Bacterivory in seawater studied with the use of inert fluorescent particles1

Fluorescent pigments were used to study the uptake of bacteria‐sized particles by laboratory and field populations of heterotrophic nanoplankton. With membrane filtration these particles can be made to mimic the natural bacterioplankton size distribution. Uptake was roughly linear for 20–40 min, and leveled off after that, with the number of particles per cell remaining constant for at least several hours. Photosynthetic nanoplankton did not take up the particles in significant numbers. Experiments with a small nonpigmented flagellate indicated some selection against the particles in favor of ingestion of unicellular cyanobacteria, but this may have been based on the size of the particles rather than quality. Preliminary field experiments indicated that nonpigmented nanoplankton was responsible for virtually all of the grazing on bacterioplankton, with microzooplankton playing a minor role.