Feeding selection based on cell size
Experiment 5 was designed to determine whether selective feeding behavior in the previous experiments was due to differences in cell size of A. anophagefferens (2–4 µm) and I. galbana (4–6 µm) or if selection was the result of physical or chemical differences. In this experiment, M. pusilla was used as the control alga since its size range overlaps with that of A. anophagefferens (∼1–3 µm). Grazing and developmental experiments were conducted in the same manner described above.
Cell densities for this experiment were determined using a Becton Dickinson FACSCalibur flow cytometer because visual differentiation between preserved cells was not possible due to the similarities in size and shape of A. anophagefferens and M. pusilla cells. Samples were taken to the Stony Brook Hospital Flow Cytometry lab and processed live, within 1 h after experiments were sampled. Distinction of cell types in mixtures was made using unique pigment fluorescence signatures in FL3 (>670 nm) versus FL4 (661 ± 16 nm) dot plots. Cell densities were obtained by adding 0.1 mL of Fluoresbrite yellow–green microspheres (6 µm) from Polysciences, Inc. to 5 mL samples from all jars at a final concentration of 230 000–240 000 spheres mL−1 (exact concentrations were determined for each experimental date). Cell densities were calculated by comparing a set number of events (5000) for a known abundance of microspheres to the number of algal events.