For determination of the cellular trace metal content, at the end of the experiment 400 mL of water were filtered onto 0.2 μm TMC-cleaned polycarbonate filters (EMD Millipore, Darmstadt, Germany) and then rinsed for 15 min with a 0.1 M oxalic acid wash to remove cell surface bound trace metals. Finally, the filters were rinsed with filtered seawater and stored in 5 mL TMC polyfluoralkoxy vials until further analysis. Samples were analyzed with an inductively coupled plasma mass spectrometry (ICP-MS) following a digestion with nitric acid (HNO3) and HF.