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Chrysolepidomonas dendrolepidota was described recently and little is known of its distribution, physiology or ecology. Although many photosynthetic chrysophytes have been identified as mixotrophic, combining photosynthesis and phagotrophy, this study provides the first evidence of mixotrophic nutrition in C. dendrolepidota. In a 2 × 2 factorial experiment with relatively high and low levels of light and nutrients, the ingestion rate was highest in the high-light, low-nutrient treatment. Growth rates and abundance increased in high- versus low-light conditions. In additional experiments with nitrogen and phosphorus modified separately, ingestion rate was different from the control only in the treatment with both macronutrients reduced. Supplementing low-nutrient treatments with additional bacteria led to an increase in algal abundance relative to controls without added bacteria. These results indicate that acquisition of major nutrients is a likely driver of mixotrophic nutrition in C. dendrolepidota. Although the distribution of C. dendrolepidota is unknown, mixotrophic chrysophytes can seasonally dominate planktonic algal abundance in some aquatic systems, and nutrient limitation is known to increase bacterivory in some of those species. Identifying and understanding how different mixotrophic species respond to changing environmental cues is crucial to understanding their roles in aquatic food webs.

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.

Between July and December 1998, mixotrophic ciliates ranged in concentration from 751 to 33,600 cells/L in a small oligotrophic lake in northeast PA. Mixotrophic ciliates, primarily oligotrichs, comprised on average 58 % of the total ciliate fauna and 88 % of the total ciliate biovolume during summer and fall. Stombidium viride appeared to be the dominant mixotroph. Average ciliate abundance did not change substantially during this time period. However in December, after lake turnover, only 5 % of all ciliates were mixotrophic, contributing 50% of the total ciliate biovolume. The cell density of mixotrophic ciliates was not significantly correlated with Chl-a concentration. Grazing experiments conducted in August and October, using fluorescently labeled beads (0.5 μm diameter) indicated that mixotrophic ciliates removed 30.6 and 17.4 bacteria ciliate -1 h -1 and heterotrophic ciliates removed bacteria at a rate of 18.2 and 3.0 bacteria ciliate -1 h -1 . Overall, the ciliates removed less than 1% of the bacterial standing stock per hour.

SUMMARY 1. Previous studies of mixotrophy in the flagellate Poterioochromonas malhamensis (Chrysophyceae) were performed on strains that had been in culture for > 30 years. This study aims to compare mixotrophy in a cultured strain with one recently isolated from a mesotrophic lake (Lacawac) in Pennsylvania, U.S.A. 2. P. malhamensis from the lake exhibited a nutritional flexibility similar to that of the culture strain, growing phototrophically but inefficiently in comparison to other nutritional modes (growth rate (m) = 0.015 h ‐1 ). Supplementing an inorganic salts medium with 1 mM glucose resulted in a doubling of m to 0.035 h ‐1 and 0.033 h ‐1 in the light and the dark, respectively. Addition of an algal prey, Nannochloris, to the inorganic salts medium increased growth to rates similar to those observed with glucose. Maximum growth of the lake strain, 0.095 h ‐1 , was achieved when bacteria was supplied as food. During growth on bacteria, cellular chlorophyll a (Chl a) decreased from 140 fg cell ‐1 to 10 fg cell ‐1 over 22 h when cultured either in the light or dark. In illuminated cultures, cell-specific Chl a concentration recovered to 185 fg cell ‐1 after bacteria became limiting. 3. In contrast to the cultured strain, however, the lake isolate exhibited an inverse relationship between light intensity and ingestion rate. Calculated grazing rates, based upon the ingestion of fluorescently labeled bacteria, were 3.2, 5.2 and 9.4 bacteria flagellate ‐1 h ‐1 , for P. malhamensis incubated in high light, low light and darkness, respectively. Phagotrophy is thus influenced by a light regime in this predominately heterotrophic mixotroph.

We examined effects of terrestrial and aquatic detritus on optical, chemical, and biological properties of a planktonic food web in mesotrophic Calder Lake, New York. The 5400-L mesocosms received detritus from terrestrial leaves or aquatic macrophytes (added 1-2 mg dissolved organic C/L). Terrestrial detritus had significant effects on UV-B absorbance (OD250 twofold greater), but humic substances (OD440) remained low. Both C sources resulted in 40% declines in oxygen saturation and two- to threefold increases in dissolved inorganic and total P but no effects on NH4+ or NO3-. Bacterial growth was stimulated by 20% in systems receiving macrophyte C but declined by 50% in systems with terrestrial C. Initially, controls had >60% greater chlorophyll a concentrations than C-amended systems, but mesocosms with macrophyte C exhibited the greatest chlorophyll a concentrations after day 10. Autotrophic nanoflagellates

Direct observation was used to measure feeding rates of the flagellate Spumella on three sizes of bacteria plus 0.3 µm latex beads using video microscopy. Feeding rate was maximum on the intermediate-sized bacteria. Maximum ingestion rates (Im) for the large- (0.53 µm3), intermediate- (0.08 µm3) and small-sized (0.02 µm3) bacteria and 0.014 µm3 latex beads were 11, 38 and 14 bacteria and 9 beads flagellate−1 h−1, respectively. The growth rates of Spumella sp. feeding on monoxenic cultures of the large- vs. the intermediate-sized bacteria were indistinguishable but Spumella sp. could not sustain its population density when feeding on the small bacterium as the sole food source. Our data are consistent with the hypothesis that Spumella sp., and possibly other flagellate protozoa, tend to feed selectively on larger prey. One consequence of this hypothesis is that differential grazing by bactivores may select for small bacteria in natural waters.

Chroococcoid cyanobacteria, (mean size = 0.79 μm, likely Synchetocystis limnetica Popovsk) and total eubacteria (mean size = 0.33 μm), from Lake Baikal, USSR, were enumerated using epifluorescence microscopy and sized with image analysis. Bacterial densities ranged from 0.44 · 106 cells ml−1 at 250 m to 2.3 · 106 cells ml−1 at the surface. Mean eubacterial abundance was 1.3 · 106 cells ml−1. Cyanobacterial densities were more variable, ranging from 0.42 · 104 cells ml−1 at 250 m to 9.8 · 104 cells ml−1 at the surface, with a mean abundance of 2.7 · 104 cells ml−1. The cyanobacteria, in particular, occurred in clusters resembling “marine snow”. Our results indicate that Lake Baikal picoplankton size and density are similar to other large lakes but may have a more diverse community structure than in other large oligotrophic lakes.

Ligh emitted by the bioluminescent bacterium, Xenorhabdus luminescens (isolated from a nematode host), can be measured to monitor reductions of these bacteria in the presence of phagotrophs. X. luminescens is relatively large (0.6 ¢ 3 μm), but comparable in size to many cyanobacteria. We used the light emission method to examine phagotroph feeding on X. luminescens using uni-specific cultures of two chrysomonads, Ochromonas sp. and Spulemma sp. From light decay rates in control and experimental vials, we computed an apparent filtration rate (FR); then, for a concentration (C) of 1 · 106 bacteria ml−1, we estimated capture rate (CR) as FR · C. The Ochromonas sp. did not ingest the bacterium. The maximum estimated FR for Spumella, observed at 6.0 · 103 flagellates ml−1 (medium density), was 0.37 ml h−1, for a volume-specific clearance rate (FR/cell volume) of 7.9 · 105 h−1 and a CR of 62 bacteria · flagellate−1 h−1. Video microscopy indicated these were accurate estimates of capture rates. Microscopic counts were used to monitor growth of a flagellate, Spumella sp., on X. luminescens as the sole food supply. The flagellate doulbed in number every 3.2, while consuming bacteria at a rate of 23 bacteria flagellate−1 h−1. The Spumella grazed the bacteria to a minimum of 5 · 105 cells ml−1, a concentration comparable to observed field densities of other bacteria.

The toxicity of the chrysophyte flagellate Poterioochromonas malhamensis to the rotifer Brachionus angularis was investigated. Fed rotifers exposed to the flagellate experienced a mortality rate indistinguishable from starvation. Unfed rotifers exposed to the flagellate experienced a higher mortality rate. The mortality rate appears to depend on the flagellate concentration. Higher doses of flagellates resulted in quicker rotifer death. These laboratory results are consistent with the hypothesis that the occurrence of B. angularis in the field may be negatively related to the presence of P. malhamensis and related flagellates.

Escherichia coli can have a filamentous growth form due to mutation or, in the current study, selection of predator-resistance when preyed on by a bactivorus protozoan. When used as food for the mixotrophic flagellate, Poterioochromonas malhamensis, filamentous E. coli appeared in continuous and batch cultures. The filamentous morphology of E. coli is persistent in plate and liquid culture in the absence of the flagellate. Typically, filament length is long in exponential growth and short in stationary phase.A major question in the morphology of these filamentous E. coli (Fig. 1) is whether they are chains of cells that have failed to separate or whether they are a continuous filament without septa. Fluorescence light microscopy using DAPI, a DNA-specific stain, indicates that the filaments show both continuous fluorescence (Fig. 2a), indicating a continuous filament, and discrete units of fluorescence (Fig. 2b), indicating separate cells. Division patterns of living filaments observed with video microscopy show that cell division does occur, but it has been observed only at 10-20 μm from the ends of filaments.