Two Fe-Mn bottle amendment experiments with two natural phytoplankton communities were performed during Polarstern expedition PS97 in 2016 in the Drake Passage. At two locations, sea water was pumped (using trace metals clean techniques) from 25m depth and used to fill polycarbonate bottles after having passed through a cleaned 200 μm mesh (removing large grazers). The Control treatment was the sampled seawater without any trace metals addition while the other three treatments were enriched with either FeCl3 alone (0.5 nM; +Fe treatment) or MnCl2 alone (1 nM; +Mn treatment) or both trace metals together (+FeMn treatment). All treatments were done in triplicate 2,5L PC bottles. All incubation bottles were maintained at 30 μmol photons m-2 s-1 under a 16:8 (light:dark) hour cycle at 1 ̊C.Autotrophic picoeukaryotes were analyzed via flow cytometry. At the start and the end of the experiments, samples were preserved with 10% buffered formalin, flash-frozen in liquid nitrogen, and analyzed flow cytometrically to assess picoplankton densities. Before running the samples, 2 μL beads (Sperotech - Rainbow Fluorescent Particles (RFPs) - 2.11 μm) were added to each treatment as a size and fluorescence reference. Then picoeukaryotes were identified based on side scatter versus FL-3. Three P subgroups (0.2 – 2 μm) were differentiated according to their size : small (P1), medium (P2) and large (P3), according to sub-cluster of events.