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Application of plant hormones is an effective strategy to further improve the accumulation of microalgae biomass. In this study, the IAA (indole-3-acetic acid) production capacity of symbiotic bacteria isolated from the culture system of Ochromonas and Chlorella was detected and compared, with the bacterial community structure revealed. The results demonstrated that Algoriphagus, Porphyrobacter, Roseococcus, and Rhizobium were the dominant bacterial genera both in Ochromonas and Chlorella system, distributed in Proteobacteria and Bacteroidetes. Sixteen of 40 strains classified into 15 genera isolated from the two genera of microalgae could produce IAA, with Agrobacterium and Rhizobium as the main representatives with highest IAA production level (22.53–34.18 mg/L in 30 h). And through determination of changes of chlorophyll content in the co-culture system of microalgae and the supernatant of IAA high-yielding bacteria suspension from medium with or without tryptophan (synthetic precursor of IAA), Rhizobium (C-3) and Agrobacterium (O-16, C-2) were proved to promote the growth of Ochromonas and Chlorella through secreting IAA. The biomass yield of Ochromonas and Chlorella could be respectively increased by 78.98% and 72.73% through the co-culture with Agrobacterium (C-2), by 68.15% and 69.93% for Rhizobium (C-3) and by 80.89% and 65.73% for Agrobacterium (O-16). Moreover, compared with the application of exogenous IAA to improve the microalgae biomass, it is more efficient and economical to construct algae bacteria symbiosis system using IAA secretory bacteria commonly existed in microalgae culture system. In addition, the growth promoting effect of IAA secretory bacteria on microalgae was different among the species of algae.

Ochromonas triangulata, the type species for a genus with over 125 taxa, was collected for only the second time, again from the type locality. Cell morphology, cell division, palmelloid stage and cyst structure generally agreed with the original description. Molecular phylogenetic analysis based on the 18S rRNA gene revealed 13 clades of Ochromonas-like flagellates as well as the clade represented by our O. triangulata strain and the nearly identical strain RCC-21/AC025. We also conducted a concatenated analysis using the 18S rRNA and the rbcL genes, and we recovered the same 14 clades. One clade, containing strains CCAP 933/27 and CCMP1861, previously named Ochromonas tuberculata, was re-identified as Chrysastrella paradoxa and Chrysastrella breviappendiculata, respectively. One clade included the Poterioochromonas strains but we were unable to convincingly connect species names to the strains because authentic strains were unknown or not examined. Organisms in the clade that included the well-...

Ochromonas sp. (Chrysophyceae) occupies different trophic levels depending on its prevalent nutritional strategy in the field. Ochromonas sp. can utilize inorganic and organic carbon to grow photoautotrophically, mixotrophically, and heterotrophically. To examine how exogenous organic carbon influences population growth and photosynthesis in Ochromonas sp., we cultured Ochromonas sp. in media with different glucose concentrations (0, 50, 100, and 150 mg L−1) and two treatment modes (single addition of glucose at the beginning and fed-batch mode with repeated addition every 12 h) for 15 days. Results showed that (1) compared to photoautotrophic growth, adding glucose significantly enhanced the growth rate and carrying capacity of Ochromonas sp. in a concentration-dependent manner; (2) photosynthesis also increased with glucose addition during the exponential growth stage; (3) repeated glucose addition caused higher biomass and photosynthetic performances than single addition, and (4) with depletion of glucose and the population reaching a stationary stage, photosynthetic parameters decreased slightly but remained at a relatively stable level, which was similar to that of photoautotrophic growth. We conclude that in comparison with pure photoautotrophy, mixotrophic growth can result in higher population growth rate and final biomass in a shorter time, especially in fed-batch cultures, indicating that this would be a highly efficient way for mass cultivation of the flagellate Ochromonas.

The Deepwater Horizon oil spill released millions of barrels of crude oil into the Gulf of Mexico, and saw widespread use of the chemical dispersant Corexit. We assessed the role of traits, such as cell size, cell wall, motility, and mixotrophy on the growth and photosynthetic response of 15 phytoplankton taxa to oil and Corexit. We collected growth and photosynthetic data on five algal cultures. These responses could be separated into resistant (Tetraselmis astigmatica, Ochromonas sp., Heterocapsa pygmaea) and sensitive (Micromonas pusilla, Prorocentrum minimum). We combined this data with 10 species previously studied and found that cell size is most important in determining the biomass response to oil, whereas motility/mixotrophy is more important in the dispersed oil. Our analysis accounted for a third of the variance observed, so further work is needed to identify other factors that contribute to oil resistance.

Ochromonas spp. strains CCMP1393 and BG-1 are phagotrophic phytoflagellates with different nutritional strategies. Strain CCMP1393 is an obligate phototroph while strain BG-1 readily grows in continuous darkness in the presence of bacterial prey. Growth and gene expression of strain CCMP1393 were investigated under conditions allowing phagotrophic, mixotrophic, or phototrophic nutrition. The availability of light and bacterial prey led to the differential expression of 42% or 45–59% of all genes, respectively. Data from strain CCMP1393 were compared to those from a study conducted previously on strain BG-1, and revealed notable differences in carbon and nitrogen metabolism between the 2 congeners under similar environmental conditions. Strain BG-1 utilized bacterial carbon and amino acids through glycolysis and the tricarboxylic acid cycle, while downregulating light harvesting and carbon fixation in the Calvin cycle when both light and bacteria were available. In contrast, the upregulation of genes related to photosynthesis, light harvesting, chlorophyll synthesis, and carbon fixation in the presence of light and prey for strain CCMP1393 implied that this species is more phototrophic than strain BG-1, and that phagotrophy may have enhanced phototrophy. Cellular chlorophyll a content was also significantly higher in strain CCMP1393 supplied with bacteria compared to those without prey. Our results thus point to very different physiological strategies for mixotrophic nutrition in these closely related chrysophyte species.

Shifts in the nutritional mode between phototrophy, mixotrophy and heterotrophy are a widespread phenomenon in the evolution of eukaryotic diversity. The transition between nutritional modes is particularly pronounced in chrysophytes and occurred independently several times through parallel evolution. Thus, chrysophytes provide a unique opportunity for studying the molecular basis of nutritional diversification and of the accompanying pathway reduction and degradation of plastid structures. In order to analyze the succession in switching the nutritional mode from mixotrophy to heterotrophy, we compared the transcriptome of the mixotrophic Poterioochromonas malhamensis with the transcriptomes of three obligate heterotrophic species of Ochromonadales. We used the transcriptome of P. malhamensis as a reference for plastid reduction in the heterotrophic taxa. The analyzed heterotrophic taxa were in different stages of plastid reduction. We investigated the reduction of several photosynthesis related pathways e.g. the xanthophyll cycle, the mevalonate pathway, the shikimate pathway and the tryptophan biosynthesis as well as the reduction of plastid structures and postulate a presumable succession of pathway reduction and degradation of accompanying structures., Different pathway repertoirs in heterotrophic chrysophytes indicate the stepwise loss of photosynthesis related pathways in the course of evolution of heterotrophy.

Collectively, phagotrophic algae (mixotrophs) form a functional continuum of nutritional modes between autotrophy and heterotrophy, but the specific physiological benefits of mixotrophic nutrition differ among taxa. Ochromonas spp. are ubiquitous chrysophytes that exhibit high nutritional flexibility, although most species generally fall towards the heterotrophic end of the mixotrophy spectrum. We assessed the sources of carbon and nitrogen in Ochromonas sp. strain BG-1 growing mixotrophically via short-term stable isotope probing. An axenic culture was grown in the presence of either heat-killed bacteria enriched with ^(15)N and ^(13)C, or unlabeled heat-killed bacteria and labeled inorganic substrates (^(13)C-bicarbonate and ^(15)N-ammonium). The alga exhibited high growth rates (up to 2 divisions per day) only until heat-killed bacteria were depleted. NanoSIMS and bulk IRMS isotope analyses revealed that Ochromonas obtained 84–99% of its carbon and 88–95% of its nitrogen from consumed bacteria. The chrysophyte assimilated inorganic ^(13)C-carbon and ^(15)N-nitrogen when bacterial abundances were very low, but autotrophic (photosynthetic) activity was insufficient to support net population growth of the alga. Our use of nanoSIMS represents its first application towards the study of a mixotrophic alga, enabling a better understanding and quantitative assessment of carbon and nutrient acquisition by this species.

Background Ochromonas is a genus of mixotrophic chrysophytes that is found ubiquitously in many aquatic environments. Species in this genus can be important consumers of bacteria but vary in their ability to perform photosynthesis. We studied the effect of light and bacteria on growth and gene expression of a predominantly phagotrophic Ochromonas species. Axenic cultures of Ochromonas sp. were fed with heat-killed bacteria (HKB) and grown in constant light or darkness. RNA was extracted from cultures in the light or in the dark with HKB present (Light + HKB; Dark + HKB), and in the light after HKB were depleted (Light + depleted HKB). Results There were no significant differences in the growth or bacterial ingestion rates between algae grown in light or dark conditions. The availability of light led to a differential expression of only 8% of genes in the transcriptome. A number of genes associated with photosynthesis, phagotrophy, and tetrapyrrole synthesis was upregulated in the Light + HKB treatment compared to Dark + HKB. Conversely, the comparison between the Light + HKB and Light + depleted HKB treatments revealed that the presence of HKB led to differential expression of 59% of genes, including the majority of genes involved in major carbon and nitrogen metabolic pathways. Genes coding for unidirectional enzymes for the utilization of glucose were upregulated in the presence of HKB, implying increased glycolytic activities during phagotrophy. Algae without HKB upregulated their expression of genes coding for ammonium transporters, implying uptake of inorganic nitrogen from the culture medium when prey were unavailable. Conclusions Transcriptomic results agreed with previous observations that light had minimal effect on the population growth of Ochromonas sp. However, light led to the upregulation of a number of phototrophy- and phagotrophy-related genes, while the availability of bacterial prey led to prominent changes in major carbon and nitrogen metabolic pathways. Our study demonstrated the potential of transcriptomic approaches to improve our understanding of the trophic physiologies of complex mixotrophs, and revealed responses in Ochromonas sp. not apparent from traditional culture studies. Electronic supplementary material The online version of this article (doi:10.1186/s12864-017-3549-1) contains supplementary material, which is available to authorized users.

There is an extensive literature on chrysophyte stomatocysts from diverse habitats and their use as paleolimnetic indicators of past environmental conditions in lakes. The majority of these cysts are unidentified, and more laboratory studies are needed to link stomatocyst morphotypes to vegetative stages. A laboratory procedure that resulted in stomatocyst production in the chrysophyte alga Ochromonas pinguis (Chrysophyceae) was tested to determine if the protocol would also stimulate the production of stomatocysts in six other chrysophytes. Chrysolepidomonas dendrolepidota, Chrysosaccus cf. sphaericus, Dermatochrysis sp., Synura cf. americana, Synura cf. macropora and a second species of Ochromonas all produced stomatocysts within two to three weeks. Morphology of the stomatocyst was described using scanning electron microscopy (SEM). This was the first report of stomatocysts from Chrysosaccus and the first SEM description of O. pinguis cysts. These autecological studies provide a greater linkage between sediment stomatocysts and their biological origin, strengthening the interpretive value of these microfossils and broadening our understanding of chrysophyte biogeography.

Mixotrophy is increasingly recognized as an important and widespread nutritional strategy in various taxonomic groups ranging from protists to higher plants. We hypothesize that the availability of alternative carbon and energy sources during mixotrophy allows a switch to photoheterotrophic growth, where the photosynthetic apparatus mainly provides energy but not fixed carbon. Because such a change in the function of the photosynthetic machinery is probably reflected in its composition, we compared the photosynthetic machinery in Ochromonas danica during autotrophic and mixotrophic growth. Compared with autotrophic growth, the total pigmentation of O.danica was reduced during mixotrophic growth. Furthermore, the photosystem I (PSI):PSII ratio increased, and the cellular content of Rubisco decreased not only absolutely, but also relative to the content of PSII. The changing composition of the photosynthetic apparatus indicates a shift in its function from providing both carbon and energy during photoautotrophy to mainly providing energy during mixotrophy. This preference for photoheterotrophic growth has interesting implications for the contribution of mixotrophic species to carbon cycling in diverse ecosystems. 10.1111/(ISSN)1469-8137

For many years Chrysastrella Chodat has been a cyst genus but recently it has been re-instated and re-defined. Currently, Chrysastrella contains Ochromonas-like flagellates including O. tuberculata Hibberd which was synonymized with the type species, Chrysastrella paradoxa Chodat. A mass development of stomatocysts, which we identified as Ch. paradoxa, was observed in a bog on the territory of “Yugyd va” National Park (the Subpolar Urals, Russia). The morphology of stomatocysts was studied by scanning electron microscopy and compared with previously described similar morphotypes. Stomatocysts #80 Hansen, #1 Wołowski & Płachno and #4 Bai & Chen seem to be identical to Chrysastrella paradoxa stomatocysts and we propose to reduce them to synonymy.

The bioaccumulation kinetics of thioglycolic acid stabilized CdTe quantum dots (TGA-CdTe-QDs) in a freshwater alga Ochromonas danica was comprehensively investigated. Their photoluminescence (PL) was determined by flow cytometry. Its cellular intensity increased hyperbolically with exposure time suggesting real internalization of TGA-CdTe-QDs. This hypothesis was evidenced by the nanoparticle uptake experiment with heat-killed or cold-treated cells and by their localization in the vacuoles. TGA-CdTe-QD accumulation could further be well simulated by a biokinetic model used previously for conventional pollutants. Moreover, macropinocytosis was the main route for their internalization. As limited by their diffusion from the bulk medium to the cell surface, TGA-CdTe-QD uptake rate increased proportionally with their ambient concentration. Quick elimination in the PL of cellular TGA-CdTe-QDs was also observed. Such diminishment resulted mainly from their surface modification by vacuolar biomolecules, considering that these nanoparticles remained mostly undissolved and their expulsion out of the cells was slow. Despite the significant uptake of TGA-CdTe-QDs, they had no direct acute effects on O. danica. Overall, the above research shed new light on nanoparticle bioaccumulation study and would further improve our understanding about their environmental behavior, effects and fate.

The metabolic theory of ecology predicts that temperature affects heterotrophic processes more strongly than autotrophic processes. We hypothesized that this differential temperature response may shift mixotrophic organisms towards more heterotrophic nutrition with rising temperature. The hypothesis was tested in experiments with the mixotrophic chrysophyte Ochromonas sp., grown under autotrophic, mixotrophic and heterotrophic conditions. Our results show that (1) grazing rates on bacterial prey increased more strongly with temperature than photosynthetic electron transport rates, (2) heterotrophic growth rates increased exponentially with temperature over the entire range from 13 to 33 °C, while autotrophic growth rates reached a maximum at intermediate temperatures and (3) chlorophyll contents during mixotrophic growth decreased at high temperature. Hence, the contribution of photosynthesis to mixotrophic growth strongly decreased with temperature. These findings support the hypothesis that mixotrophs become more heterotrophic with rising temperature, which alters their functional role in food webs and the carbon cycle.

This study aimed to investigate the influence of temperature on the ability of the mixotrophic flagellate Ochromonas to eliminate a toxic Microcystis population and degrade microcystins. We exposed Microcystis to cultures with or without Ochromonas YZ1 at 20, 25, and 30 °C for 10 days. Results showed that increased temperature promoted the growth of Ochromonas YZ1 and Microcystis, with the latter achieving high abundance without grazing. With increased temperature, Ochromonas YZ1 clearance rate increased, and Microcystis populations were earlier eliminated. Importantly, Ochromonas YZ1 degraded both intracellular and extracellular microcystins by grazing effects. The reduction ratios of Microcystis abundances and microcystins were both approximately 100% after 6 days at high temperature. In addition, more microcystins were released outside at 20 °C than at the higher temperatures. Overall, this study showed that high temperature favors elimination of toxin-producing Microcystis and degradation of microcystins by mixotrophic Ochromonas.

Summary 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

In this study, the concept of functional groups was applied for the first time to the seasonal dynamics of the phytoplankton community of Mudanjiang River (northeast China). The environmental variables as well as the phytoplankton communities were determined in 2007. The river showed strong seasonal fluctuations in water physical and chemical condition. A total of 11 phytoplankton functional groups (C, D, E, J, MP, P, T, TB, W1, X2 and Y) were identified that included 30 species. Groups C (Asterionella formosa and Cyclotella meneghiniana), MP (Cocconeis placentula, Cymatopleara solea, Diatoma vulgare, Ochromonas fragilis, Pinnularia major and Ulothrix variabilis), P (Fragilaria capucina, Fragilaris sp., Aulacoseira granulate and Aulacoseira granulate var. angustissima), TB (Aulacoseira varians and Navicula exigua, Navicula radiosa), X2 (Chlamydomonas sp.) and Y (Chroomonas acuta, Cryptomonas ovata, and Cryptomonas sp.) were the dominant groups during the study period. Mudanjiang River phytoplankton functi...

We investigated the effect of pH on the competition of two closely related chrysomonad species, Poterioochromonas malhamensis originating from circumneutral Lake Constance, and Ochromonas sp. isolated from a highly acidic mining lake in Austria (pH ∼2.6). We performed pairwise growth experiments between these two species at four different pH ranging from 2.5 to 7.0. Heterotrophic bacteria served as food for both flagellates. Results were compared to growth rates measured earlier in single species experiments over the same pH range. We tested the hypothesis that the acidotolerant species benefits from competitive release under conditions of acid stress. The neutrophilic strain numerically dominated over the acidotolerant strain at pH 7.0, but was the inferior competitor at pH 2.5. At pH 3.5 and 5.0 both strains coexisted. Surprisingly, P. malhamensis prevailed over Ochromonas sp. under moderately acidic conditions, i.e. at the pH where growth rates of the latter peaked when grown alone. Since bacterial food was not limiting, resource competition is improbable. It appears more likely that P. malhamensis ingested cells of its slightly smaller competitor. Adverse effects mediated via allelopathy, either directly on the competing flagellate or indirectly by affecting its bacterial food, might also have affected the outcome of competition.

Strains of the green alga Chlamydomonas acidophila and two chrysomonads, Ochromonas spp., isolated from each of two similar acid mining lakes (AMLs) with extremely low pH (∼2.6) were investigated to consider a possible synergistic stress effect of low pH and unfavourable temperature. We measured flagellate growth rates over a combination of four pH (2.5, 3.5, 5.0 and 7.0) and three temperatures (10, 17.5 and 25°C) in the laboratory. Our hypothesis was that, under highly acidic conditions (pH 27°C. The lowest temperature tested reduced growth rates of all three chrysomonad strains significantly. Since all chrysomonads were tolerant to high temperature, growth rate of one Ochromonas spp. strain was measured exemplarily at 35°C. Only at this high temperature was the realized pH niche significantly narrowed. We also recorded significant intraspecific differences within the C. acidophila strains from the two AML, illustrating that the niche width of a species is broader than that of individual clones.

Ecological stoichiometry focuses on the balance between multiple nutrient elements in resources and in consumers of those resources. The major consumers of bacteria in aquatic food webs are heterotrophic and mixotrophic nanoflagellates. Despite the importance of this consumer-resource interaction to understanding nutrient dynamics in the aquatic food web, few data are available addressing the element stoichiometry of flagellate consumers. Ochromonas danica, a mixotrophic bacterivore, was used as a model organism to study the relationships among temperature, growth rate and element stoichiometry. Ochromonas danica was grown in chemostats at dilution rates ranging between 0.03 and 0.10 h(-1) and temperatures ranging between 15 and 28 °C. Cells accumulated elements as interactive functions of temperature and growth rate, with the highest element concentrations corresponding to cells grown at a low temperature and high growth rates. The highest concentrations of elements were associated with small cells. Temperature and growth rate affected the element stoichiometry (as C:N, C:P and N:P) of O. danica in a complex manner, but the growth rate had a greater effect on ratios than did temperature.

SUMMARY A cDNA library was constructed from the chrysophycean alga, Ochromonas danica E. G. Pringsheim. 5′-end sequencing of about 600 cDNA clones yielded 476 authentic expressed sequence tags (EST) of which 275 showed significant matches (E-value

The new type blue light (BL) receptor aureochrome (AUREO) was recently discovered in a stramenopile alga, Vaucheria (Takahashi et al. Proc Natl Acad Sci USA 104(49):19625-19630, 2007). AUREO has a bZIP (basic region/leucine zipper) and BL-sensing light-oxygen-voltage (LOV) domain and functions as a BL-activated transcription factor. It mediates BL-induced branching and regulates the development of the sex organ in V. frigida. Although AUREO sequences have previously been found in Fucus and some diatoms, here we report that AUREO orthologs are commonly conserved in photosynthetic stramenopiles. Five AUREO orthologs were isolated from three stramenopile genera (Fucus, Ochromonas, and Chattonella). By BLAST search, several AUREO sequences were also detected in genomes in Aureococcus anophagefferens (Pelagophyceae). However, AUREO was not found in heterotrophic stramenopiles or in closely related phyla, such as haptophytes and cryptophytes, or in green plants. Stramenopiles do not possess phototropin, the well-known BL receptor for phototropism of green plants. From comparative analysis of LOV domains, together with kinship analysis of AUREO bZIP domains, AUREO can be regarded as the BL receptor specific to phototrophic stramenopiles. The evolution of AUREO and the phylogeny of LOV domains in stramenopiles and green plants are discussed.

Colonial Microcystis aeruginosa were obtained when the unicellular algae were exposed to flagellate Ochromonas sp. filtrate. To investigate the benefit of this morphological change, flagellates were added into cultures of unicellular and colonial M. aeruginosa, respectively. The clearance rates of flagellates on algae were markedly decreased when they were cultivated with induced colonial M. aeruginosa. This result indicated that colony formation in M. aeruginosa was a predator-induced defense, which could reduce predation risk from flagellate. The increased content of soluble extracellular polysaccharide (sEPS) and bound extracellular polysaccharide (bEPS) may play an important role in adhering M. aeruginosa cells together to form colonies. The decrease of WPS II and the increase of sinking rates of induced colonial M. aeruginosa showed that the costs of grazed-induced colony formation in M. aeruginosa may reflect in the photosystem II efficiency, and in the sinking rates.

Summary We document a remarkable morphological transformation, attributable to grazing by nanoflagellate Ochromonas sp. DS, of a phycocyanin-rich freshwater Cyanobium sp. (10-NR 98.2% similar 16S rRNA gene sequence to the type species Cyanobium gracile). The single cells aggregated into microcolonies (average size 40 μm) in the presence of the protist. Colonies were characterized by hundreds of tubes (spinae), 100 nm to 1 μm long and 63 ± 6 nm wide, on the surfaces of the Cyanobium cells co-cultured with Ochromonas. Spinae production, previously unknown for the freshwater Cyanobium species, suggests that picocyanobacterial life strategies are more flexible than previously thought.

Flagellates are important bacterial grazers in most planktonic food webs. The prey-size preference of the mixotrophic flagellate, Ochromonas sp. (Chrysophyceae), isolated from an extremely acidic lake, Lake 111 (pH 2.6), was determined using fluorescently labelled microspheres (beads). According to grazing experiments with cultured bacteria, also isolated from Lake 111, the potential grazing impact on Lake 111"s single-celled bacterial production was calculated. Ochromonas sp. ingested the smallest beads offered (0.5 µm diameter) at the highest rate. Ingestion rate declined with increasing bead size. The highest prey volume-specific ingestion was measured for Ochromonas sp. feeding on intermediate-sized beads (1.9 µm). Ingestion rates were low due in part to the large fraction of inactive flagellates observed. According to the bacterial ingestion rate, a mean of 88% (epilimnion) and 68% (hypolimnion) of in situ single- celled bacterial production is potentially grazed daily by Ochromonas sp. In the epilimnion of Lake 111, the heterotrophic carbon gain is three times higher than the autotrophic production. Alongside carbon uptake, Ochromonas sp. also benefits from ingesting bacteria through the uptake of phosphorus. A biovolume minimum corresponding to the prey size at which Ochromonas sp. feeds most efficiently occurred in the Lake 111 epilimnetic bacterial community, implying top-down control of the bacterial community by Ochromonas sp.

Phagotrophy and competitive ability of the mixotrophic Ochromonas minima were investigated in a three-factorial experiment where light intensity (low: 1.0 micromol m(-2)s(-1) and high: 60 micromol m(-2)s(-1) PPFD), nutrient concentration (ambient: 7.0 micromolNl(-1), 0.11 micromol P l(-1) and enriched: 88 micromol N l(-1), 6.3 micro mol P l(-1)) and DOC supply (without and with enrichment, 250 micromol C l(-1)) were manipulated. Ochromonas minima and bacterial abundance were monitored for 12 days. We found significant and interacting effects of light and nutrients on Ochromonas minima growth rate and abundance. At high light intensity, nutrient enrichment resulted in increased growth rates and population sizes. In contrast, reduced growth rates and population sizes were observed for nutrient enrichment when light intensity was low. Although, Ochromonas minima was able to ingest bacteria under both high and low light conditions, it grew only when light intensity was high. At high light intensity, Ochromonas minima grew exponentially under nutrient conditions that would have been limiting for photoautotrophic microalgae. In non-enriched low light treatments, Ochromonas minima populations survived, probably by using background DOC as an energy source, indicating that this ability can be of relevance for natural systems even when DOC concentrations are relatively low. When competing with photoautotrophic microalgae, the ability to grow under severe nutrient limitation and to survive under light limitation should be advantageous for Ochromonas minima.

1. The in situ abundance, biomass and mean cell volume of Actinophrys sol (Sarcodina: Heliozoa), the top predator in an extremely acidic German mining lake (Lake 111; pH 2.65), were determined over three consecutive years (spring to autumn, 2001-03). 2. Actinophrys sol exhibited pronounced temporal and vertical patterns in abundance, biomass and mean cell volume. Increasing from very low spring densities, maxima in abundance and biomass were observed in late June/early July and September. The highest mean abundance recorded during the study was 7 x 10(3) Heliozoa L-1. Heliozoan abundance and biomass were higher in the epilimnion than in the hypolimnion. Actinophrys sol cells from this acidic lake were smaller than individuals of the same species found in other aquatic systems. 3. We determined the growth rate of A. sol using all potential prey items available in, and isolated and cultured from, Lake 111. Prey items included: single-celled and filamentous bacteria of unknown taxonomic affinity, the mixotrophic flagellates Chlamydomonas acidophila and Ochromonas sp., the ciliate Oxytricha sp. and the rotifers Elosa worallii and Cephalodella hoodi. Actinophrys sol fed over a wide-size spectrum from bacteria to metazoans. Positive growth was not supported by all naturally available prey. Actinophrys sol neither increased in cell number (k) nor biomass (k(b)) when starved, with low concentrations of single-celled bacteria or with the alga Ochromonas sp. Positive growth was achieved with single- celled bacteria (k = 0.22 +/- 0.02 d(-1); k(b) = -0.06 +/- 0.02 d(-1)) and filamentous bacteria (k = 0.52 +/- < 0.01 d(- 1); k(b) = 0.66 d(-1)) at concentrations greater than observed in situ, and the alga C. acidophila (up to k = 0.43 +/- 0.03 d(-1); k(b) = 0.44 +/- 0.04 d(-1)), the ciliate Oxytricha sp. (k = 0.34 +/- 0.01 d(-1)) and in mixed cultures containing rotifers and C. acidophila (k = 0.23 +/- 0.02-0.32 +/- 0.02 d(-1); maximum k(b) = 0.42 +/- 0.05 d(-1)). The individual- and biomass-based growth of A. sol was highest when filamentous bacteria were provided. 4. Existing quantitative carbon flux models for the Lake 111 food web can be updated in light of our results. Actinophrys sol are omnivorous predators supported by a mixed diet of filamentous bacteria and C. acidophila in the epilimnion. Heliozoa are important components in the planktonic food webs of 'extreme' environments

We studied the effects of 2 bacterivorous protistan predators on the phenotypic and taxonomic successions of an experimental bacterial community growing in continuous culture with the cryptophyte Cryptomonas phaseolus. Two predators were inoculated to the cultivation system, the mixotrophic flagellate Ochromonas sp., an interception feeder, and the filter-feeding heterotrophic ciliate Cyclidium glaucoma. The system was sampled every second day over a period of 3 mo. The competition between algae and bacteria for limiting phosphorus (20 μg P l - 1 d - 1 ) was reflected in pronounced fluctuations of the ratio of algal versus bacterial biomass. High grazing pressure exerted on bacteria by the flagellate led to a strong decrease of bacterial abundance and biomass, with only few cells left freely dispersed. A succession of various grazing-resistant morphologies was observed, such as microcolonies, large aggregates and filaments. A population affiliated with Aquabacterium sp. formed grazing-resistant co-colonies with a Caulobacter spp., indicating that these 2 bacteria might find a common refuge from protistan grazing within aggregates or biofilms. During the course of the experiment these large colonies were succeeded by grazing-protected morphotypes related to the Flexibacter sancti-Flavobacterium ferrugineum lineage of the Sphingobacteriales. In contrast, the ciliate induced only a slight reduction of bacterial abundance, and bacteria even recovered to higher than original numbers after a few weeks. However, pronounced successions of microbial populations were also observed in the presence of the ciliate.

The significance of food concentration for selec- tivity was analyzed by video microscopy for 3 species of inter- ception-feeding bacterivorous nanoflagellates of the genera Spumella, Ochromonas and Cafeteria. Inert beads and live bacteria were offered simultaneously at 5 different concentra- tions. The fate of individual prey particles was recorded dur- ing the stages of the particle-flagellate interaction: capture, ingestion, digestion and egestion. The experiments revealed passive and active selection mechanisms that were regulated separately. Selective food uptake depended strongly on food concentration, whereas differential digestion was indepen- dent of the food concentration and independent of the num- ber of previously ingested food particles. In addition to active selection of food items, passive selection occurred due to the different contact probabilities of prey. In contrast to the chrysomonads Spumella sp. and Ochromonas sp., the bico- soecid Cafeteria sp. showed no significant active selection, neither during food uptake nor during digestion. The results imply that it is more efficient for some interception-feeding flagellates to feed unselectively all particles that can be mor- phologically ingested and then to attempt to digest these par- ticles. Active selection in advance may only be efficient when the particle concentration is sufficiently high such that vac- uole formation becomes time limiting.

The growth and grazing characteristics of Poterioochromonas malhamensis (Pringsheim) Peterfi (= Ochromonas malhamensis Pringsheim) (ca. 8 μm) feeding on phytoplankton, including the cyanobacteria Synechococcus sp. (ca. 2 μm) and Microcystis viridis (A. Brown) Lemmermann (ca. 6 μm) and the green alga Chlorella pyrenoidosa Chick (ca. 13 μm), were investigated in laboratory experiments involving the following treatments: (1) light without added algal prey (autotrophy), (2) light with added algal prey (mixotrophy), and (3) dark with added algal prey (phagotrophy). There were significantly higher cell numbers under mixotrophic and phagotrophic growth than under autotrophic growth. With phytoplankton as food, growth rates under both mixotrophy and phagotrophy were about two or three times higher than those under autotrophy, indicating that the algal diets were readily able to support the population growth of P. malhamensis. There were no significant differences in growth rate between mixotrophic and phagotrophic cultures during exponential growth. The ingestion rate of P. malhamensis with algal prey was also similar under both continuous light and dark. Poterioochromonas malhamensis ingested on average 0.27 M. viridis cells·flagellate−1·h−1 and 0.18 C. pyrenoidosa cells·flagellate−1·h−1 in continuous light and 0.25 M. viridis cells·flagellate−1·h−1 and 0.18 C. pyrenoidosa cells·flagellate−1·h−1 in continuous dark during exponential growth. The results showed that light had no effect on the growth and ingestion rates of P. malhamensis for phagotrophy during exponential growth. However, phagotrophic populations of P. malhamensis were incapable of growth in continuous darkness for longer than 5 days. Populations of P. malhamensis showed no increase when prey was added again after 4 days in continuous darkness, indicating that light is necessary for sustained phagotrophic growth of P. malhamensis. The study suggests that P. malhamensis, which has strong tolerance for light, is light dependent for phagotrophy.

The influence of prey characteristics such as motility and size as well as of predator characteristics such as satiation and preculturing diet on the feeding process of interception feeding heterotrophic nanoflagellates was investigated. Three species of gram-negative bacteria, one species of gram-positive bacteria, two species of cyanobacteria (Synechococcus) and inert latex particles were fed as prey particles for three species of heterotrophic nanoflagellates (Spumella, Ochromonas, Cafeteria). Ingestion rates depended on the satiation of the flagellates and especially on the filling status of the food vacuoles. In addition, the ingestion rates depended on the characteristics of the food particle and were modified by pre-culturing the flagellates on either Pseudomonas putida or Bacillus subtilis. Digestion was found to be particle-specific. Cyanobacteria were excreted a few minutes after ingestion whereas heterotrophic bacteria were stored and digested in the food vacuoles. The spectrum of ingested particles is not identical to that of digested particles and thus neither the diet of the flagellates nor their impact on bacterial communities can be calculated simply from food vacuole content. "Selective digestion" could be shown to be an important selection mechanism concerning natural food particles. The digestion strategies of Cafeteria on the one hand and Spumella and Ochromonas on the other hand may be an important factor to explain protozoan species composition and succession in the field. In addition to bacterial abundance and grazing pressure by metazooplankton, the bacterial speciescomposition as well as biochemical variations within bacterial species may influence protozoan species composition and abundance.

The aim of this study was to investigate the interactions between the cyanobacteria (Microcystis aeruginosa) and the potential grazer (Ochromonas sp.) with regard to colony formation. Two kinds of treatments were carried out: (i) In the dialyse experiment Microcystis aeruginosa and Ochromonas sp. were physically separated by a dialyse tubing. (ii) In the contact experiment interactions between Microcystis aeruginosa, heterotrophic bacteria and Ochromonas sp. in different concentrations were investigated. In one treatment where the predator Ochromonas sp. came in direct contact with Microcystis, aggregates were formed.

Most of the flooded, open-cast lignite mining lakes of Lusatia (Germany) impacted by the oxidation of iron sulphides (pyrite and marcasite) are extremely acidic. Of 32 lakes regularly studied from 1995 to 1998, 14 have a pH

Epifluorescence microscopy reveals the presence of fluorescence in the living cells of at least three classes of flagellates. In Ochromonas cells, the fluorescence is blue-green in color and is found only in the short flagellum, both in the flagellar swelling and throughout the length of the flagellum. As recognized by the locale and color of the flagellar fluorescence, the same fluorescence is observed in only certain other heterokont algal groups but is also found in one of the two isokont flagella of the prymnesiophyte Prymnesium parvum.

Iron is unique among biologically essential trace metals in having a higher particulate than dissolved concentration in ocean surface waters1. Uptake of dissolved iron is generally considered to be the norm for phytoplankton, as even the smallest iron-bearing particles are unavailable for transport into cells2,3. But the oceanic dissolved fraction is so small, and the particulate fraction so inert2, that phytoplankton production is limited by a dearth of available iron in some regions4. Here we use incubation experiments to show that Ochromonas sp., a common photosynthetic flagellate from the Pacific Ocean, can obtain iron directly in particulate form, by ingesting bacteria. Iron acquisition is highly efficient; Ochromonas assimilates 30% of the ingested ration, acquiring a high intracellular iron concentration and maintaining a significantly faster growth rate than when iron is provided in the dissolved phase. Phytoplankton capable of such phagotrophy (so-called mixotrophic species) may thus be able to assimilate iron in both particulate and dissolved forms in the ocean. Moreover, when iron availability is limited, the iron ‘cost’ of growth is diminished because Ochromonas derives a greater fraction of its energy from the bacteria. Analysis of standing stocks and clearance rates of plankton in the equatorial Pacific shows that the iron flux through mixotrophic flagellates can amount to 35–58% of the total Fe uptake by the entire autotrophic community. Our results suggest that the phagotrophic ingestion of bacteria may be an effective adaptive strategy for photosynthetic organisms to obtain iron for growth in iron-limited regions of the sea.

Summary Predation was a powerful selective force promoting increased morphological complexity in a unicellular prey held in constant environmental conditions. The green alga, Chlorella vulgaris, is a well-studied eukaryote, which has retained its normal unicellular form in cultures in our laboratories for thousands of generations. For the experiments reported here, steady-state unicellular C. vulgaris continuous cultures were inoculated with the predator Ochromonas vallescia, a phagotrophic flagellated protist (‘flagellate’). Within less than 100 generations of the prey, a multicellular Chlorella growth form became dominant in the culture (subsequently repeated in other cultures). The prey Chlorella first formed globose clusters of tens to hundreds of cells. After about 10‐20 generations in the presence of the phagotroph, eight-celled colonies predominated. These colonies retained the eight-celled form indefinitely in continuous culture and when plated onto agar. These selfreplicating, stable colonies were virtually immune to predation by the flagellate, but small enough that each Chlorella cell was exposed directly to the nutrient medium.

Mixotrophic flagellates can compete with obligately heterotrophic flagellates for the uptake of food particles, whereas mixotrophs and obligately phototrophic phytoplankton can compete for soluble nutrients. Competition for soluble nutrients is expected when photosynthesis covers a significant portion of the mixotrophs' carbon metabolism. When heterotrophy predominates, however, mixotrophs may release soluble nutrients and facilitate phototrophs. Mechanistic resource competition theory predicts that, due to their ability to utilize substitutable C and P sources, mixotrophs should be able to coexist with their more specialized competitors under certain conditions of resource supply. In laboratory experiments, the mixotrophic flagellate Ochromonas excluded heterotrophic flagellates when only phototrophic growth was possible. However, Ochromonas was excluded by heterotrophic flagellates when only phagotrophic growth was possible. Both coexisted when food bacteria and light were supplied simultaneously. Ochromonas coexisted with a P-limited phytoplankter, Cryptomonas sp., when soluble reactive phosphorus (SRP) and bacterial phosphorus were available as alternative P sources. In nature, the mixotrophic strategy may be successful when resources are limiting. This is supported by published data on the occurrence of mixotrophic chrysophytes.

In laboratory experiments, I studied the influences of bacterial density and light on the ingestion and growth rates, pigment contents, and the carbon and phosphorus turnover rates of the mixotrophic flagellate Ochromonas sp. The investigated strain is a bacterivorous flagellate that can enhance its photosynthetic apparatus and grow phototrophically when bacterial densities are low. This was also evident from significantly higher chlorophyll a contents during active photosynthetic growth phases. Moderate phototrophic growth should be possible even if bacteria were absent. Bacterial ingestion rates increased hyperbolically with bacterial density, and there was no difference between light- and darkadapted cells. Ochromonas released soluble reactive phosphorus (SRP) when growth was predominantly phagotrophic, but it took up SRP when growth was phototrophic. The mixotrophic strategy in Ochromonas appears to be bound up with costs and trade-offs : Ochromonas needs high bacterial densities to reach maximum growth rates, its basic metabolic costs are higher than for obligately phagotrophic flagellates, and its phototrophic growth rates are lower than for obligately phototrophic phytoplankton of comparable size.

Summary We have examined the common soil zooflagellate Oikomonas mutabilis Kent in the light and electron microscope and sequenced its 18s rRNA gene. It has tubular mitochondrial cristae and a single anterior tinsel cilium clothed in a unilateral row of bipartite (or possibly tripartite) hairs (retronemes) with single terminal filaments, as well with simple non-tubular hairs; it is therefore clearly a heterokont. It differs from previously studied colourless chrysomonads in the apparent absence of a vestigial leucoplast with eyespot, in the probably bipartite retronemes, in the absence of a vestigial smooth cilium, and in the unilateral arrangement of the retronemes. Scales appear to be absent. Phylogenetic analysis of its 18s rRNA sequence, however, clearly shows that it is most closely related to the chrysomonads, but it is not specifically related to Paraphysomonas . On most trees it is the sister group to chrysomonads; though its sequence is, therefore, significantly divergent from those of all genuine chrysomonads, with the maximum parsinomy method it can group weakly with Ochromonas . A possible relationship with Ochromonas is also suggested by the presence of numerous large cortical vacuoles which fluoresce brightly when exposed to ultraviolet or blue light. Bright short wavelength light kills the cells in a few seconds. Oikomonas appears to be a third independent example of plastid loss and the consequent secondary origin of zooflagellates within the Heterokonta: our present analysis suggests that Pseudofungi (oomycetes and hyphochytriomycetes) also evolved from ochristan algae, by the loss of chloroplasts, thus providing evidence for at least four separate losses of chloroplasts within Heterokonta. We create a new order (Oikomonadales), validate the class Oikomonadea for Oikomonas , and include it with the classes Chrysomonadea, Eustigmatophycea, and Raphidomonadea in the superclass Limnistia, for which we present a revised classification.

SUMMARY Sulcochrysis biplastida gen. et sp. nov., a golden, marine, mixotrophic flagellate is described. Cells resemble Ochromonas in light microscopic features, but they are distinct at the electron microscopic level from Ochromonas or any other typical chrysophyte. Ultrastructural features that discriminate Sulcochrysis from the Chrysophyceae are: (i) a proximal helix in the flagellar transition region; (ii) basal bodies situated in the anterior depression of the nucleus; (iii) the lack of the rhizoplast; and (iv) simple flagellar hairs lacking lateral filaments. These features suggest that Sulcochrysis is a relative of the Pedinellophyceae, Dictyochophyceae and Pelagophyceae. However, Sulcochrysis has a flagellar root system similar to that of the Ochromonas-type cell and it may use the R3 root for prey capture, as do ochromonadalean algae. The R3 root and the phagotrophic mechanism using the R3 root are interpreted as a plesiomorphy, because these are also distributed in primitive heterokonts such as the bicosoecids. Sulcochrysis has one microtubule probably homologous with the x-fibre of Bicosoeca maris Picken. Based on these features it is suggested that Sulcochrysis is an organism that links bicosoecids (Bicosoeco-phyceae), Pedinellophyceae, Dictyochophyceae and Pelagophyceae.

Summary The presence of Ochromonas sp. from an extremely acidic site in Italy is reported for the first time. The results of ecophysiological tests allow a probable identification of this strain with O. vulcania, the only other species which has been described from low-pH environments, while some of the morphological and ultrastructural characters are not in agreement with those reported in literature.

A simple-defined medium was formulated that allows robust axenic-growth of the model mixotrophic protist Ochromonas danica at a neutral pH. This new defined medium, with a minimum number of constituents, facilitates more highly controlled studies of mixotrophic metabolism and nutrient regeneration than have previously been possible.

Bacterivory in obligate phototrophic algal flagellates may be an important strategy for acquiring nutrients during periods of inorganic nutrient limitation. Several marine algal flagellates were shown to increase bactivory when phosphate was limited. Grazing on bacteria by algal flagellates was found during blooms of Prymnesium parvum in Sandsljorden, western Norway, in 1989 and Chrysochromulina polylepis on the south and west coast of Norway in 1988. Dissolved phosphate was not detectable in these situations. Algal flagellates may graze bacteria to obtain phosphate, which may permit these algal flagellates to develop blooms when phosphate becomes limited. True mixotrophy, defined as use of particulate organic matter for cellular growth, has been conclusively demonstrated for only a few species in the genus Ochromonas (Fenchel 1982) and for Poteiroochromonas malhamensis (Caron et al. 1990). Much more widespread is the ability to use a restricted range of organic substances, available at high concentrations, as a dietary supplement or sole C source in the dark (Antia 1980). Mixotrophy in algal flagellates has been considered mainly a strategy for gaining C during low light (Bird and Kalff 1987; Sanders and Porter 1988) and therefore has been studied in algae that use bacteria as their primary C source. Doddema and Van der Veer ( 198 3) suggested that phagocytosis might permit utilization of particulate organic N and P when inorganic nutrients are in limited supply. Obligate phototrophs are the appropriate test organisms to evaluate this hypothesis. Bacteria have a high P content even when phosphate is limited (Andersen et al. 1986), although their P content can be reduced when they are drastically starved (Tezuka 1990). Bacteria are therefore a potential source of P when obligate phototrophic algal flagellates are subjected to

Stalks and loricae of chrysomonads were studied by fluorescence microscopy employing Calcoflor White-Evans blue (CW-Eb) staining, and by uranyl acetate staining of dried, whole mount preparations for electron microscopy. These structures were composed of microfibrils approximately 4 nm in diameter embedded in a matrix. The organization of the loricae of Poterioochromonas malhamenesis, "Amimonas minuta", Poterioochromonas stipitata and Ochromonas gloeopara showed a similar structural plan, consisting of a foot, stalk and cup region that together resemble a wine glass. CW-Eb-stained, microfibrillar stalks were identified also in Paraphysomonas vestita, Anthophysa vegetans and "Felimonas flocculans". These results suggest that CW-Eb-stained structures composed of microfibrils approximately 4 nm in diameter may be more common in chrysomonads than previously recognized. In cultures, these structures participate in the formation of cell aggregates and attachment of cells to substrates, and thus may be of ecological importance. Additionally, non-siliceous, CW-Eb-stained cysts were identified for the first time in P. malhamensis.

SUMMARY 1. In order to test the effect of Ochromonas sp., a mixotrophic chrysophyte, on cyanobacteria, grazing experiments were performed under controlled conditions. We studied grazing on three Microcystis aeruginosa strains, varying in toxicity and morphology, as well as on one filamentous cyanobacterium, Pseudanabaena sp. Furthermore, we analysed the co-occurrence of Ochromonas and Microcystis in natural systems in relation to various environmental parameters (TP, TN, DOC, temperature, pH), using data from 460 Norwegian lakes. 2. Ochromonas was able to feed on all four cyanobacterial strains tested, and grew quickly on all of them. The chrysophyte caused net growth reductions in all three Microcystis strains (the very toxic single-celled strain PCC 7806; the less toxic colony-forming Bear AC and the less toxic single-celled Spring CJ). The effect of Ochromonas was strongest on the Spring CJ strain. Although the effect of Ochromonas grazing on the growth of Pseudanabaena was relatively smaller, it also reduced the net growth of this cyanobacterium significantly. 3. After 4 days of incubation with Ochromonas the total amount of cyanotoxins in the three Microcystis strains was reduced by 91.1–98.7% compared with the controls. 4. Ochromonas occurred in similar densities across all 460 Norwegian lakes. Microcystis occurred only at higher TN, TP, temperature and pH values, although its density was often several orders of magnitude higher than that of Ochromonas. Ochromonas co-occurred in 94% of the samples in which Microcystis was present. 5. From our study it is not clear whether Ochromonas could control Microcystis blooms in natural lakes. However, our study does demonstrate that Ochromonas usually occurs in lakes with Microcystis, and our small scale experiments show that Ochromonas can strongly reduce the biomass of Microcystis and its toxin content.

The 16S-like ribosomal RNA genes from Mallomonas papillosa Harris et Bradley (Synurophyceae) and Tribonema aequale Pascher (Xanthophyceae) were sequenced and compared to those of other eukaryotes. Mallomonas is closely related to Ochromonas (Chrysophyceae) and supports the general hypothesis of a close phylogenetic relationship between the Synurophyceae and Chrysophyceae. Tribonema is specifically related to Costaria costata (C. A. Agardh) Saunders (Phaeophyceae) demonstrating an unexpected phylogenetic relationship between the Xanthophyceae and Phaeophyceae. Distance and parsimony analysis place these four chromophyte genera in a complex evolutionary assemblage that includes the Bacillariophyceae and Oomycetes but excludes the Dinophyceae. The close relationship between the chromophyte algae and the Oomycete fungi supports the hypothesis that protists with tripartite hairs form a natural assemblage.

Summary An unialgal culture of Ochromonas globosa SKUJA has been obtained from the plankton of a river in Karelia. The cell body of O. globosa is ovoid or globular, the beak and the tail have not been observed, but the platform is characteristic of the anterior end of the cell. Bacteria can be engulfed. The details of the structure of cell organelles in general correspond to the known cytology of Ochromonas . An unusual ultrastructural feature is the backward directed microtubular rootlet R-I. It may be connected with the reduction of the beak in the small-celled species of Ochromonas .

Three cultured species and mixed seawater communities of heterotrophc or mixotrophic flagellates were fed with non-cultivated aquatic bacteria labelled with I4C-protein hydrolysate, i.e. the method of radioactive labelled bacteria (RLB). Seawater bacteria incorporated protein-hydrolysate more rapidly and far more efficiently than 14~-glucose. Loss of label was less in the protein-labelled bacteria compared with those labelled with glucose. All tested flagellate species were found to ingest bacteria; individual clearance rates ranged from 0.014 $ ind.-' h-' (36 bacteria ind.-' h-'; Ochromonas minima) to 0.156 $ ind.-' h-' (156 bacteria ind.-' h-'; Paraphysomonas sp.). Uptake of bacteria by 0. minima and Bodo sp. was linear for ca 10 min, and levelled off between 10 to 40 min. Paraphysomonas sp. had a linear uptake for 20 min. Grazing rates obtained with 0. minima fed labelled bacteria and grazing rates calculated from chemostat culture were not significantly different. RLB proved rapid and efficient for estimating clearance by small heterotrophs and mixotrophs. Grazing rates in Bodo sp, and Paraphy- somonas sp, from a 'direct-count' method using DTAF-stained bacteria were 10 % of those obtained with RLR. RLB is shown to work well even with small size differences between predator and prey. This is obtained by use of pre-starved bacteria, which are reduced to a size where they may be separated from predators using a 1.0 pm filter. Field experiments with mixed seawater communities gave 10 to 30 times higher grazing rates using RLB compared to stained bacteria. The method accounts for food vacuoles containing bacteria, egested upon preservation of the bacterivory flagellates.

We studied the impact of grazing and substrate supply on the size structure of a freshwater bacterial strain (Flectobacillussp.) which showed pronounced morphological plasticity. The cell length varied from 2 to >40 μm and encompassed rods, curved cells, and long filaments. Without grazers and with a sufficient substrate supply, bacteria grew mainly in the form of medium-sized rods (4 to 7 μm), with a smaller proportion (Ochromonassp. showed that freely suspended cells of 80% filamentous cells. These attained a biomass comparable to that of populations in chemostats without grazers, which were composed of medium-sized rods and c-shaped cells. Carbon starvation resulted in a fast decrease in cell length and a shift towards small rods, which were highly vulnerable to grazing. Dialysis bag experiments in combination with continuous cultivation revealed that filament formation was significantly enhanced even without direct contact of bacteria with bacterivores and was thus probably stimulated by grazer excretory products.