Simultaneous determination of iron reduction and uptake by phytoplankton.

Yeala Shaked, Adam B. Kustka, Francois M. M. Morel,Yigal Erel

Limnol. Oceanogr. Methods 2:137-145 (2004) | DOI: 10.4319/lom.2004.2.137

ABSTRACT: 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 (C18 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 retained the Fe(II)FZ3 complex. The method was calibrated using 55Fe(II) standards in NaCl solution, yielding high recovery efficiency and linear behavior across the entire range tested from 3 x 10-11 M to 2 x 10-6 M Fe(II). Special attention was given to possible artifacts due to 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 EDTA as a model ligand.