Chlorophyll a-content of all (total), the small (0.2-2 μm) and the large (> 2 μm) cells determined from the community sampled at station West and East (PS97) before and after exposure to different Fe and Mn availabilities

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. Chlorophyll a samples were taken at the beginning and the end of both experiments. In order to compare the contribution of large (>2 μm) relative to small cells (0.2-2 μm), 250 mL (on average) of samples were filtered onto 0.2 μm (for the total fraction) and 2 µm (for the large fraction) polycarbonate filters, hence the small fraction was calculated as the difference of the total and the large fraction. All samples were directly flash frozen into liquid nitrogen (N~2~) and then stored at −80 ̊C in the dark until further analysis. After being homogenized, samples were extracted in 90% acetone for 24h at 4 ̊C in the dark and analyzed fluorometrically on a Trilogy Fluorometer.