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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.

Autotrophic and heterotrophic growth characteristics of a nano-flagellate were investigated. The flagellate,Ochromonas sp., was isolated from the northern Baltic Sea. Autotrophic growth was poor. Axenically pregrown flagellates did not increase significantly in cell number during incubation in different inorganic media. The number of flagellates remained constant 3-5 weeks in cultures kept in the light (100μmol m(-2) sec(-1)), whereas in the dark, a high mortality rate was found. Uptake of inorganic(14)C into an acid-stable fraction indicated thatOchromonas had a functional photosynthetic apparatus. Heterotrophic growth in both liquid medium and medium containing bacteria was rapid. The maximum growth rate corresponded to a generation time of 5.3 hours. Light had no effect on heterotrophic growth. Cells pregrown onEscherichia coli minicells survived without additional bacteria as food when kept in the light, but rapid death occurred in darkness. In conclusion, heterotrophy is the major mechanism to support growth in this species ofOchromonas, but under poor environmental conditions photoautotrophy might be a strategy for survival rather than growth.

Ingestion and growth rates of the nanoflagellate predator Ochromonas danica feeding on the bacterium Pseudomonas fluorescens were quantified in laboratory cultures. Bacterial prey were grown under four nutritional conditions with respect to macronutrient elements: C-limited, N-limited, P-limited, and balanced. Ingestion and growth rates were saturating functions of prey abundance when preying upon nutritionally balanced, C-limited, and P-limited bacteria but were unimodal functions of abundance when preying on N-limited bacteria. At saturating prey concentrations, the ingestion rate of C-limited prey was about twice that of prey in other nutritional states, while at subsaturating prey concentrations, the ingestion rates of both C- and N-limited prey were higher than those of prey in other nutritional states. Over all prey concentrations, growth was most rapid on balanced and C-limited prey and generally lowest for P-limited prey. Due to the unimodal response of growth rate to abundance of N-limited prey, growth rate on N-limited prey approached that obtained on balanced and C-limited prey when prey were available at intermediate abundances. The accumulation of recycled N increased with the growth rate of O. danica. Recycling of N was highest when O. danica was feeding upon P-limited prey. The accumulation of recycled P increased with growth rate for balanced and N-limited prey, but not for P-limited prey, which consistently had low accumulation of recycled P. The low growth rate and negligible recycling of P for O. danica preying on P-limited prey is consistent with the theory of ecological stoichiometry and resembles results found for crustacean zooplankton, especially in the genus Daphnia. Potentially, the major predators of bacterioplankton and a major predator of phytoplankton play analogous roles in the trophic dynamics and biogeochemistry of aquatic ecosystems.

Axenic growth of a mixotrophic alga, Ochromonas sp., was compared in several inorganic and organic media, and in the presence of live bacteria under nutrient-replete and low-nutrient conditions. Axenic growth in the light was negligible in inorganic media with or without the addition of glucose. Addition of vitamins increased growth rate, but average cell size declined, resulting in no net increase in biomass. Supplementing axenic cultures with a more complex organic substrate resulted in moderate growth and higher maximal abundance (and biomass) than in the inorganic media with added vitamins. The absence of light did not greatly affect population growth rate in the presence of complex dissolved organic compounds, although cell size was significantly greater in the light than in the dark. The highest growth rates for the alga (up to 2.6 d?1) were measured in treatments containing live bacteria. Increases in cell number of Ochromonas sp. in the presence of bacterial prey were similar in the light and dark, although chloroplast and cell sizes differed. Bacterial abundance was reduced and dissolved phosphorus and ammonia were rapidly released in bacterized cultures in the light and dark, indicating high rates of bacterial ingestion and suggesting an inability of the alga to store or utilize N and P in excess of the quantities required for heterotrophic growth. Low-nutrient conditions in the presence of bacteria were promoted by adding glucose to stimulate bacterial growth and the uptake of N and P released by algal phagotrophy. Subsequent decreases in dissolved N and P following the addition of glucose corresponded to a second period of rapid growth of the alga in both light and dark. This result, combined with evidence for slow axenic growth of this strain, indicated that nutrient acquisition for this species in the presence of bacteria was accomplished primarily via ingestion of bacteria.