists and germination of resting stages from untreated sediment samples with emphasis on dinoflagellates

Abstract Incubation of untreated sediment samples from the northern part of the Swedish west coast resulted in a conspicuous proliferation of various protists. Samples from ten coastal sites were incubated in filtered seawater and the resulting vegetative stages were recorded for a period of two weeks. At least 47 different dinoflagellate taxa were encountered as vegetative stages, more than 46 different ciliate taxa and at least 64 living diatom taxa were present. There were also cyanobacteria, haptophytes, cryptophytes, euglenophytes, prasinophytes, chlorophytes and amoebae. As total 263 taxa were identified from less than 100 cm3 sediment, suggesting that most microorganisms are ubiquitous, present almost everywhere within their geographic region, but often rare or cryptic. All dinoflagellates present in the area that are known to be cyst-producing were found, corresponding to 25% of the known dinoflagellates at the Swedish west coast. Approximately 26% of the planktonic and 22% of the pennate diatoms ...

Impact of microzooplankton on the progression and fate of the spring bloom in fjords of northern Norway

The objective of the study was to determine the role of microzooplankton in the fate of primary production and progression of the spring phytoplankton bloom in high latitude fjords. The 3 fjords, Balsfjord, Malangen and Ullsfjord (Norway), varied in oceanic influence and in the rate of development of the spring bloom. The abundance of Phaeocystis pouchetii was relatively low in the spring of 1997 compared to previous years, and diatoms dominated the biomass of the phytoplankton assemblage in all 3 fjords. The mean biomass of microzooplankton in the top 20 m averaged 72, 66 and 80 mg C m -3 and values integrated to 170 m averaged 4560, 4450 and 6820 mg C m -2 in Balsfjord, Malangen and Ullsfjord, respectively. The composition of the microzooplankton was consistent among the fjords and over time, with the proportion of biomass split evenly between nanoflagellates, dinoflagellates and ciliates. Grazing rates of the microzooplankton community were measured with the dilution technique. The impact of microzooplankton grazing was similar among the fjords, accounting for on average 68, 63 and 55% of the production of the < 200 µm phytoplankton in Bals- fjord, Malangen and Ullsfjord, respectively. When integrated to 20 m, based on a microzooplankton biomass-specific ingestion rate, microzooplankton grazing accounted for an estimated 12 to 26% of the gross primary production. However, this does not account for the carbon requirements of a sub- stantial proportion of the microzooplankton that occurred below 20 m. Clearance rates by ciliates of nanophytoplankton cells of a similar size to the single cells of P. pouchetii were determined from the uptake rates of fluorescently labelled algae (FLA). Generally, the taxa of ciliates that were found to ingest FLA accounted for ♢ 50% of the abundance of the ciliate population in each fjord. Taxon-spe- cific ciliate clearance rates of FLA in surface waters varied with ciliate size from 5.6 to 1.3 µl cell -1 h -1 . The FLA-consuming ciliate population cleared a total of 27 〈 10 3 to 141 〈 10 3 µl l -1 d -1 in surface waters. The ingestion rates of the FLA-consuming ciliates were equivalent to between 11 and 29% of the total microzooplankton consumption. It is possible that the grazing pressure exerted by the micro- zooplankton on single cells and small colonies was high enough to decrease the overall competitive- ness of the P. pouchetii populations and contribute to their low abundance.

Ecological role and bacterial grazing of Halteria spp.: small freshwater oligotrichs as dominant pelagic ciliate bacterivores

We conducted extensive studies on bacterivory and bacterial production over several seasons in 2 reservoirs: the meso-eutrophic R ˇ imov Reservoir in the Czech Republic and the highly eu- trophic Sau Reservoir in Spain. Based on abundance, seasonal dynamics, and cell-specific uptake rates of different ciliate taxa, as well as heterotrophic nanoflagellate bacterivory, we were able to quantify bacterivory by individual ciliate species, total ciliates, and aggregated protists in these sys- tems. With increasing trophic status, a higher portion of bacterial production was consumed by pro- tists, and there was a greater importance of ciliate grazing, accounting for 40 and 50% of the total protistan bacterivory in the epilimnion of the R ˇ imov and Sau reservoirs, respectively. Increases were attributable to the oligotrichs of the genus Halteria that often numerically dominate freshwater pelagic ciliate communities. In both reservoirs, the most important ciliate bacterivores in order of im- portance were: oligotrichs, primarily the bacterivorous Halteria spp., peritrichs, and scuticociliates. We also examined food vacuole content in natural populations of Halteria spp. to estimate the propor- tion of cells that had ingested algae. Our results and a review of previous reports on the abundance of Halteria spp. suggest that small halteriids are ecologically important bacterial consumers in meso- to eutrophic freshwater systems due to: (1) efficient uptake of prey over a large size spectrum (approxi- mately 0.4 to 5 µm), (2) high clearance rates on picoplankton-sized particles along with (3) high potential growth rate, and (4) lower vulnerability to metazooplankton predation compared to other common pelagic ciliates. Correspondingly, we suggest a revised concept of planktonic ciliate bacteri- vory, where the principal role is attributed to small omnivorous filter-feeding oligotrichous ciliates.