Total dissolved Fe (dFe) and Mn (dMn) concentrations were determined in the culture medium (without cells). To this end, 100 mL of seawater were filtered through HCl-cleaned polycarbonate filters (0.2 μm pore size, EMD Millipore, Darmstadt, Germany) using a TMC filtration system and the filtrate collected in a HCl-cleaned polyethylene bottle, which was stored triple bagged at 2 ̊C for analysis. Concentrations of total dFe and dMn were determined on a SeaFast system (Elemental Scientific, Omaha, NE, USA; (Hathorne et al., 2012; Rapp et al., 2017)) coupled to an inductively coupled plasma mass spectrometer (ICP-MS, Element2, Thermo Fisher Scientific, resolution of 2000).