Simultaneous determination of iron reduction and uptake by phytoplankton

• Published in: Limnology and Oceanography: Methods
• Main Authors: Shaked, Yeala, Kustka, Adam B., Morel, François M.M., Erel, Yigal
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2004
• Subjects: Bering Sea
• Online Access: http://dx.doi.org/10.4319/lom.2004.2.137; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.4319%2Flom.2004.2.137; https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lom.2004.2.137

We have developed a new method to assess simultaneously the rates of biological reduction of Fe(III)‐ligand complexes at nanomolar concentrations and iron uptake in marine phytoplankton. Other methods for measuring iron reduction require micromolar iron concentrations, which can saturate iron uptake in these organisms. In the present study, high sensitivity was obtained by combining radiometric techniques, ligand competition, and selective retention on solid phase extraction columns (C 18 Sep‐Pak©). Following the addition of the Fe(II) specific ligand ferrozine (FZ) to seawater spiked with nanomolar concentrations of radiolabeled iron, the rate of iron uptake is determined by filtering aliquots of cultures, and the rate of Fe(II) binding to FZ by passing the filtrate through a column, which retains the Fe(II)FZ 3 complex. The method was calibrated using 55 Fe(II) standards in NaCl solution, yielding high recovery efficiency and linear behavior across the entire range tested from 3 × 10 −11 M to 2 × 10 −6 M Fe(II). Special attention was given to possible artifacts caused by reduction of Fe(III) in the presence of FZ and Fe(III) adsorption onto the column, which are critical when measuring picomolar concentrations of Fe(II). The method was successfully applied to Thalassiosira weissflogii cultures and to natural phytoplankton populations in the Bering Sea using ethylenediaminetetraacetic acid as a model ligand.