• plastidic protists;
  • planktonic picoeukaryotes;
  • cell sorting;
  • electron microscopy


Flow cytometric sorting, based on cellular optical properties and macromolecule content, has been successfully employed to taxonomically affiliate bacterioplankton. However, this approach has not been much used for eukaryotic plankton. To redress this imbalance, we identified a conspicuous group of red autofluorescent picoplankton in surface waters of the South Atlantic Ocean. Using catalysed reporter deposition fluorescence in situ hybridization, virtually, all cells sorted from that group were affiliated with the Mamiellales clade II (84 ± 4%, division Chlorophyta) with a size of 1.6 ± 0.03 μm. Based on electron microscopy, the Mamiellales clade II–sorted cells have a simple morphology with apparently no scales, flagella or surface features. Their latitudinal distribution resembled the distribution of Synechococcus with very low concentrations in the surface waters of the Southern subtropical gyre (0.6–1.6 × 103 cells mL−1) and increased concentrations in the Southern temperate waters 8.3 × 103 cells mL−1. Identification of the flow cytometric group as Mamiellales clade II allowed us to characterize the morphology of these enigmatic uncultured picoplanktonic cells by electron microscopy and to determine their apparent preference for temperate rather than subtropical oceanic photic waters.